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diff --git a/37199.txt b/37199.txt new file mode 100644 index 0000000..d6e20f7 --- /dev/null +++ b/37199.txt @@ -0,0 +1,1824 @@ +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: ASCII + +*** 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 + + + + + + + + + + UNIVERSITY OF KANSAS PUBLICATIONS + + MUSEUM OF NATURAL HISTORY + + + Volume 7, No. 2, pp. 307-338, 5 figures in text + + August 24, 1953 + + + + Ecology of the Opossum on a Natural Area + in Northeastern Kansas + + + BY + + HENRY S. FITCH + + AND + + LEWIS L. SANDIDGE + + + + UNIVERSITY OF KANSAS + LAWRENCE + 1953 + + + + + UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY + + Editors: E. Raymond Hall, Chairman, A. Byron Leonard, Robert W. Wilson + + + Volume 7, No. 2, pp. 307-338, 5 figures in text + + Published August 24, 1953 + + + + UNIVERSITY OF KANSAS + + Lawrence, Kansas + + + PRINTED BY + FERD VOILAND, JR., STATE PRINTER + TOPEKA, KANSAS + 1953 + + 24-7812 + + + + +Ecology of the Opossum on a Natural Area in Northeastern Kansas + +BY + +HENRY S. FITCH and LEWIS L. SANDIDGE + + +On the 590-acre University of Kansas Natural History Reservation where +our study was made, the opossum, _Didelphis marsupialis virginiana_ +Kerr, is the largest predatory animal having a permanently resident +population. The coyote, raccoon 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. + +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 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. + + [Illustration: FIGURE 1. Map of the University of Kansas + Natural History Reservation showing locations where opossums + were live-trapped.] + +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' x 8" x 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 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. + +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. + +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. + + + + +HABITAT + + +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 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. + +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. + +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, +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. + + + + +BEHAVIOR + + +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. + +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. + +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. + + [Illustration: FIGURE 2. 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).] + +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. + + [Illustration: FIGURE 3. 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.] + +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. + + + + +MOVEMENTS + + +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 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." + +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 (_loc. cit._) 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. Occasionally 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. + +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 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 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. + + [Illustration: FIGURE 4. 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.] + +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. + +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 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. + +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. + +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 +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. + + + + +DISPERSAL OF YOUNG + + +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. + +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. + + Date of capture and Date of Distance + Sex marking as pouch young recapture in feet + + Female April 14, 1951 September 22, 1951 1870 + Female May 6, 1950 February 28, 1952 1800 + Female May 14, 1950 December 31, 1950 1750 + Female March 28, 1951 January 23, 1952 1700 + Female May 11, 1951 November 9, 1951 1700 + Female May 11, 1951 March 2, 1952 1450 + Female April 2, 1950 October 7, 1950 1160 + Female April 14, 1951 May 19, 1952 1100 + Male May 11, 1951 February 3, 1952 800 + Female May 11, 1951 January 9, 1952 700 + Female April 2, 1950 October 3, 1950 700 + Female May 6, 1950 April 3, 1951 650 + Female March 28, 1951 February 2, 1952 500 + Male April 18, 1952 July 6, 1952 120 + Female April 2, 1950 April 14, 1951 10 + +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 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. + + + + +FEEDING HABITS + + +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. + +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 (_Vitis vulpina_) is an abundant woodland vine on the +area and often forms dense tangles both in deep woods and in edge +situations. 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. + +Hackberry fruit (_Celtis occidentalis_) 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. + +Wild plum (_Prunus americanus_) and wild crabapple (_Pyrus ioensis_) +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 (_Prunus virginiana_) and climbing bittersweet (_Celastrus +scandens_), 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. + +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. + + 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%). + + 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 of + 2 others); rabbit in two, making up most of both; insects + (grasshopper, and large black beetle) in two making up small + percentages. + + 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. + + 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. + + 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. + + 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. + + 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 (_Vespula_, about 6 individuals, all in one + scat), toe bones and fur of cottontail rabbit; a few scales of + immature copperhead; and a snail. + + November, 1952, 2 scats. Grape in both, making up all of one + and about 90% of the other. + +Sandidge (_loc. cit._) 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. + +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 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. + +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 (_Eumeces +fasciatus_ and _E. obsoletus_) and the racerunner (_Cnemidophorus +sexlineatus_). Both Sandidge (1953) and Reynolds (1945) recorded the +five-lined skink (_E. fasciatus_) 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 _Pasimachus_ and _Brachinus_) and occasionally, +snails, centipedes and millipedes. + +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. Of the 8 kinds of frogs and toads +breeding at the pond, the bullfrog (_Rana catesbeiana_), leopard frog +(_Rana pipiens_) and cricket frog (_Acris gryllus_) were most +abundant, throughout the season and especially when drying occurred. +All three probably are important foods of the opossum locally. + + + + +WEIGHTS + + +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. + +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. + +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. + +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. + +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 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. + + [Illustration: FIGURE 5. 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.] + +The few opossums caught in summer were thin and appeared to be +suffering from infestations of ectoparasites, especially chiggers +(_Eutrombicula alfreddugesi_) and ticks (_Dermacentor variabilis_). +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. + +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. + + + + +BREEDING SEASON + + +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 others bore litters as late as early March. Two +lots of small young found in early June may have been second litters. + +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. + + Weight of + Date female Number of + in grams young Development of young + + March 1, 1952 2000 9 Newborn + March 2, 1952 1450 6 Newborn + March 2, 1952 1230 7 Newborn + March 5, 1950 1200 10 About 16 mm. snout to vent + March 5, 1950 1300 1 About 14 mm. snout to vent + March 6, 1951 1110 4 Newborn + March 18, 1952 1930 8 Not present when female + was trapped on March 1 + March 18, 1952 1520 6 + March 18, 1952 1230 12 About 40 mm. snout to vent + March 19, 1951 1000 8 Estimated 1 week old + March 22, 1950 1040 9 About 34 mm. snout to vent + March 24, 1950 1280 10 74 mm. snout to vent + March 24, 1950 1480 8 + March 27, 1950 965 8 Total length 26 mm., + weight .8 g. + March 28, 1951 820 7 20 mm. crown to rump; born + since previous capture of + female on March 7 + March 30, 1950 1325 9 Total length 33 mm. + March 31, 1952 1930 8 + March 31, 1952 1630 5 Total length 73 mm. + +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. + +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. + +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. + +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. + +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, were taken. Dates +of capture, weights in grams and sexes were as follows: + + Oct. 3, 1950 400 grams male + Oct. 6, 1950 510 grams female + Oct. 8, 1950 260 grams female + Oct. 8, 1950 350 grams female + Oct. 18, 1950 350 grams[A] female + Dec. 5, 1951 630 grams female + Dec. 30, 1950 710 grams female + Jan. 1, 1951 660 grams female + Jan. 1, 1950 700 grams[A] male + Jan. 9, 1950 550 grams male + Jan. 11, 1950 550 grams male + + [A] estimated + +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. + +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. + + + + +NUMBERS OF YOUNG + + +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, 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. + +Lay (_loc. cit._) 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. + +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. + + + + +COMPOSITION OF THE POPULATION + + +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. + + 1949-1950 1950-1951 1951-1952 Total + Old adults 11(25%) 9(26.4%) 11(39.2%) 31(29.2%) + Yearlings: + Born in late winter 29(66%) 18(53.0%) 13(46.5%) 60(56.6%) + Born in late spring 4(9.1%) 7(20.6%) 4(14.3%) 15(14.2%) + Total 44 34 28 106 + +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. + + + + +POPULATION DENSITY + + +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. + + +CENSUS WITH HALF-MONTHLY SAMPLING PERIODS + + Number of Number of Number of Computed + individuals individuals recaptures population + Sampling period taken taken in in for + in following following sampling + period period period period + + Early November 1949 3 7 1 21 + Late November 1949 7 8 3 18.7 + Early December 1949 8 11 3 29.3 + Late December 1949 11 7 4 19.2 + Early January 1950 7 3 1 21 + Early March 1950 5 8 2 20 + Late March 1950 8 6 3 16 + Early April 1950 6 3 1 18 + Late April 1950 3 6 2 9 + Early May 1950 6 3 2 9 + Early November 1950 1 3 1 3 + Late December 1950 3 6 1 18 + Early February 1951 4 13 3 17.3 + Late February 1951 13 6 3 26 + Early March 1951 6 4 3 8 + Late March 1951 4 5 2 10 + Early April 1951 5 1 1 5 + Late April 1951 1 5 1 5 + Early May 1951 5 3 2 7.5 + Early February 1952 9 4 2 18 + Late February 1952 4 9 1 36 + Early March 1952 9 6 2 27 + Late March 1952 6 5 2 15 + + +CENSUS WITH MONTHLY SAMPLING PERIODS + + Number of Number of Number of Computed + individuals individuals recaptures population + Sampling period taken taken in in for + in following following sampling + period period period period + + November 1949 9 16 7 21 + December 1949 16 9 3 48 + March 1950 11 9 3 33 + April 1950 9 7 2 32 + October 1950 9 3 3 9 + November 1950 3 3 1 9 + December 1950 3 7 3 7 + January 1951 7 14 3 33 + February 1951 14 7 4 25 + March 1951 7 5 3 12 + April 1951 5 6 3 10 + November 1951 3 6 1 18 + December 1951 6 5 1 30 + January 1952 5 11 3 18 + February 1952 11 13 4 36 + March 1952 13 9 5 23 + April 1952 9 3 1 27 + +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 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. + +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. + +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 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. + + + + +MORTALITY FACTORS + + +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. + +Most of the opossums caught in summer and early fall had eye +infections, and all of them were infested with ticks (_Dermacentor +variabilis_). Sometimes ticks were attached in dense clusters of +several dozen on the animal's ears and scattered over other parts of +the body. + +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. + +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. + +Natural enemies of the opossum on the area include the red-tailed +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. + + + + +SUMMARY + + +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. + +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. + +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. + +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 +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. + +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. + +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. + +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. + +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 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. + + + + +LITERATURE CITED + + +FITCH, H. S. + + 1950. A new style live-trap for small mammals. Jour. Mamm., 31:364-365. + + 1952. The University of Kansas Natural History Reservation. Univ. + Kansas Mus. Nat. Hist., Misc. Publ., 4:1-38, 4 pls. + + +HALL, E. R., and KELSON, K. R. + + 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. + + +HARTMAN, C. G. + + 1923. Breeding habits, development and birth of the opossum. + Smithsonian Report 1921:347-363. + + 1928. The breeding season of the opossum (_Didelphis virginiana_) + and the rate of intrauterine and postnatal development. + Jour. Morph. and Physiol., 46:143-215. + + 1952. Possums. Univ. of Texas Press, Austin. xvi + 174 pp. + + +LAY, D. W. + + 1942. Ecology of the opossum in eastern Texas. Jour. Mamm., 23:147-159. + + +LEONARD, A. B., and GOBLE, R. C. + + 1952. Mollusca of the University of Kansas Natural History Reservation. + Univ. Kansas Sci. Bull., 34:1013-1055. + + +REYNOLDS, H. C. + + 1945. Some aspects of the life history and ecology of the opossum in + central Missouri. Jour. Mamm., 26:361-379. + + +SANDIDGE, L. L. + + 1953. Food and dens of the opossum (_Didelphis virginiana_) in + northeastern Kansas. Trans. Kansas Acad. Sci., 59:97-106. + + +WISEMAN, G. L., and HENDRICKSON, G. O. + + 1950. Notes on the life history and ecology of the opossum in + southeast Iowa. Jour. Mamm., 31:331-337. + + +_Transmitted May 4, 1953._ + + + * * * * * + + + + +Transcriber's Notes + + +Other than two possible typographical errors listed below, the title +and verso (second) page 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, ..." + + Page Correction + ==== =========================================================== + 316 Occasionaly => Occasionally + 338 Possible typo: Didelphis Virginiana => Didelphis virginiana + +Emphasis Notation + + _Text_ - Italics + + + + + + + +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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