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diff --git a/37199-h/37199-h.htm b/37199-h/37199-h.htm new file mode 100644 index 0000000..5192336 --- /dev/null +++ b/37199-h/37199-h.htm @@ -0,0 +1,2448 @@ +<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN" + "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd"> + <html xmlns="http://www.w3.org/1999/xhtml"> + <head> + <meta http-equiv="Content-Type" content="text/html;charset=ISO-8859-1" /> + <meta http-equiv="Content-Style-Type" content="text/css" /> + <title> + The Project Gutenberg eBook of Ecology of the Opossum on a Natural Area in Northeastern Kansas, by Henry S. Fitch and Lewis L. Sandidge. + </title> + <style type="text/css"> + + .book {margin-left: 10%; margin-right: 10%;} + p {text-align: justify; text-indent: 1.5em;} + table {margin-left: auto; margin-right: auto;} + td {padding-left:6px;} + ins {background-color: #e0ffe0; text-decoration: none;} + .pagenum {position: absolute; left: 92%; text-indent:0; + font-size: 0.75em; text-align: right; color: #b0b0b0;} + .pagenum2 {position: absolute; left: 92%; text-align: right; + color: #ffffff;} + .pagenum3 {position: absolute; left: 92%; text-indent:0; + text-align: right; color: #b0b0b0;} + .reference {margin-left: 5.5em; text-indent: -3em;} + .vtop {vertical-align: top;} + .center {text-align: center; text-indent:0; } + .text_lf {text-align: left;} + .text_rt {text-align: right;} + .smaller {font-size: 0.75em;} + .smcap {font-variant: small-caps;} + .caption1 {font-weight: bold; font-size:2.00em; text-align: center;} + .caption2 {font-weight: bold; font-size:1.50em; text-align: center;} + .caption3 {font-weight: bold; font-size:1.15em; text-align: center;} + .caption3nc {font-size:1.15em;} + .caption4 {font-weight: bold; font-size:0.75em; text-align: center;} + .trans_notes {background:#e0ffe0; padding: 7px; border:solid black 1px;} + .footnote {font-size: 0.8em;} + .fnanchor {vertical-align: super; font-size: .8em; + text-decoration: none;} + .fig_center {margin-left: auto; margin-right: auto; text-align:center;} + .fig_caption {font-size:0.85em; text-indent:2.3em; text-align: justify;} + </style> + </head> +<body> + + +<pre> + +The Project Gutenberg EBook of Ecology of the Opossum on a Natural Area in +Northeastern Kansas, by Henry S. Fitch and Lewis L. Sandidge + +This eBook is for the use of anyone anywhere at no cost and with +almost no restrictions whatsoever. You may copy it, give it away or +re-use it under the terms of the Project Gutenberg License included +with this eBook or online at www.gutenberg.org + + +Title: Ecology of the Opossum on a Natural Area in Northeastern Kansas + +Author: Henry S. Fitch + Lewis L. Sandidge + +Release Date: August 24, 2011 [EBook #37199] + +Language: English + +Character set encoding: ISO-8859-1 + +*** START OF THIS PROJECT GUTENBERG EBOOK ECOLOGY OF OPOSSUM--N.E. KANSAS *** + + + + +Produced by Chris Curnow, Tom Cosmas, Joseph Cooper and +the Online Distributed Proofreading Team at +https://www.pgdp.net + + + + + + +</pre> + + +<div class="book"><!-- Begin book --> +<p><span class="pagenum2"><a name="Page_307" id="Page_307">[Pg 307]</a></span></p> +<div class="center"> +<img src="images/bar_double.png" width="100%" height="15" alt="double bar" /> +<div class="caption2"><div class="smcap">University of Kansas Publications<br /> +Museum of Natural History</div></div> +<img src="images/bar_single.png" width="30%" height="15" alt="single bar" /> +<div class="caption2">Volume 7, No. 2, pp. <ins title='Correction: was "305-338"'>307-338</ins>, 5 figures in text</div><br /> +<div class="center"> + <img src="images/bar_single.png" width="30%" height="15" alt="single bar" /> + <span class="caption2">August 24, 1953</span> + <img src="images/bar_single.png" width="30%" height="15" alt="single bar" /></div> +<br /> + +<div class="caption1">Ecology of the Opossum on a Natural Area<br /> +in Northeastern Kansas</div> +<br /> +<br /> +<div class="caption3">BY</div> +<br /> +<div class="caption2">HENRY S. FITCH</div> + +<div class="caption3">AND</div> + +<div class="caption2">LEWIS L. SANDIDGE</div> +<br /> +<br /> + +<span class="smcap">University Of Kansas<br /> +Lawrence</span><br /> +1953 +</div> +<br /> +<br /> +<br /> +<br /> + +<p><span class="pagenum2"><a name="Page_308" id="Page_308">[Pg 308]</a></span></p> +<div class="center"> +<br /> +<br /> +<span class="smcap">University of Kansas Publications, Museum of Natural History</span><br /> +<br /> +Editors: E. Raymond Hall, Chairman, A. Byron Leonard, Robert W. Wilson<br /> +<br /> +<br /> + +<div class="caption4">Volume 7, No. 2, pp. <ins title='Correction: was "305-338"'>307-338</ins>, + 5 figures in text<br /> +Published August 24, 1953</div> +<br /> +<br /> +<br /> + +<div class="caption4"><span class="smcap">University of Kansas</span><br /> +Lawrence, Kansas</div><br /> +<br /> +<br /> + +<div class="caption5">PRINTED BY<br /> +FERD VOILAND, JR., STATE PRINTER<br /> +TOPEKA, KANSAS<br /> +1953<br /> +<img src="images/union_label.png" width="74" height="27" alt="Look for the Union Label!" /><br /> +24-7812<br /></div> +</div> +<br /> +<br /> + +<p><span class="pagenum"><a name="Page_309" id="Page_309">[Pg 309]</a></span></p> +<br /> +<br /> +<div class="center"> +<div class="caption2">Ecology of the Opossum on a Natural Area<br /> +in Northeastern Kansas</div> + +<div class="caption3">BY</div> + +<div class="caption3">HENRY S. FITCH and LEWIS L. SANDIDGE</div> +</div> + +<p>On the 590-acre University of Kansas Natural History Reservation +where our study was made, the opossum, <i>Didelphis marsupialis virginiana</i> +Kerr, is the largest predatory animal having a permanently +resident population. The coyote, racoon and red fox also occur on +the area but each ranges widely, beyond the Reservation boundaries. +With the passing nearly a century ago of the larger animals of the +original fauna, the buffalo, elk, deer, antelope, wild turkey, gray +wolf and others, lesser herbivores and carnivores including the opossum +and animals of similar size fell heir to their key positions of +predominance at the peak of the food pyramid. These smaller animals, +however, exert less powerful effects in controlling the general +aspect of the biotic community, and affect it in different directions. +The over-all ecology is greatly altered. The flora and fauna both +are undergoing successional changes which will continue for a long +time and probably will culminate in a biotic community much different +from the original climax.</p> + +<p>The opossum plays an important part in this process of change; +being relatively large, numerous, and of omnivorous habits, it variously +influences, directly and indirectly, the populations of its plant +and animal associates, through a complex web of interrelationships. +Several excellent field- and laboratory-studies of the opossum have +been published (Hartman, 1928, 1952; Lay, 1942; Reynolds, 1945; +Wiseman and Hendrickson, 1950) and the life history of this remarkable +marsupial is already well known. The purpose of our +study, therefore, was to gain a better understanding of the ecological +relationships of the opossum in the particular region represented +by the study area. To accomplish this, we gathered data concerning +the animal's responses to climate and varying weather conditions; +its annual cycle of breeding, growth and activity, movements, +principal food sources, numbers, population turnover, and natural +enemies. Although we did gain a somewhat better understanding +of the opossum's ecology, results are remarkably meager in proportion +to the large amount of time expended. The hours of work daily +in setting and tending a line of live-traps ordinarily were rewarded +with only a few records, sometimes none. Comparable time and effort +<span class="pagenum"><a name="Page_310" id="Page_310">[Pg 310]</a></span> +directed to the study of smaller and more abundant kinds of animals +has been far more productive of data. Field work was carried +on in parts of 1949, 1950, 1951 and 1952.</p> + +<div class="fig_center" style="width: 653px"> +<img src="images/fig1.png" width="653" height="665" alt="" title="" /><br /> +<div class="fig_caption"> +<span class="smcap">Figure 1.</span> Map of the University of Kansas +Natural History Reservation showing locations where opossums +were live-trapped. +</div></div> + + +<p>Because opossums are nocturnal and rarely seen in the course of +their regular activities, the present study is based mainly on information +gained by live-trapping them. Several different sizes of traps +of the type described by Fitch (1951) were used. The most successful +were 2' × 8" × 8" in dimensions although many of the larger ones +were also used. They were constructed of hardware cloth having +a half-inch mesh. Live-trapping was begun in October 1949 by +Fitch with a line of about a dozen traps. In the following month +Sandidge joined in the field work. The trapping was continued +throughout the winter and spring of 1949-1950 and was resumed the +<span class="pagenum"><a name="Page_311" id="Page_311">[Pg 311]</a></span> +following fall and more traps were added from time to time until +a maximum line of approximately 60 was attained. Sandidge's participation +ended in December, 1950. The live-trapping was continued +on a reduced scale by Fitch through the winter and spring +of 1951 and some was done sporadically in the fall, winter and spring +of 1951 to 1952.</p> + +<p>Traps were baited with a variety of foods such as carcasses of +small vertebrates, meat scraps, canned dog food, ground horse meat +and bacon grease. At each capture, sex, weight, and individual +formula of the opossum, based on toe-clipping and ear-clipping +(Fitch, 1952), were recorded. Also recorded was the exact site of +capture as located in one of 84 divisions of the Reservation and estimated +in feet from some named landmark. Notes on breeding condition, +pelage, injuries, parasites and general appearance were also +taken at the time of capture. For opossums caught in 1951 and 1952, +the hind foot measurement was recorded.</p> + +<p>Often, attempt was made to follow the released opossum to determine +the direction and distance of its homeward travel but this was +difficult because of brushy terrain and secretive habits of the animal. +An opossum being followed would almost invariably take refuge in a +tree if it caught sight of the observer. Other information regarding +the animal's habits was obtained from tracks in snow or soft soil +and from the distribution and contents of scats. Carcasses of opossums +which had fallen victim to predators were found on a few +occasions and in some instances clues as to the identity of the predator +were obtained. One hundred and seventeen opossums were live-trapped +and handled a total of 276 times. Six of these were dead +when first found in the traps. The remaining 111 were marked and +released. In addition, 207 pouch-young carried by adult females +were recorded and 115 of these were individually marked by toe-clipping. +Some of the opossums that were marked while in the +mother's pouch were subsequently recaptured when they were well-grown, +independent young, or adults, affording information on +growth and dispersal.</p> +<br /> + +<div class="caption2"><a name="HABITAT" id="HABITAT"></a> +HABITAT</div> + +<p>The habitats of the Reservation have been described briefly by +Fitch (1952) and by Leonard and Goble (1952). More than half +the area consists of steep wooded slopes with mixed second growth +forest, consisting of elm, hickory, oak, walnut, ash, honey locust, +hackberry and osage orange, in about that order of abundance, with +thickets of blackberry, crabapple, wild plum and grape. Fallow +<span class="pagenum"><a name="Page_312" id="Page_312">[Pg 312]</a></span> +fields and pastures of the upland and valley floors alternate with the +woodland. The varied habitat provides numerous different food +sources. Along the edges of the hilltops there is a nearly continuous +limestone outcrop with a lower outcrop paralleling it. These +rock ledges, well distributed throughout the area, provide an +abundance of den sites and most of the opossums definitely trailed +to a home base were found to be utilizing dens in the rock ledges. +Two small creeks on the area have some water for most of the year. +As compared with wooded bottomland of larger stream courses in +Douglas County and those counties adjoining it, the Reservation +area probably supports a relatively low population density of opossums. +"Sign" has been found in much greater abundance in near-by +areas supporting a heavier woodland.</p> + +<p>Every part of the Reservation is used by opossums, but their +activity is concentrated in the woodland, and all dens found were +in woodland. Most parts of the fields are within 100 yards of the +edge of the woodland and no point is more than 700 feet from the +edge. Most of the opossums' foraging in fields was concentrated +along the edge; otherwise they tended to follow creeks and gullies +and they follow well worn trails more often than they do in the +woods. Within the woodland, activity tended to be concentrated +along the small streams, and along the rock ledges where den sites +were plentiful. Throughout the annual cycle, and from year to +year, there were minor shifts in areas of concentrated activity depending +on seasonal changes in food sources such as thickets of wild +plum, crabapple, blackberry and grape, with fruits ripening at +slightly different times of year. The areas adjoining the Reservation +offer somewhat similar habitat conditions, part woodland, part pasture +land and some cultivated fields with corn or other crops which +provide food sources for the opossum.</p> + +<p>Under original conditions the area that is now the Reservation +probably was marginal habitat for opossums, consisting mainly of +open grassland with trees in small and scattered clumps, if indeed +they were present at all. There has been steady encroachment of +shrubs and trees, originally chiefly confined to near-by bottomlands +such as those of the Kaw and Wakarusa valleys. Concurrently, the +original hardwood forest of the bottomlands has mostly disappeared, +and the land has been taken over for intensive agricultural use. +The new upland forest provides a habitat different in many respects +from the original bottomland forest. The species composition, in +trees and other plants, is somewhat different, with more xeric types, +<span class="pagenum"><a name="Page_313" id="Page_313">[Pg 313]</a></span> +especially on steep south slopes. Logs and large old hollow trees +are scarce. The lack of such potential den sites is compensated for +by the abundance of holes and crevices along hilltop rock ledges.</p> +<br /> + +<div class="caption2"><a name="BEHAVIOR" id="BEHAVIOR"></a> +BEHAVIOR</div> + +<p>Undisturbed opossums were seen in the course of their normal +activities on only a few occasions, and behavior is known to us +mainly from the sign and from observations made on those that were +live-trapped. Ordinarily those taken in live-traps were found curled +up in deep sleep from which they did not arouse until touched or +until the trap was moved or jarred. Reactions to humans varied +greatly in individuals and was not necessarily correlated with age +or sex. Adult males were uniformly hostile to the trapper and reacted +with harsh, low growls, with back arched and hair bristling. Although +many adult females and young of both sexes were similarly +hostile in behavior, others were not. Some cowered silently in the +trap. Others showed hardly any uneasiness. A small proportion of +them feigned death when handled or even before they were touched. +Feigning was especially frequent in response to clipping of toes +and ears when the animal was marked. In some that were handled, +the feigning reaction was weak or incomplete, the animal arising +almost immediately after collapsing or beginning to collapse in the +feint.</p> + +<p>Those that feigned death usually maintained the deception for +not more than two or three minutes after a person had moved away +out of sight. The opossum first raised its head and sniffed, listened, +and looked about cautiously for a short time, with body and limbs +still relaxed in the feigning posture. Failing to detect any sign of +danger, it gradually shifted to a sitting position, and then to a standing +one, from which it began moving away with many short pauses +at first, and then more rapidly.</p> + +<p>Upon being released, some opossums scrambled for shelter immediately; +others stood their ground defiantly with back arched, hair +bristling and fangs bared. One that was put on the defensive would +usually maintain its stance for less than a minute if not further disturbed +by movements of the trapper. It would then slowly turn its +head and begin walking away with deliberate gliding movements, +often pausing abruptly in the middle of its stride with one or two +feet off the ground in a pose reminiscent of that of a bird dog making +its "point." After moving away a few yards, it would gradually +accelerate its pace in a scramble for shelter, but an occasional individual +moved away unhurriedly, even foraging as it went.</p> + +<p><span class="pagenum"><a name="Page_314" id="Page_314">[Pg 314]</a></span></p> + +<div class="fig_center"> +<img src="images/fig2.png" width="604" height="603" alt="" title="" /> +</div> +<div class="fig_caption"> +<span class="smcap">Figure 2.</span> Half-mile-square area on Reservation, +showing dates and successive sites of capture for two subadult +male opossums; one opossum on upper half of map and other +opossum on lower half. Arrows from circles show courses taken +by released opossums that were followed to dens. (crosses). +</div> + +<p>On the few occasions when opossums were seen at night, their +relative alertness and speed of movement contrasted with the sluggishness +and seeming stupidity of those observed in daylight. Several +were seen on roads in the beam of automobile headlights. +These were quick to escape, running into thick roadside vegetation +or woods to elude pursuit. Others were found in woodland, with +the aid of a powerful flashlight as the investigator moved about on +foot. They did not permit close approach, and escaped by running. +One hid in a blackberry thicket. Several that were chased climbed +trees when hard pressed. One that was overtaken, and others that +were shaken out of trees and caught, showed fight, standing on the +defensive, and slashing at the pursuer with a rapidity and vigor +never encountered in those removed from traps in the daytime.</p> + +<p><span class="pagenum"><a name="Page_315" id="Page_315">[Pg 315]</a></span></p> + +<div class="fig_center"> +<img src="images/fig3.png" width="639" height="628" alt="" title="" /><br /> +<div class="fig_caption"> +<span class="smcap">Figure 3.</span> Half-mile-square area on Reservation, +showing dates and successive sites of capture of an old adult +male in upper half of map and an adult female in lower half. +</div> +</div> + +<p>Nocturnal tendencies of the opossum were emphasized by the infrequency +with which undisturbed individuals were seen in the daytime. +In more than a thousand days of field work on the Reservation, +opossums were found out on only four occasions. These occasional +daytime forays seem to occur almost always in animals driven by +hunger on winter days, when the temperature has suddenly risen +after periods of severely cold weather that have imposed inactivity +and fasting.</p> +<br /> + +<div class="caption2"><a name="MOVEMENTS" id="MOVEMENTS"></a> +MOVEMENTS</div> + +<p>Earlier field studies of the opossum have produced somewhat +conflicting evidence and conclusions regarding the extent and manner +of the opossum's travels. Lay (1942:158) live-trapped and +marked 117 opossums on an 86-acre study area in eastern Texas +over a two-year period and caught 29 of them at three or more different +<span class="pagenum"><a name="Page_316" id="Page_316">[Pg 316]</a></span> +trapping stations. He found that "The average minimum area +between the stations in these 29 home ranges was 11.5 acres. The +mean of the greatest distances traveled between stations was 1460 +feet, which would form a theoretical circle of 38.4 acres.... +Separate individual territories are not important to opossums as +home ranges overlapped in every instance." Reynolds, in central +Missouri, concluded that: "The subsequent recovery of only 5 of +68 released animals, the reported capture of one individual 7 miles +from the point of release nine months later, and the rapid repopulation +of an area devoid of opossums at the close of the hunting season +indicate that most opossums are nomadic." In southeastern Iowa, +Wisemann and Hendrickson (1950:336) found that: "Recaptures, +in 1942, of three opossums tagged in 1941 indicated a yearly mobility +of one-fourth mile; four tagged in 1942 were recaptured within one-half +mile from sites of tagging."</p> + +<p>Opossums, like other animals, obviously make various types of +movements. Ordinarily one tends to keep within a relatively small +area that is familiar to it and that satisfies all its ecological requirements. +This constitutes its home range. Many other animals, including +various mammals, are characterized by territoriality; individuals, +pairs or groups occupy definite areas, defended as territories, +to the exclusion of other members of their species. Like Lay (<i>loc. +cit.</i>) we found no evidence of territoriality in the opossum. In general, +opossums are unsocial but not intolerant in their behavior. In +the present study numerous individuals of both sexes and various +sizes and ages were found to be occupying the same area simultaneously, +with overlapping but no exact correspondence in home +ranges. <a name="Occasionally"></a><ins title='Correction: was Occasionaly"'>Occasionally</ins> +two or more opossums may use the same den, +but each goes its own way on its foraging and it seems that no sociability +is involved.</p> + +<p>On many occasions opossums were tracked in soft snow or mud +which retained footprints. Under conditions prevailing locally, it +was difficult to follow such a trail for any great distance but trailing +did divulge information concerning the type of route followed and +the method of foraging. Opossums were found to have little inclination +to follow beaten trails, either their own or those of other animals. +A foraging opossum moved about in an extremely circuitous +and erratic route, seldom taking more than a few steps without a +change of direction, and frequently crossing its own course in a +series of loops, some only a few feet or a few inches in diameter. +In moving about, it is guided partly by the tactile and olfactory +<span class="pagenum"><a name="Page_317" id="Page_317">[Pg 317]</a></span> +stimuli of objects on or beneath the ground surface which are potential +food sources. Foraging consists of a succession of tests of such +objects, as the animal moves from one to another. Opossums may +habitually follow intermittent creeks or gullies or even roads when +these provide better foraging than does the adjoining habitat. +Metamorphosing amphibians may provide such a food source along +a creek and the supply of crushed insects or other small animals +along a road attracts the opossum. Food is found by turning chips +and leaves, and by poking and probing in chinks and crevices with +its snout and paws. On a few occasions short, well worn trails made +<span class="pagenum"><a name="Page_318" id="Page_318">[Pg 318]</a></span> +by opossums were found, from dens to near-by feeding areas where +grape tangles provided an abundant and readily available food +source over periods of weeks. More often, an opossum follows no +trail in its search for food, but seems to wander at random within +its home range.</p> + +<div class="fig_center"> +<img src="images/fig4.png" width="608" height="615" alt="" title="" /><br /> +<div class="fig_caption"> +<span class="smcap">Figure 4.</span> Quarter-mile-square areas on +Reservation showing dates and successive sites of capture of +individual opossums; (A) subadult male; (B) subadult male; (C) +subadult male; (D) adult female. Arrows from circles show +courses that were taken by released opossums that were +followed; crosses show location of dens to which they were +traced. +</div> +</div> + +<p>Evidence of the existence and extent of home range was obtained +for those opossums that were trapped on several or many occasions. +Records of each were usually well scattered over an area hundreds +of feet in diameter. Limits of home ranges are not sharply defined +and at any time the opossum may extend its range into new areas. +It may shift to a new den from which areas beyond its original home +range are readily accessible, and may then occupy a new home range +overlapping part of the old one. Or, it may make a relatively long +shift, to an area entirely distinct from the original home range and +well separated from it. That such shifts are frequent was indicated +by the brief span of records for most of the opossums live-trapped +on the Reservation. After the first capture and marking an individual +was often caught consistently over periods of weeks, only to +drop out suddenly either having been eliminated or having moved +elsewhere. Of the 111 opossums marked and released, 62 were +caught only once and 25 others were recaptured only within a period +of one or two months. Relatively few, only 24 (14 males and 10 females), +had records extending over more than two months. Many +of the opossums trapped were probably at or near the edges of their +home ranges which barely overlapped the study area; consequently +the chances of recapturing them were poor. Those caught well +within the trapping area were much more likely to be recaptured.</p> + +<p>Tracking of opossums suggested that having once left the home +den, an animal ordinarily did not return until it had finished its +nightly foraging, and wandered more or less at random over its +home range. Successive capture sites for any one opossum might +be near together or far apart with respect to its over-all range, but +on the average, they would be separated by approximately half the +breadth of the home range assuming the animal's activity to be +evenly distributed over the whole area. Each of twenty-two opossums +was caught at only two different trapping stations. For this +group, the average distance between stations was 761 feet (657 +feet for seven males and 810 feet for 15 females) indicating home +ranges of approximately 42 acres in extent. Each of ten opossums +was caught at three different stations; for these the distances between +the first and second stations, between the first and third and +between the second and third comprise three distinct movement +<span class="pagenum"><a name="Page_319" id="Page_319">[Pg 319]</a></span> +records, and the average of all three probably affords a more reliable +figure for the radius of the home range than does the single +movement available for each of the 22 animals captured at only two +stations. For these average individual movements the mean of +this whole group of 10 was 841.5 feet. Each of five opossums was +taken at 4 different trapping stations, and for each of these a record +of six different movements was available. The average was +1016 feet. For the 37 opossums caught at two, three or four different +trapping stations, the mean distance was 817 feet; this is an +indication of home ranges of approximately 48 acres in extent. +Each of thirteen opossums was caught at five or more trapping stations. +The distribution of these stations affords a crude idea of +the extent and position of each animal's home range, but ordinarily +it might be expected that the area included between capture sites +would be less than the animal's actual home range, because relatively +few of the sites of capture would be on the margin of the +home range. For this group, maximum distances between trapping +stations averaged 1954 feet suggesting a home range of nearly 70 +acres, larger than that computed for the opossums caught at only +two, three, or four stations. However, for those caught at five or +more stations, the time involved averaged longer and probably some +had altered their ranges to invade new areas. Ranges may have +been broadly oval rather than circular so that the maximum diameter +measured between stations exceeded somewhat the average range +diameter for each animal.</p> + +<p>The opossums having home ranges entirely within the study area +were those most likely to be caught repeatedly and at different locations, +while those with ranges centering near the edge of the area, +or outside of it tended to be caught at fewer locations and less frequently. +For those animals with ranges partly outside the study +area, the captures recorded would represent only one sector of the +home range and would tend to be near together, so that many of +the radii computed for individual home ranges are too small. Each +average figure for home range is perhaps erroneously low for this +reason. The error tends to be greatest for those taken at only two +locations, and least for those trapped at the greatest number of different +locations.</p> + +<p>Approximate size of the usual home range is apparent from the +several figures although various unknown or unmeasurable factors +distort the data. The usual home range of the opossum in the area of +the study is in the neighborhood of 50 acres or a little less. With the +data available no significant differences in sizes of home ranges are +<span class="pagenum"><a name="Page_320" id="Page_320">[Pg 320]</a></span> +discernible between males and females nor between adults and +young of the year. Shifts occur frequently, contributing to population +turnover, which may result in almost complete replacement of +individuals in the course of a year's time, on an area of less than a +square mile.</p> +<br /> + +<div class="caption2"><a name="DISPERSAL_OF_YOUNG" id="DISPERSAL_OF_YOUNG"></a> +DISPERSAL OF YOUNG</div> + +<p>One hundred and fifteen small young of 14 different litters were +marked while still attached to the mother's teats in the pouches. +A fairly high rate of mortality probably is normal in the small dependent +young and further mortality probably resulted from the +deleterious effects of examining and handling them and the females +that carried them. At any rate, 47 of 208 young recorded, were +missing at subsequent recaptures of the females, before the young +were old enough to become independent. It is almost certain that +the actual losses were much higher, because the records for each +female cover only part of the period during which young are carried +in the pouch.</p> + +<p>Fifteen of these marked young of seven different litters were recaptured +after periods of months, when they were well grown or +adult and the locations of these recaptures afford information concerning +the animals' dispersal. Their records are summarized below. +Opossums that wandered much more than half a mile or at +most three-fourths of a mile from the place of original capture were +unlikely to be recaptured, and some originally recorded at sites near +the edge of the study area might have moved beyond its boundary +with much shorter shifts.</p> +<br /> + +<div class="center"> +<table summary="Table1: Recapture Data"> +<tr> + <th> Sex </th> + <th>Date of capture and<br />marking as pouch young</th> + <th>Date of<br />recapture</th> + <th>Distance<br />in feet</th> +</tr> +<tr> + <td class="text_lf">Female</td> + <td class="text_lf">April 14, 1951</td> + <td class="text_lf">September 22, 1951</td> + <td class="text_rt">1870</td> +</tr> +<tr> + <td class="text_lf">Female</td> + <td class="text_lf">May 6, 1950</td> + <td class="text_lf">February 28, 1952</td> + <td class="text_rt">1800</td> +</tr> +<tr> + <td class="text_lf">Female</td> + <td class="text_lf">May 14, 1950</td> + <td class="text_lf">December 31, 1950</td> + <td class="text_rt">1750</td> +</tr> +<tr> + <td class="text_lf">Female</td> + <td class="text_lf">March 28, 1951</td> + <td class="text_lf">January 23, 1952</td> + <td class="text_rt">1700</td> +</tr> +<tr> + <td class="text_lf">Female</td> + <td class="text_lf">May 11, 1951</td> + <td class="text_lf">November 9, 1951</td> + <td class="text_rt">1700</td> +</tr> +<tr> + <td class="text_lf">Female</td> + <td class="text_lf">May 11, 1951</td> + <td class="text_lf">March 2, 1952</td> + <td class="text_rt">1450</td> +</tr> +<tr> + <td class="text_lf">Female</td> + <td class="text_lf">April 2, 1950</td> + <td class="text_lf">October 7, 1950</td> + <td class="text_rt">1160</td> +</tr> +<tr> + <td class="text_lf">Female</td> + <td class="text_lf">April 14, 1951</td> + <td class="text_lf">May 19, 1952</td> + <td class="text_rt">1100</td> +</tr> +<tr> + <td class="text_lf">Male</td> + <td class="text_lf">May 11, 1951</td> + <td class="text_lf">February 3, 1952</td> + <td class="text_rt">800</td> +</tr> +<tr> + <td class="text_lf">Female</td> + <td class="text_lf">May 11, 1951</td> + <td class="text_lf">January 9, 1952</td> + <td class="text_rt">700</td> +</tr> +<tr> + <td class="text_lf">Female</td> + <td class="text_lf">April 2, 1950</td> + <td class="text_lf">October 3, 1950</td> + <td class="text_rt">700</td> +</tr> +<tr> + <td class="text_lf">Female</td> + <td class="text_lf">May 6, 1950</td> + <td class="text_lf">April 3, 1951</td> + <td class="text_rt">650</td> +</tr> +<tr> + <td class="text_lf">Female</td> + <td class="text_lf">March 28, 1951</td> + <td class="text_lf">February 2, 1952</td> + <td class="text_rt">500</td> +</tr> +<tr> + <td class="text_lf">Male</td> + <td class="text_lf">April 18, 1952</td> + <td class="text_lf">July 6, 1952</td> + <td class="text_rt">120</td> +</tr> +<tr> + <td class="text_lf">Female</td> + <td class="text_lf">April 2, 1950</td> + <td class="text_lf">April 14, 1951</td> + <td class="text_rt">10</td> +</tr> +</table></div> + +<p>Most of these opossums were recaptured within a year of the time +they were marked as small young in the females' pouches, and on +the average they had moved a little less than 400 yards. While the +<span class="pagenum"><a name="Page_321" id="Page_321">[Pg 321]</a></span> +sex ratio was equal in the pouch young that were marked, it is noteworthy +that all but two of the recaptured opossums were females; +and of the two males, one was recaptured early, before it could have +had time to wander far. The young males, after becoming independent +must tend to wander much more widely, and to settle in +new areas far removed from the mother's home range. It is unlikely +that this dispersal of the young males is motivated either by rivalry +and intolerance of larger males or by sexual drive. The dispersal +occurs in late summer when there is no breeding activity, and when +food is present in greatest abundance and variety.</p> +<br /> + +<div class="caption2"><a name="FEEDING_HABITS" id="FEEDING_HABITS"></a> +FEEDING HABITS</div> + +<p>The feeding habits of the opossum in Douglas County, northeastern +Kansas, have been discussed by Sandidge (1953). His data +were obtained from stomach analysis of specimens caught in steel +traps. In the present study no stomachs were available for analysis +as the opossums on the Reservation were not sacrificed for this purpose +and effort was made to avoid mortality in those that were live-trapped. +Information concerning their feeding habits was obtained +mainly by examination of scats in the field. On this 590-acre tract +maintained as a Natural Area with human disturbance kept to a +minimum, the available food sources differed somewhat from those +of other woodland areas and especially from those of cultivated or +suburban areas as reported upon by Sandidge.</p> + +<p>The feces or "scats" of the opossum are not liable to be confused +with those of other mammals except possibly with those of the +striped skunk or raccoon, both relatively uncommon on the Reservation. +Favorite sites for deposition of opossum scats were at the +bases of large trees, usually honey locusts or elms, near the animal's +den. Accumulations of several dozen scats may collect in +such situations. Often the opossums live-trapped were found to +have deposited scats and many of these were saved for examination, +although they were usually trampled, broken and mixed with earth +and hair. Few scats were seen in the field throughout the summer. +Their disintegration is rapid at that time of year because of the +high temperature, frequent heavy rains, and abundance of dung-feeding +insects. Scats were seen in greatest abundance in the fall, +partly because the opossum population was then at its annual high +point. During fall, wild fruits made up the greater part of the diet +and were represented in almost every scat that was seen. Wild +grape (<i>Vitis vulpina</i>) is an abundant woodland vine on the area +and often forms dense tangles both in deep woods and in edge situations. +<span class="pagenum"><a name="Page_322" id="Page_322">[Pg 322]</a></span> +Grape was the most abundant single item, and a large number +of scats consisted exclusively of grape seeds and skins. In November +and December opossums could be trapped most effectively +by making sets in or near grapevine tangles where the animals were +attracted by the abundant ripe fruits. The crops of wild grapes +were especially heavy in 1948 (before live-trapping was begun) +and in 1949, and scats containing them were noticed in those years +especially. Opossums, too, were more numerous on the Reservation +in 1948 and 1949 than they were in 1950, 1951, and 1952.</p> + +<p>Hackberry fruit (<i>Celtis occidentalis</i>) was second to grape in importance +and large numbers of scats were found to be composed +mainly or entirely of the skins and seeds of this fruit. In the fall +of 1951, these fruits were especially important and were the principal +food source.</p> + +<p>Wild plum (<i>Prunus americanus</i>) and wild crabapple (<i>Pyrus +ioensis</i>) also are important in fall and winter and are present in +many scats. In summer, blackberry, abundant on some parts of the +Reservation, is an important food. Other wild fruits noticed in scats +include those of cherry (<i>Prunus virginiana</i>) and climbing bittersweet +(<i>Celastrus scandens</i>), and mast (acorn ?). In the fall of +1948, corn made up a large part of the contents of scats noticed. +Crops of corn were grown on two fields of the Reservation in that +year. In following years, corn was noticed less frequently in scats +but still continued to be one of the important food items. Several +cornfields adjoined the Reservation, and the scats containing the +grain were observed mainly along the borders of these fields.</p> + +<p>The crayfish is evidently the most important animal food, at least +during the cooler half of the year when scats are seen in greatest +numbers. Remains of crayfish were far more conspicuous than those +of other invertebrates, and often made up the greater part of the +scat. The sample of scats examined in the field, as noted below, +are thought to be representative of the much larger number noticed +but not examined in detail.</p> + +<div class="smaller"> +<p>August 19, 1951, 16 scats. Food items in their approximate order of importance +were: blackberry in six (100% in 5, 95% in 1); grape in five (100% in 2, +97% in 1, 95% in 1, 50% in 1); crayfish in three (100% in 1, 60% in 1, 40% in 1); +wild plum in two (85% in 1, 5% in 1); wild crabapple in two (100% in both); +insects in three (scarabaeid beetle 10% in 1, cicada 2% in 1, unidentified insect +fragments in 5); fox squirrel in one (15%); unidentified plant fibers in one +(40%).</p> + +<p>September, 1951, 16 scats. Grape in seven (all or most of 5 scats and small +percentages of 2 others); cherry in seven (all or most of 5 scats and small percentages +of 2 others); crayfish in seven (all or most of 5 and small percentages +<span class="pagenum3"><a name="Page_323" id="Page_323">[Pg 323]</a></span> +of 2 others); rabbit in two, making up most of both; insects (grasshopper, and +large black beetle) in two making up small percentages.</p> + +<p>October, 1951, 8 scats. Hackberry in three, making up nearly all of them; +grape in two (all of 1 and most of the other); wild plum in one (100%); mast +(acorn?) in one, making up 100%; crayfish in one making up about half; fox +squirrel in one making up the remainder of the scat containing crayfish; rabbit +in one making up a small percentage.</p> + +<p>November, 1951, 12 scats. Hackberry in five, making up all or most of four +and a small part of the fifth; grape in five, making up all or most of four and +a small part of the fifth; wild crabapple in three, making up all of two and most +of the third; and cottontail in one, making up all of it.</p> + +<p>January, 1952, 3 scats. Hackberry in all, making up all of two and most of +the third; copperhead (scales of medium-sized adult) making up a fraction of +the third scat. Pile of more than a dozen scats not individually separable, nearly +all consisted mainly or entirely of hackberry fruits estimated at 2000; other +contents chiefly crabapple and corn.</p> + +<p>September, 1952, 8 scats. Grape in all, making up all of six and 90% of the +seventh, and about 20% of the eighth; wild plum seeds in one making up 40%; +blue feathers, evidently of a jay, in one, making up a trace; carabid beetles in +one making up a trace.</p> + +<p>October, 1952, about 14 scats, two separate (both consisting exclusively of +grape) and the remainder mixed in two approximately equal piles, one pile consisting +of grape, except for small quantity of fine fur; second pile consisting +mainly of grape (about 90%) with small percentages of yellowjackets (<i>Vespula</i>, +about 6 individuals, all in one scat), toe bones and fur of cottontail rabbit; a +few scales of immature copperhead; and a snail.</p> + +<p>November, 1952, 2 scats. Grape in both, making up all of one and about +90% of the other.</p> +</div> + +<p>Sandidge (<i>loc. cit.</i>) found remains of cottontail rabbit in some +of the stomachs he examined, but followed Reynolds (1945) in regarding +these as carrion since the opossum was considered to be +too inefficient a predator to catch and kill cottontails—prey approximating +its own size and much superior in speed. Adult cottontails +seem to be secure from opossum predation under ordinary circumstances. +However, the opossum obtains some of its food by raiding +the nests of small animals, including those of rabbits. At the Reservation, +on May 21, 1951, at 9:00 P. M., distressed squealing of a +rabbit was heard in high brome grass. Investigation revealed that +a large male opossum had killed a young cottontail, weighing approximately +150 grams, and had started to eat it. This young rabbit, +about the minimum size of young wandering outside the nest, evidently +was pounced upon as it hid beneath the high grass.</p> + +<p>Live-traps for mice, in lines or grids of 100 or more, often were set +on the Reservation, and predators, including opossums, disturbed +them on many occasions. Attacks sometimes resulted in release and +escape of the trapped animal, and in other instances resulted in its +<span class="pagenum"><a name="Page_324" id="Page_324">[Pg 324]</a></span> +being caught and eaten. In many instances identity of the predator +could not be determined, but it is believed that such attacks by the +opossum were relatively infrequent and inefficient. Steel traps set +beside the mouse traps after consistent raids, to catch or discourage +the predator, caught opossums on several occasions. These opossums +usually had overturned mouse traps without opening them and +when the trapped mouse was missing from the trap no evidence of +its having been eaten was obtained. On other occasions raccoons +were caught in the steel traps, and their raids were characterized +by systematic and dextrous opening of the mouse traps and, frequently, +by predation on the small mammals inside them.</p> + +<p>Wire funnel traps set for reptiles along rock ledges also were +often disturbed by predators, mainly skunks and opossums, both of +which were caught on several occasions, when steel traps were used +as a protective measure. The opossums often were attracted to the +funnel traps by large insects such as camel crickets, grasshoppers +and beetles, but also by trapped lizards including the skinks +(<i>Eumeces fasciatus</i> and <i>E. obsoletus</i>) and the racerunner (<i>Cnemidophorus +sexlineatus</i>). Both Sandidge (1953) and Reynolds +(1945) recorded the five-lined skink (<i>E. fasciatus</i>) in opossum +stomachs. On the Reservation this common lizard probably is one +of the most frequent items of vertebrate prey of the opossum. Flat +rocks a few inches in diameter frequently have been found flipped +over; larger flat rocks and those solidly anchored in the ground +often have been found partly undermined by opossums scratching +away the loose dirt at their edges. Flat rocks similar to those found +disturbed by opossums are the favorite resting places of the skinks, +which, in cold or wet weather, are sluggish when beneath such +shelters; this is especially true of female skinks that are nesting. +The shape and size of some of the excavations suggested predation +on skink nests. Other possible food sources in the same situation, +in loose soil beneath flat rocks, include narrow-mouthed toads, +lycosid spiders, beetles (mainly carabids such as <i>Pasimachus</i> and +<i>Brachinus</i>) and occasionally, snails, centipedes and millipedes.</p> + +<p>A pond, a little more than an acre in size, was a focal area for +opossums and more were caught there than on any other part of +the Reservation. Opossums that were trapped and marked on other +parts of the Reservation were likely to be caught here sooner or +later. Tracks in the mud showed that the edge was patrolled almost +nightly by one or more opossums and this activity was especially +noticeable when the pond was drying. Frogs were obviously +the chief attraction inducing the opossums to forage there. +<span class="pagenum"><a name="Page_325" id="Page_325">[Pg 325]</a></span> +Of the 8 kinds of frogs and toads breeding at the pond, the bullfrog +(<i>Rana catesbeiana</i>), leopard frog (<i>Rana pipiens</i>) and cricket frog +(<i>Acris gryllus</i>) were most abundant, throughout the season and +especially when drying occurred. All three probably are important +foods of the opossum locally.</p> +<br /> + +<div class="caption2"><a name="WEIGHTS" id="WEIGHTS"></a> +WEIGHTS</div> + +<p>Opossums were weighed in the field, with small spring scales of +2000-gram capacity, graduated in 25-gram intervals. Weights recorded +were accurate within a margin of about 10 grams. After +other data were recorded, the opossum was offered the hook at the +base of the scale, and usually bit and held fast. Then it could be +suspended off the ground and a reading taken.</p> + +<p>When the same opossum was trapped two or more times within a +few days, weight was usually found to fluctuate sometimes more +than 200 grams, or more than 10 per cent of the animal's body +weight. Opossums recaptured soon after their original capture and +toe-clipping were generally found to have lost weight, reflecting the +deleterious effect of marking by this method. The temporary laming +of the animals prevented them from traveling as far or as fast as +they normally would have; consequently they probably obtained +correspondingly less food. They were also handicapped in digging, +grasping and climbing. Nineteen such animals taken within a +month of the original capture and marking, averaged 94 per cent +of their original weights. The minimum was 82 per cent. Only 2 +of the 19 had gained.</p> + +<p>The stumps of amputated toes did not heal rapidly in opossums—contrary +to experiences with many other kinds of mammals, reptiles, +and amphibians also marked by toe-clipping. For many weeks +the toes remained unhealed, sore and swollen. In several instances +after periods of months the clipped toe stumps were unhealed. +This was observed even in some of the opossums that were marked +as pouch young and recaptured when grown to nearly adult size.</p> + +<p>Some adult opossums trapped were heavier than the 2000-gram +capacity of the spring scale usually used in the field, and no definite +weights were recorded for most of these animals. Some of them +that were caught near the laboratory were brought there for +weighing.</p> + +<p>Even within the same age- and sex-group at any one time, opossums +varied widely in general condition and in weight. Some were +emaciated and sickly in appearance with sparse, ragged pelage, +while others were in excellent condition, fat and with thick, glossy +<span class="pagenum"><a name="Page_326" id="Page_326">[Pg 326]</a></span> +pelage. Seasonal trends are partly obscured by these differences in +individuals, by the tendency to lose weight in those recently marked, +and by the irregular fluctuations that occur in each animal.</p> + +<div class="fig_center"> +<img src="images/fig5.png" width="690" height="575" alt="" title="" /><br /> +<div class="fig_caption"> +<span class="smcap">Figure 5.</span> Weight changes in opossums +live-trapped; lines connect successive weight records of the +same individual, showing, in most, a downward trend throughout +the winter and early spring, and an upward trend in late +spring. +</div> +</div> + +<p>The few opossums caught in summer were thin and appeared to +be suffering from infestations of ectoparasites, especially chiggers +(<i>Eutrombicula alfreddugesi</i>) and ticks (<i>Dermacentor variabilis</i>). +Those trapped in October and November were mostly fat and in +good condition. For individuals caught at different seasons, maximum +weights were generally recorded in these two months. The +maximum weight record of the study was one of an adult male +weighing 5000 grams on December 23, 1950. The weight records +of this individual were more complete than most and are recorded +below to illustrate seasonal trends for adults. May 10, 1950, 1925 +grams; May 14, 1830 grams; May 17, 1940 grams; November 5, 4540 +grams; November 28, 4540 grams; December 23, 5000 grams; February +18, 1951, 3300 grams; March 6, 3080 grams; March 28, 3080 +grams; May 28, 3080 grams; June 18, 2620 grams.</p> + +<p><span class="pagenum"><a name="Page_327" id="Page_327">[Pg 327]</a></span> +Of opossums that were trapped alive, the weight ranged from the +maximum of 5000 grams to a minimum of 126 grams. The maximum +in males was higher than in females. In fall, three rather +poorly defined age-size groups were discernible in each sex: adults +more than a year old and including all the largest individuals; large +young born late the preceding winter and approaching small adult +size; smaller young born in early summer and still less than half-grown. +After November, young cease to gain, or gain slowly and +irregularly through the winter and spring and adults tend to decline +in weight, as food becomes scarce and frequent fasting is enforced +by cold or stormy weather. The smaller young probably are subject +to drastic reduction in numbers as a result, directly or indirectly, +of severe winter weather. Many of these smaller young, weighing +considerably less than 1000 grams, did not survive overnight when +caught in live-traps in cool autumn weather, whereas adults and +well-grown young generally survived exposure even for several successive +nights in various extremes of weather conditions.</p> +<br /> + +<div class="caption2"><a name="BREEDING_SEASON" id="BREEDING_SEASON"></a> +BREEDING SEASON</div> + +<p>Hartman (1928:154) stated that there were at least two litters of +young per year in the southern states with a small percentage +of unusually fecund females producing a third litter. Lay, in eastern +Texas, concluded (1942:155) that "The present investigation +substantiates Hartman's deduction of two litters being normal, but +fails to disclose any evidence of a third litter." He found females +carrying young in the pouch only within the seven-months period +January to July with definite peaks in February and June, and stated +that second litters appear in the pouch from early April to as late +as May 20 to 23. Reynolds (1945:362) found that the breeding +season in central Missouri in 1941 and 1942 began about the first +of February, with known or calculated birth dates of 42 litters +rather evenly distributed throughout the periods February 12 to +April 2, and May 16 to June 4. Eight of these females had given +birth to young between March 16 and April 2, approximately six to +nine weeks after the beginning of the breeding season. Reynolds +assumed that these were individuals that had failed to find mates +during the first oestrus of the season and that after completing the +regular dioestrus of about 28 days they had then mated and borne +young. Wiseman and Hendrickson (1950:333) in southeastern +Iowa recorded a female with a litter no more than two days old on +February 23, and several other females with young were estimated +to have borne litters at approximately this same date, while still +<span class="pagenum"><a name="Page_328" id="Page_328">[Pg 328]</a></span> +others bore litters as late as early March. Two lots of small young +found in early June may have been second litters.</p> + +<p>For the region represented by the present study, the data indicate +a breeding season with later onset and sharply circumscribed limits +as compared with an earlier onset and less circumscribed limits in +Texas, central Missouri, and even southeastern Iowa, which is a +little farther north. The available data indicate that there are two +distinct and well-defined breeding seasons in the course of the annual +cycle on the University of Kansas Natural History area. The +whole population, including young of the preceding year, some still +far below average adult size, breeds from about the middle of February +into early March, and first litters are born mainly in early +March. Individual females may vary as much as two to three weeks +in the time of breeding, and varying weather conditions from year +to year may hasten or delay onset of the breeding season. Data are +recorded below for all females caught in March that were carrying +litters.</p> +<br /> + +<div class="center"> +<table summary="Table 2: Breeding Data" width="80%"> +<tr> + <th>Date</th> + <th>Weight of female <br />in grams</th> + <th>Number of young</th> + <th class="text_lf"> Development of young</th> +</tr> +<tr> + <td class="text_lf">March 1, 1952</td> + <td class="center">2000</td> + <td class="center">9</td> + <td class="text_lf">Newborn</td> +</tr> +<tr> + <td class="text_lf">March 2, 1952</td> + <td class="center">1450</td> + <td class="center">6</td> + <td class="text_lf">Newborn</td> +</tr> +<tr> + <td class="text_lf">March 2, 1952</td> + <td class="center">1230</td> + <td class="center">7</td> + <td class="text_lf">Newborn</td> +</tr> +<tr> + <td class="text_lf">March 5, 1950</td> + <td class="center">1200</td> + <td class="center">10</td> + <td class="text_lf">About 16 mm. snout to vent</td> +</tr> +<tr> + <td class="text_lf">March 5, 1950</td> + <td class="center">1300</td> + <td class="center">1</td> + <td class="text_lf">About 14 mm. snout to vent</td> +</tr> +<tr> + <td class="text_lf">March 6, 1951</td> + <td class="center">1110</td> + <td class="center">4</td> + <td class="text_lf">Newborn</td> +</tr> +<tr> + <td class="text_lf">March 18, 1952</td> + <td class="center">1930</td> + <td class="center">8</td> + <td class="text_lf">Not present when female was <br />trapped on March 1</td> +</tr> +<tr> + <td class="text_lf">March 18, 1952</td> + <td class="center">1520</td> + <td class="center">6</td> + <td> </td> +</tr> +<tr> + <td class="text_lf">March 18, 1952</td> + <td class="center">1230</td> + <td class="center">12</td> + <td class="text_lf">About 40 mm. snout to vent</td> +</tr> +<tr> + <td class="text_lf">March 19, 1951</td> + <td class="center">1000</td> + <td class="center">8</td> + <td class="text_lf">Estimated 1 week old</td> +</tr> +<tr> + <td class="text_lf">March 22, 1950</td> + <td class="center">1040</td> + <td class="center">9</td> + <td class="text_lf">About 34 mm. snout to vent</td> +</tr> +<tr> + <td class="text_lf">March 24, 1950</td> + <td class="center">1280</td> + <td class="center">10</td> + <td class="text_lf">74 mm. snout to vent</td> +</tr> +<tr> + <td class="text_lf">March 24, 1950</td> + <td class="center">1480</td> + <td class="center">8</td> + <td> </td> +</tr> +<tr> + <td class="text_lf">March 27, 1950</td> + <td class="center">965</td> + <td class="center">8</td> + <td class="text_lf">Total length 26 mm., weight .8 g.</td> +</tr> +<tr> + <td class="text_lf">March 28, 1951</td> + <td class="center">820</td> + <td class="center">7</td> + <td class="text_lf">20 mm. crown to rump; born since <br /> + previous capture of female on March 7</td> +</tr> +<tr> + <td class="text_lf">March 30, 1950</td> + <td class="center">1325</td> + <td class="center">9</td> + <td class="text_lf">Total length 33 mm.</td> +</tr> +<tr> + <td class="text_lf">March 31, 1952</td> + <td class="center">1930</td> + <td class="center">8</td> + <td> </td> +</tr> +<tr> + <td class="text_lf">March 31, 1952</td> + <td class="center">1630</td> + <td class="center">5</td> + <td class="text_lf">Total length 73 mm.</td> +</tr> +</table> +</div> + +<p>None of the females trapped in February was carrying young in +the pouch, but probably some early litters are born in the last week +of February or even earlier. By late March most of the females +are carrying young in their pouches, and those which do not have +young, have their pouches enlarged and vascularized for accommodation +of the young. Presumably such females have already borne +young and then lost them. Nearly all the litters seen in the latter +half of March had young that were much larger than at birth.</p> + +<p><span class="pagenum"><a name="Page_329" id="Page_329">[Pg 329]</a></span> +Of 13 females examined in April, 12 were carrying young, and the +remaining one was known to have been carrying a single young on +March 1, but had lost it. Eleven females were examined in May, +four of which were the same ones examined in April. Eight of the +eleven females were carrying young; of the remaining three, one had +lost the litter of young that it had been carrying when trapped in +April. Two had empty pouches on May 19 and 20, but probably +had successfully reared the litters of young which they had been +carrying when trapped in April. The young of all those females +trapped on different dates in April and May were in stages of growth +indicative of birth about the first week in March. The latest date +on which a female was recorded with first-litter young in the pouch +was May 22, 1951, and these were the largest pouch young observed. +Their eyes were recently opened, they were estimated to weigh +60 grams each with hind feet 20 mm. long. Young continue to +grow rapidly after leaving the female's pouch. A young female +caught on June 16, 1949, weighed 126 grams. For seven young +caught on July 5 and 6, 1952, weights and hind-foot measurements +were, for males: 660 grams, 52 mm.; 560 grams, 46 mm.; 550 grams, +48 mm.; 450 grams, 44 mm.; 370 grams, 44 mm.; 330 grams, 37 mm.; +and for the one female: 430 grams, 46 mm.</p> + +<p>The wide variation in size in this small group of young of nearly +the same age is noteworthy. Size and condition of the females +carrying them, number of competing litter mates, and early success +or handicap in independent life causes so much divergence in size +that at the age of four months some young are twice as large as +others.</p> + +<p>By late fall the young grow to small-adult size. For example, the +female that weighed 126 grams when first caught on June 16, 1949, +was recaptured on November 29, 1949, and on that date weighed +1710 grams.</p> + +<p>A second breeding season ensues soon after the young of the first +litter leave the pouch, and these young probably soon learn to shift +for themselves. Second litters are usually born in early June. On +June 14, 1952, a female was taken with young only a few days old +in her pouch. On July 5, 1952, two females last taken on May 19 +and May 20, with their pouches recently vacated by first litters, were +found to have young the size of half-grown mice, evidently two to +three weeks old. In the months of October, November, December +and January, a total of 11 young, thought to represent second litters, +<span class="pagenum"><a name="Page_330" id="Page_330">[Pg 330]</a></span> +were taken. Dates of capture, weights in grams and sexes were as +follows:</p> + +<div class="center"> +<table summary="Table 3: Second Litter Data"> +<tr> + <td class="text_lf">Oct. 3, 1950</td> + <td class="text_lf">400 grams</td> + <td class="text_lf">male</td> +</tr> +<tr> + <td class="text_lf">Oct. 6, 1950</td> + <td class="text_lf">510 grams</td> + <td class="text_lf">female</td> +</tr> +<tr> + <td class="text_lf">Oct. 8, 1950</td> + <td class="text_lf">260 grams</td> + <td class="text_lf">female</td> +</tr> +<tr> + <td class="text_lf">Oct. 8, 1950</td> + <td class="text_lf">350 grams</td> + <td class="text_lf">female</td> +</tr> +<tr> + <td class="text_lf">Oct. 18, 1950</td> + <td class="text_lf">350 grams<a name="FNanchor_A_1" id="FNanchor_A_1"></a><a href="#Footnote_A_1" class="fnanchor">[A]</a></td> + <td class="text_lf">female</td> +</tr> +<tr> + <td class="text_lf">Dec. 5, 1951</td> + <td class="text_lf">630 grams</td> + <td class="text_lf">female</td> +</tr> +<tr> + <td class="text_lf">Dec. 30, 1950</td> + <td class="text_lf">710 grams</td> + <td class="text_lf">female</td> +</tr> +<tr> + <td class="text_lf">Jan. 1, 1951</td> + <td class="text_lf">660 grams</td> + <td class="text_lf">female</td> +</tr> +<tr> + <td class="text_lf">Jan. 1, 1950</td> + <td class="text_lf">700 grams<a href="#Footnote_A_1" class="fnanchor">[A]</a></td> + <td class="text_lf">male</td> +</tr> +<tr> + <td class="text_lf">Jan. 9, 1950</td> + <td class="text_lf">550 grams</td> + <td class="text_lf">male</td> +</tr> +<tr> + <td class="text_lf">Jan. 11, 1950</td> + <td class="text_lf">550 grams</td> + <td class="text_lf">male</td> +</tr> +</table> +</div> +<br /> +<div class="center"> +<a name="Footnote_A_1" id="Footnote_A_1"></a> +<a href="#FNanchor_A_1"><span class="footnote">[A]</span></a> estimated +</div> +<br /> + +<p>The hind foot measured 48 mm. and 51 mm., respectively, in the +young weighing 630 grams and 660 grams. These young, born in +early summer have grown, by October, to a size comparable with +that attained in July by young of the early spring litters. The +variation in size is also similar but with a little wider range. The +summer breeding season may be somewhat more protracted than +the breeding season in early spring.</p> + +<p>Too few females were caught in summer to compare the summer +breeding season with the early spring breeding season, with respect +to size of litters, percentage of non-breeders, and other factors which +might affect the size of the crop of young produced. It is not clear +why, among opossums trapped in winter, the young born in early +spring outnumber those born in early summer by about four to one. +Some females are eliminated after rearing the first litter, and others, +exhausted by rearing large first litters may fail to participate in the +second breeding season. However, it seems that the young of the +summer litters must be subject to other unusual and selective mortality +factors which eliminate most of them by fall. That such factors +vary from year to year is indicated by the changing ratio of +summer-born young to other opossums in each of the three winter +seasons when trapping was carried on.</p> +<br /> + +<div class="caption2"><a name="NUMBERS_OF_YOUNG" id="NUMBERS_OF_YOUNG"></a> +NUMBERS OF YOUNG</div> + +<p>Hartman (1952) has summarized his own findings and those of +other authors regarding the embryology, birth, and early development +of the opossum, and has corrected numerous popular misconceptions. +He states that an average litter consists of about 21 +eggs, but mentions much larger litters of up to as many as 56. +However, many of these may fail to develop. The female normally +has 13 functional nipples in her pouch and each one accommodates +a single young. Excess young beyond this number are doomed, and +soon perish from starvation if they reach the pouch after all the +nipples are occupied. None of the females examined in the present +study had a full complement of 13 young. Under unfavorable conditions, +<span class="pagenum"><a name="Page_331" id="Page_331">[Pg 331]</a></span> +most or all of the young may fail to make the trip from the +vaginal orifice to the pouch. Also, the pouch young are subject to +heavy mortality, but observations concerning the time and cause of +mortality are lacking.</p> + +<p>Lay (<i>loc. cit.</i>) found an average of 6.8 pouch young in 65 litters +examined in eastern Texas; Reynolds found an average of 8.9 (5 to +13) in 42 litters from Boone County, central Missouri; Wiseman and +Hendrickson found an average of 9 (6 to 12) in southeastern Iowa. +In the present study, 28 of the female opossums examined were +carrying litters in their pouches, and all these females were caught +in the months of March, April, May, June and July. The number +of young varied from one to 12. Seven females each had seven +young, six each had eight, three had six, three had five, and there +were two each with nine, 10, and 12 young, and one each with one, +four and 11 young. The average was 7.4 per litter. On several occasions +females captured with young in their pouches and recaptured +one or more times within a few weeks, were found to have +lost some or all of the young. Some of the females examined probably +had already lost parts of their litters. For instance, the female +recorded with just one small young on March 1, probably had lost +most of her litter and when recaptured a month later she did not +have any young.</p> + +<p>Nineteen yearling opossums were taken in the fall-winter-spring +season of 1951-52; 42 per cent of the total, and 67 per cent of the +females were individuals marked as pouch young the preceding +spring. In the course of live-trapping, that spring, some first litters +may have been missed. No second litters were marked because +trapping was not continued into June and July when second litters +are being carried by females. These figures suggest that the breeding +population of females on an area consists chiefly of those born +there the preceding spring.</p> +<br /> + +<div class="caption2"><a name="COMPOSITION_OF_THE_POPULATION" id="COMPOSITION_OF_THE_POPULATION"></a> +COMPOSITION OF THE POPULATION</div> + +<p>Sex ratio of opossums trapped was approximately 1:1; 59 males +to 58 females. Age groups for opossums caught in the three seasons +are shown in the following tabular fashion. For a few individuals +age status was doubtful.</p> +<br /> + +<div class="center"> +<table summary="Table 4: Age Data"> +<tr> + <th> </th> + <th>1949-1950</th> + <th>1950-1951</th> + <th>1951-1952</th> + <th>Total</th> +</tr> +<tr> + <td class="text_lf">Old adults</td> + <td class="text_lf">11(25%)</td> + <td class="text_lf"> 9(26.4%)</td> + <td class="text_lf">11(39.2%)</td> + <td class="text_lf"> 31(29.2%)</td> +</tr> +<tr> + <td class="text_lf">Yearlings:</td> +</tr> +<tr> + <td class="text_lf"> Born in late winter</td> + <td class="text_lf">29(66%)</td> + <td class="text_lf">18(53.0%)</td> + <td class="text_lf">13(46.5%)</td> + <td class="text_lf"> 60(56.6%)</td> +</tr> +<tr> + <td class="text_lf"> Born in late spring</td> + <td class="text_lf"> 4(9.1%)</td> + <td class="text_lf"> 7(20.6%)</td> + <td class="text_lf"> 4(14.3%)</td> + <td class="text_lf"> 15(14.2%)</td> +</tr> +<tr> + <td class="text_lf"> Total</td> + <td class="text_lf">44</td> + <td class="text_lf">34</td> + <td class="text_lf">28</td> + <td class="text_lf">106</td> +</tr> +</table> +</div> + +<p><span class="pagenum"><a name="Page_332" id="Page_332">[Pg 332]</a></span> +In the 1950-51 season, small young of the summer brood seemed +unusually numerous. In the 1951-52 period, young of both age +classes were relatively scarce and old adults made up an unusually +high proportion of the population. Excluding the 14 marked pouch +young that were later recaptured, there were only four of the total +of 106 that were trapped in each of two seasons. One young less +than a quarter grown, that was accidentally caught in a live-trap +set for woodrats, was recaptured as a breeding adult the following +winter. An adult male and two adult females each caught in the +1949-50 season were each recaptured repeatedly in the 1950-51 season. +Ninety-five per cent replacement of the breeding population +by the following breeding season is indicated by our figures. Only +3 (or 5 per cent) of the individuals of the population trapped and +marked in the season of 1949-50, were recaptured among the 62 +opossums recorded in the two subsequent seasons. Various mortality +factors including predation, disease, and accidents account +for some 70 per cent. These are replaced by first-year young which +make up the greater part of the breeding population. The remaining +25 per cent presumably shift their ranges sufficiently in the course +of a year to have moved beyond the limits of an area of the size encompassed +by the present study.</p> +<br /> + +<div class="caption2"><a name="POPULATION_DENSITY" id="POPULATION_DENSITY"></a> +POPULATION DENSITY</div> + +<p>No precise measurement of the population density on the study +area was obtained. It was not practical to capture every individual +present there, and rapid population turnover, due to mortality and +wandering, obscured the trends. The information obtained concerning +movements of opossums suggest that one may habitually +forage as much as 900 feet from its home base. Assuming that 900 +feet is the typical cruising radius, the areas drawn upon by the trap +lines in the three different seasons were approximately as follows: +1949-50—400 acres; 1950-51—350 acres; 1951-52—220 acres. In +these same three seasons the numbers of opossums caught were, respectively, +46, 37, and 30. If these figures represent the numbers +actually present, densities of one to 8.7 acres, one to 9.5 acres, and +one to 7.3 acres are indicated. However, some opossums using the +area probably were missed; and on the other hand, not all those +caught in the course of a season were present there simultaneously. +Many of those present early in the season would have moved away +a few months later, and others would have moved in, replacing +them. The number present at any one time could scarcely have been +more than half the number caught in the entire season.</p> + +<p><span class="pagenum"><a name="Page_333" id="Page_333">[Pg 333]</a></span></p> + +<div class="center smcap">Census With Half-monthly Sampling Periods</div> +<br /> +<div class="center"> +<table summary="Table 5A: Half-monthly Census Data"> +<tr> + <td class="text_lf">Sampling period</td> + <td class="center">Number of <br />individuals taken <br />in period</td> + <td class="center">Number of <br />individuals taken <br />in following period</td> + <td class="center">Number of <br />recatures in <br />following period</td> + <td class="center">Computed population <br />for sampling period</td> +</tr> +<tr> + <td class="text_lf">Early November 1949</td> + <td class="center">3</td> + <td class="center">7</td> + <td class="center">1</td> + <td class="center">21</td> +</tr> +<tr> + <td class="text_lf">Late November 1949</td> + <td class="center">7</td> + <td class="center">8</td> + <td class="center">3</td> + <td class="center">18.7</td> +</tr> +<tr> + <td class="text_lf">Early December 1949</td> + <td class="center">8</td> + <td class="center">11</td> + <td class="center">3</td> + <td class="center">29.3</td> +</tr> +<tr> + <td class="text_lf">Late December 1949</td> + <td class="center">11</td> + <td class="center">7</td> + <td class="center">4</td> + <td class="center">19.2</td> +</tr> +<tr> + <td class="text_lf">Early January 1950</td> + <td class="center">7</td> + <td class="center">3</td> + <td class="center">1</td> + <td class="center">21</td> +</tr> +<tr> + <td class="text_lf">Early March 1950</td> + <td class="center">5</td> + <td class="center">8</td> + <td class="center">2</td> + <td class="center">20</td> +</tr> +<tr> + <td class="text_lf">Late March 1950</td> + <td class="center">8</td> + <td class="center">6</td> + <td class="center">3</td> + <td class="center">16</td> +</tr> +<tr> + <td class="text_lf">Early April 1950</td> + <td class="center">6</td> + <td class="center">3</td> + <td class="center">1</td> + <td class="center">18</td> +</tr> +<tr> + <td class="text_lf">Late April 1950</td> + <td class="center">3</td> + <td class="center">6</td> + <td class="center">2</td> + <td class="center">9</td> +</tr> +<tr> + <td class="text_lf">Early May 1950</td> + <td class="center">6</td> + <td class="center">3</td> + <td class="center">2</td> + <td class="center">9</td> +</tr> +<tr> + <td class="text_lf">Early November 1950</td> + <td class="center">1</td> + <td class="center">3</td> + <td class="center">1</td> + <td class="center">3</td> +</tr> +<tr> + <td class="text_lf">Late December 1950</td> + <td class="center">3</td> + <td class="center">6</td> + <td class="center">1</td> + <td class="center">18</td> +</tr> +<tr> + <td class="text_lf">Early February 1951</td> + <td class="center">4</td> + <td class="center">13</td> + <td class="center">3</td> + <td class="center">17.3</td> +</tr> +<tr> + <td class="text_lf">Late February 1951</td> + <td class="center">13</td> + <td class="center">6</td> + <td class="center">3</td> + <td class="center">26</td> +</tr> +<tr> + <td class="text_lf">Early March 1951</td> + <td class="center">6</td> + <td class="center">4</td> + <td class="center">3</td> + <td class="center">8</td> +</tr> +<tr> + <td class="text_lf">Late March 1951</td> + <td class="center">4</td> + <td class="center">5</td> + <td class="center">2</td> + <td class="center">10</td> +</tr> +<tr> + <td class="text_lf">Early April 1951</td> + <td class="center">5</td> + <td class="center">1</td> + <td class="center">1</td> + <td class="center">5</td> +</tr> +<tr> + <td class="text_lf">Late April 1951</td> + <td class="center">1</td> + <td class="center">5</td> + <td class="center">1</td> + <td class="center">5</td> +</tr> +<tr> + <td class="text_lf">Early May 1951</td> + <td class="center">5</td> + <td class="center">3</td> + <td class="center">2</td> + <td class="center">7.5</td> +</tr> +<tr> + <td class="text_lf">Early February 1952</td> + <td class="center">9</td> + <td class="center">4</td> + <td class="center">2</td> + <td class="center">18</td> +</tr> +<tr> + <td class="text_lf">Late February 1952</td> + <td class="center">4</td> + <td class="center">9</td> + <td class="center">1</td> + <td class="center">36</td> +</tr> +<tr> + <td class="text_lf">Early March 1952</td> + <td class="center">9</td> + <td class="center">6</td> + <td class="center">2</td> + <td class="center">27</td> +</tr> +<tr> + <td class="text_lf">Late March 1952</td> + <td class="center">6</td> + <td class="center">5</td> + <td class="center">2</td> + <td class="center">15</td> +</tr> +</table> +</div> +<br /> + +<div class="center smcap">Census with Monthly Sampling Periods</div> +<br /> +<div class="center"> +<table summary="Table 5B: Monthly Census Data"> +<tr> + <td class="text_lf">Sampling period</td> + <td class="center">Number of <br />individuals taken <br />in period</td> + <td class="center">Number of <br />individuals taken <br />in following period</td> + <td class="center">Number of <br />recatures in <br />following period</td> + <td class="center">Computed population <br />for sampling period</td> +</tr> +<tr> + <td class="text_lf">November 1949</td> + <td class="center">9</td> + <td class="center">16</td> + <td class="center">7</td> + <td class="center">21</td> +</tr> +<tr> + <td class="text_lf">December 1949</td> + <td class="center">16</td> + <td class="center">9</td> + <td class="center">3</td> + <td class="center">48</td> +</tr> +<tr> + <td class="text_lf">March 1950</td> + <td class="center">11</td> + <td class="center">9</td> + <td class="center">3</td> + <td class="center">33</td> +</tr> +<tr> + <td class="text_lf">April 1950</td> + <td class="center">9</td> + <td class="center">7</td> + <td class="center">2</td> + <td class="center">32</td> +</tr> +<tr> + <td class="text_lf">October 1950</td> + <td class="center">9</td> + <td class="center">3</td> + <td class="center">3</td> + <td class="center">9</td> +</tr> +<tr> + <td class="text_lf">November 1950</td> + <td class="center">3</td> + <td class="center">3</td> + <td class="center">1</td> + <td class="center">9</td> +</tr> +<tr> + <td class="text_lf">December 1950</td> + <td class="center">3</td> + <td class="center">7</td> + <td class="center">3</td> + <td class="center">7</td> +</tr> +<tr> + <td class="text_lf">January 1951</td> + <td class="center">7</td> + <td class="center">14</td> + <td class="center">3</td> + <td class="center">33</td> +</tr> +<tr> + <td class="text_lf">February 1951</td> + <td class="center">14</td> + <td class="center">7</td> + <td class="center">4</td> + <td class="center">25</td> +</tr> +<tr> + <td class="text_lf">March 1951</td> + <td class="center">7</td> + <td class="center">5</td> + <td class="center">3</td> + <td class="center">12</td> +</tr> +<tr> + <td class="text_lf">April 1951</td> + <td class="center">5</td> + <td class="center">6</td> + <td class="center">3</td> + <td class="center">10</td> +</tr> +<tr> + <td class="text_lf">November 1951</td> + <td class="center">3</td> + <td class="center">6</td> + <td class="center">1</td> + <td class="center">18</td> +</tr> +<tr> + <td class="text_lf">December 1951</td> + <td class="center">6</td> + <td class="center">5</td> + <td class="center">1</td> + <td class="center">30</td> +</tr> +<tr> + <td class="text_lf">January 1952</td> + <td class="center">5</td> + <td class="center">11</td> + <td class="center">3</td> + <td class="center">18</td> +</tr> +<tr> + <td class="text_lf">February 1952</td> + <td class="center">11</td> + <td class="center">13</td> + <td class="center">4</td> + <td class="center">36</td> +</tr> +<tr> + <td class="text_lf">March 1952</td> + <td class="center">13</td> + <td class="center">9</td> + <td class="center">5</td> + <td class="center">23</td> +</tr> +<tr> + <td class="text_lf">April 1952</td> + <td class="center">9</td> + <td class="center">3</td> + <td class="center">1</td> + <td class="center">27</td> +</tr> +</table> +</div> + +<p>Crude census-figures were obtained by utilizing the Lincoln Index +and computing the total on the basis of the ratio of marked (and +recognizable) individuals to others caught in a sampling period. +A large number of census figures were obtained over the three-year +period of the study. Each separate census, however, was based +on an inadequate sample as the number of marked individuals taken +<span class="pagenum"><a name="Page_334" id="Page_334">[Pg 334]</a></span> +at each sampling, as recaptures from the previous sampling period, +varied from one to five. While little confidence can be placed in +any one census computation, the trends of figures from series of such +computations reveal the approximate number of opossums on the +area if due allowance is made for certain distorting factors. Presumably +the differences in figures obtained at different samplings +result chiefly from the margin of error in the data, although it is +true that there is rapid change in the actual number of opossums.</p> + +<p>The number of active opossums in the region of the study reaches +a peak in late summer and early fall, when second litters of young +have grown large enough to become independent. At this season +the population contains a high proportion of young of the year. +During the ensuing months of fall and winter there is a steady decrease +in numbers, through various mortality factors, with no replacement +until young are born about the first week of March. +These young do not become independent until late May or early +June, and during the intervening months there is a further reduction +of the adults and yearlings, so that the active population reaches +its annual low point in late spring. At that time of year most opossums +are in poor physical condition.</p> + +<p>The area represented by the opossums trapped totaled more than +500 acres, but not more than 400 acres were within the area drawn +upon by the trap line at any one time. Usually the area represented +at any one time by the trap line was less—100 to 350 acres, with +from 25 to 45 traps. Traps were moved from time to time depending +on the distribution of opossum sign and food sources, the +weather, and the time available for this study. As a result, successive +samples are not strictly comparable and a major source of +error is introduced into the census computations. Lack of exact +correspondence in the area represented by successive samples would +result in a disproportionally small number of recaptures, and an +erroneously high census computation. While adequate adjustment +cannot be made, examination of the data suggests that census figures +are too high, by as much as 50 per cent in many instances as a result +of this factor, while in some other instances when there was little or +no alteration of a trap line from one period to another, the census +figure was not affected. In the winter of 1949-50, the area covered +was most extensive, from 350 to 400 acres, and the numbers of +opossums taken were correspondingly larger. In the 1950-51 season +the area involved was approximately 220 acres, and in the 1951-52 +season it was a little less than 200 acres. In view of the census +figures obtained and the probable errors, it appears that the opossum +<span class="pagenum"><a name="Page_335" id="Page_335">[Pg 335]</a></span> +population in early autumn is about one to 20 acres, and that by late +spring it is reduced to not much more than half that number.</p> +<br /> + +<div class="caption2"><a name="MORTALITY_FACTORS" id="MORTALITY_FACTORS"></a> +MORTALITY FACTORS</div> + +<p>Many of the opossums trapped were suffering from injury, disease, +or parasite infestation, and some were in critical conditions. +A large adult male trapped on April 2, 1952, seemed to be dying from +disease. It was much emaciated and the pelage was sparse and +ragged, as if the animal had been sick for a long time. The skin +had numerous light-colored pustules 1 to 2 mm. in diameter, and +these were especially prominent on the ears, lips, and penis. When +released, the opossum was too weak to move away. It was excited +by movements of the trapper, and stood erect with violent involuntary +rocking movements. After a few seconds it gradually slumped +to the ground and subsided into quiescence. On the next day no +trace of it could be found.</p> + +<p>Most of the opossums caught in summer and early fall had eye +infections, and all of them were infested with ticks (<i>Dermacentor +variabilis</i>). Sometimes ticks were attached in dense clusters of +several dozen on the animal's ears and scattered over other parts of +the body.</p> + +<p>In March and April, 1950, seven adult opossums were found dead +in the traps. None of these showed any evidence of disease or injury +and they were normal in appearance except that they were thin. +It was concluded that death had resulted from exposure and starvation +in the traps in these animals already in critical condition as a +result of winter food scarcity and frequent fasting. Up to this time +the procedure had been to check the trap line only on alternate +days and no mortality had resulted, even in the coldest part of the +winter. The implication is that by spring, opossums are in a condition +so critical that they are unable to withstand exposure or fasting +and die whenever weather conditions are unusually severe.</p> + +<p>After these losses in the spring of 1950, trap lines were checked +daily. However, in October, 1950, further mortality in traps resulted +in the loss of three or more opossums. All three of these were +rat-sized young of second litters. These young lacked the abundant +supply of fat characteristic of larger opossums in fall, and seemingly +were unable to withstand exposure to chilly nights. Such susceptibility +to cold might result in heavy mortality in retarded second-litter +young when cold weather of autumn is unseasonably early or +is unusually severe.</p> + +<p>Natural enemies of the opossum on the area include the red-tailed +<span class="pagenum"><a name="Page_336" id="Page_336">[Pg 336]</a></span> +hawk, horned owl and coyote. Because of the opossum's +nocturnal habits it is rarely exposed to hawk predation. Food habits +of the coyote on the area have not yet been investigated. Numerous +instances of horned owl predation on opossums have been recorded +in the literature. On January 15, 1950, an owl attacked an opossum +caught in a live-trap. The trap was found overturned, and a few +feet away were entrails and a quantity of opossum hair where the +animal was eaten. Low vegetation in the vicinity had many fine +down feathers of the owl clinging to it. On December 24, 1950, the +carcass of a small adult opossum was found in a pasture near the +edge of the woods. The head and tail were intact, but otherwise +little more remained than the spinal column, girdles and larger limb +bones. White excreta of a large bird beside the carcass indicated +predation by a raptor, probably a horned owl.</p> +<br /> + +<div class="caption2"><a name="SUMMARY" id="SUMMARY"></a> +SUMMARY</div> + +<p>On a natural area, the University of Kansas Natural History Reservation, +in Douglas County, northeastern Kansas, the population of +opossums was studied, chiefly by live-trapping, in the fall-winter-spring +seasons of 1949-50, 1950-51 and 1951-52. The study area +provided a varied habitat of elm-oak-hickory woodland, pastureland, +and fallow fields. Opossums use all parts of it, but concentrate their +activities in the woodland.</p> + +<p>Opossums being mainly nocturnal were rarely seen in the daytime, +except when caught in traps. Reactions to humans varied; some +were indifferent, some feigned death, others merely tried to escape, +and some defended themselves vigorously, snarling and snapping.</p> + +<p>No evidence of territorial behavior was found in the opossum. +Many individuals of both sexes and various sizes, occurred together +on the same area. Successive captures of individuals revealed the +usual extent of home ranges, which averaged approximately 50 acres, +and tended to a circular or broadly oval shape. No significant difference +in size of home ranges between males and females, or between +adults and well-grown young, was found. Of 115 young +marked by toe-clipping while still in the females' pouches, 15 were +recaptured after periods of months. All but two of these recaptured +young were females which had settled down within a few hundred +feet of the locations where they were born. The young males seem +to wander much more extensively than do the females.</p> + +<p>Feeding habits were investigated by field examination of scats +found mainly in fall and winter. These consisted mainly of wild +fruits, especially grape, blackberry, wild crabapple, wild plum, and +<span class="pagenum"><a name="Page_337" id="Page_337">[Pg 337]</a></span> +hackberry. Crayfish was the most important animal food. No comparable +data for spring or summer were obtained because scats +deteriorate rapidly in warm weather and were seldom found then. +Clues as to the summer food were gained from sign. On many occasions +opossums disturbed live-traps set for small animals, to obtain +the voles, mice, skinks, or insects caught in them. Evidence of opossum +activity such as digging and scratching was frequently noticed +at the edges of rocks and in crevices, where such prey as skinks, +narrow-mouthed toads, beetles, spiders and centipedes seek shelter. +One opossum was observed to catch and kill a young cottontail.</p> + +<p>The opossums trapped ranged in weight from 126 grams to 5000 +grams but most weighed between 1000 and 2000 grams. After being +trapped and marked by toe-clipping, animals usually lost weight, up +to as much as 18 per cent of the original weight. Food scarcity and +enforced fasting in cold weather caused a weight loss from November +until the arrival of warm spring weather. By late April and +May some opossums were emaciated and in critical condition.</p> + +<p>The entire population of opossums, including the majority less +than a year old, breeds in February, and litters are born mainly in +the first half of March. The young develop rapidly in the female's +pouch, and become independent in late May, and there is a second +breeding season with young born mainly in the first half of June. +By the onset of cool fall weather, young born in early spring have +grown so that most are as large as small adults. The young born in +early summer are still less than half-grown. The young of the second +litter are less successful than those of the first litter and make +up only a small part of the breeding population the following year. +In 28 litters of young the average was 7.4, but probably some of +these litters had already sustained losses.</p> + +<p>In each of three different winters, the largest age group in the +population of opossums was that of the newly matured young born +in early spring. The old adults were the next most numerous group, +and the second-litter young born in early summer were the least +numerous. The figures obtained from live-trapping indicate an +annual population turnover of approximately 95 per cent, with some +70 per cent eliminated by various mortality factors and replaced by +young, the remaining 25 per cent shifting to new areas, with compensatory +shifts of individuals replacing them.</p> + +<p>The various mortality factors which regulate the numbers of opossums +are not well known, and even less is known regarding the relative +importance of the factors. Food supply and weather are obviously +of major importance and closely interrelated in their effect on +<span class="pagenum"><a name="Page_338" id="Page_338">[Pg 338]</a></span> +the population. One large adult opossum that was trapped seemed +to be dying from disease and was scarcely able to stand; but others +caught near-by before and after were unaffected. The horned owl +is perhaps the most important natural enemy of the opossum on the +Reservation, and instances of owl predation on opossums were noted.</p> +<br /> + +<div class="caption2"><a name="LITERATURE_CITED" id="LITERATURE_CITED"></a> +LITERATURE CITED</div> + +<span class="smcap">Fitch, H. S.</span><br /> +<div class="reference">1950. A new style live-trap for small mammals. Jour. Mamm., 31:364-365.</div> +<div class="reference">1952. The University of Kansas Natural History Reservation. Univ. Kansas + Mus. Nat. Hist., Misc. Publ., 4:1-38, 4 pls.</div> +<br /> +<span class="smcap">Hall, E. R.</span>, and <span class="smcap">Kelson, K. R.</span><br /> +<div class="reference">1952. Comments on the taxonomy and geographic distribution of some + North American marsupials, insectivores and carnivores. Univ. + Kansas Publ., Mus. Nat. Hist., 5:319-341.</div> +<br /> +<span class="smcap">Hartman, C. G.</span><br /> +<div class="reference">1923. Breeding habits, development and birth of the opossum. Smithsonian<br /> + Report 1921:347-363.</div> +<div class="reference">1928. The breeding season of the opossum (<i>Didelphis virginiana</i>) and the + rate of intrauterine and postnatal development. Jour. Morph. and + Physiol., 46:143-215.</div> +<div class="reference">1952. Possums. Univ. of Texas Press, Austin. xvi + 174 pp.</div> +<br /> +<span class="smcap">Lay, D. W.</span><br /> +<div class="reference">1942. Ecology of the opossum in eastern Texas. Jour. Mamm., 23:147-159.</div> +<br /> +<span class="smcap">Leonard, A. B.</span>, and <span class="smcap">Goble, R. C.</span><br /> +<div class="reference">1952. Mollusca of the University of Kansas Natural History Reservation. + Univ. Kansas Sci. Bull., 34:1013-1055.</div> +<br /> +<span class="smcap">Reynolds, H. C.</span><br /> +<div class="reference">1945. Some aspects of the life history and ecology of the opossum in central + Missouri. Jour. Mamm., 26:361-379.</div> +<br /> +<span class="smcap">Sandidge, L. L.</span><br /> +<div class="reference">1953. Food and dens of the opossum + (<ins title='Correction: was "Didelphis Virginiana"'><i>Didelphis virginiana</i></ins>) in northeastern + Kansas. Trans. Kansas Acad. Sci., 59:97-106.</div> +<br /> +<span class="smcap">Wiseman, G. L.</span>, and <span class="smcap">Hendrickson, G. O.</span><br /> +<div class="reference">1950. Notes on the life history and ecology of the opossum in southeast + Iowa. Jour. Mamm., 31:331-337.</div> +<br /> + +<p><i>Transmitted May 4, 1953.</i></p> +<br /> +<br /> + +<div class="trans_notes"> +<br /> +<div class="caption2">Transcriber's Notes</div> + +<p>Other than two possible typographical errors listed below, <a href="#Page_307">the title +and verso (second) page</a> specifies the pages are 305-338; but the first +numbered page (the third one) is numbered "309". The content provider examined +the text at page breaks and looked for evidence of a missing leaf; but found none. +So, this appears to be a printer's error in the pagination as the numbering +sequence otherwise follows the normal format for these scientific texts. Therefore, +the numbering was changed in the descriptions to read "... pp. 307-338, ..."</p> + + +<table summary="Corrections"> +<tr><td>Page</td><td>Correction</td></tr> +<tr><td>316</td><td>Occasionaly => <a href="#Occasionally">Occasionally</a></td></tr> +<tr><td>338</td><td>Possible typo: <i>Didelphis Virginiana</i> => <i>Didelphis virginiana</i></td></tr> +</table> +<br /> +</div> +</div><!-- End book --> + + + + + + + + +<pre> + + + + + +End of the Project Gutenberg EBook of Ecology of the Opossum on a Natural +Area in Northeastern Kansas, by Henry S. Fitch and Lewis L. Sandidge + +*** END OF THIS PROJECT GUTENBERG EBOOK ECOLOGY OF OPOSSUM--N.E. 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