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diff --git a/37450.txt b/37450.txt new file mode 100644 index 0000000..d8195b3 --- /dev/null +++ b/37450.txt @@ -0,0 +1,2115 @@ +The Project Gutenberg EBook of Aspects of Reproduction and Development in +the Prairie Vole (Microtus ochrogaster), by Henry S. Fitch + +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: Aspects of Reproduction and Development in the Prairie Vole (Microtus ochrogaster) + +Author: Henry S. Fitch + +Release Date: September 17, 2011 [EBook #37450] + +Language: English + +Character set encoding: ASCII + +*** START OF THIS PROJECT GUTENBERG EBOOK ASPECTS OF REPRODUCTION AND *** + + + + +Produced by Chris Curnow, Tom Cosmas, Joseph Cooper and +the Online Distributed Proofreading Team at +http://www.pgdp.net + + + + + + + + + + UNIVERSITY OF KANSAS PUBLICATIONS + MUSEUM OF NATURAL HISTORY + + Volume 10, No. 4, pp. 129-161, 8 figs. in text, 6 tables + + December 19, 1957 + + Aspects of Reproduction and Development + in the Prairie Vole (Microtus ochrogaster) + + BY + HENRY S. FITCH + + UNIVERSITY OF KANSAS + LAWRENCE + 1957 + + + + + UNIVERSITY OF KANSAS PUBLICATIONS, MUSEUM OF NATURAL HISTORY + + Editors: E. Raymond Hall, Chairman, Henry S. Fitch, + Harrison B. Tordoff + + Volume 10, No. 4, pp. 129-161, 8 figs. in text, 6 tables + Published December 19, 1957 + + UNIVERSITY OF KANSAS + Lawrence, Kansas + + PRINTED IN + THE STATE PRINTING PLANT + TOPEKA, KANSAS + 1957 + + [Illustration: Look for the Union label!] + + 26-7561 + + + + + +Aspects of Reproduction and Development +in the Prairie Vole (Microtus ochrogaster) + +BY + +HENRY S. FITCH + + + + +INTRODUCTION + + +The prairie vole is by far the most abundant mammal on the +University of Kansas Natural History Reservation and on grassland +areas throughout northeastern Kansas. This vole therefore affects +the vegetation, perhaps more than any other native vertebrate, and +it is an important food source for most of the vertebrate predators. +Since the Reservation was established, in 1948, more data have been +accumulated concerning this vole than for any other species of +animal there. From February, 1950, to February, 1954, a grid of +live-traps at 50-foot intervals was set for several days each month +in a three-acre field inhabited by voles, and the population of +marked individuals was studied throughout the four-year period. From +November, 1953, to June, 1956, a half-acre trap grid with 20-foot +interval was used on an area adjoining the three-acre field. Other +trap lines in somewhat different habitats were maintained for +shorter periods as a basis for comparison. By June, 1956, a total of +some 3550 voles had been caught and recorded 14,750 times in all. +The present report is a preliminary attempt to analyze, in part, +these extensive data, and is concerned with certain phases of the +species' reproduction and growth that have bearing on the observed +population changes from month to month and from year to year on the +Reservation. + +Through the studies of Jameson (1947) and Martin (1956), both made +in the same general area as my own, and several earlier studies, the +life history and ecology of the prairie vole are already well known. +The present report, with much larger amounts of data, further +clarifies certain phases of the ecology; and by using types of data +not available to Jameson and Martin I have dealt with some topics +not included in their reports. + +Previous studies of growth in _Microtus_ have been based almost +entirely on weights. However, the weight of an individual vole may +fluctuate widely over a short period, depending on pregnancy and +parturition, length of time in a trap without food, availability of +moisture, and other factors. In the course of my study, in 1954 +and 1955, and parts of 1953 and 1956, measurements of total length, in +addition to weights, were recorded for most of the voles +live-trapped. + +To test the accuracy of measurements, successive readings were +compared in individual voles that were already of large adult size +and that presumably either had stopped growing or were growing so +slowly that the gain was scarcely detectable in the relatively short +periods involved. For 200 such readings 33 per cent were just the +same as previous records for the same animals, 24 per cent deviated +by 1 mm., 22 per cent deviated by 2 mm., 15 per cent by 3 mm., 4.5 +per cent by 4 mm., .5 per cent by 5 mm., 1 per cent by 6 mm., and .5 +per cent by 7 mm. On the average, successive measurements varied by +1.43 mm., somewhat less than one per cent of the adult vole's total +length. Occasional errors of two to four per cent were easily +eliminated because for the voles used for growth records, series of +measurements were available, with clearly defined trends. The +occasional readings that deviated from the general trend for the +individual were discarded. + +Measurements were recorded along with other data in the field at the +point of capture. Obtaining a reasonably accurate measurement on a +live and struggling vole required patience and practice. With the +thumb and forefinger of the left hand, I grasped the vole by loose +skin of the nape, and simultaneously grasped the tail at a point +approximately three-fourths of the distance to the tip. Then, with +gentle but steady pressure, I stretched the vole out to its full +length, meanwhile manipulating a millimeter ruler with the free +fingers, so that the vole was pressed against it, with the nose pad +at the end of the ruler. + +The total length measurement is considered the best index to +over-all size. The relative tail-length varies slightly between +individuals, averaging approximately 22 per cent of the total +length. Individuals having broken tails, or having the distal parts +of their tails missing, were not included. The total length can be +measured with greater accuracy than can either the head-and-body +length or the tail-length separately. + + + + +GENERAL SOCIAL BEHAVIOR + + +As compared with other mammals, voles are tolerant and somewhat +social. That individuals are not mutually exclusive (territorially) +in areas occupied was demonstrated on many occasions when more than +one individual was caught simultaneously in the same live-trap. +Injury of a vole by a trap-mate was a rare occurrence. + +Multiple captures often involved a female in oestrus and one or more males, +or a female and her young, but other instances involved various +combinations of sex and age groups. As many as five adults have been +caught in a trap simultaneously at times when the population density +was high. At such times, the meadow habitat is crossed by a maze of +interconnecting surface runways and one runway may be traced +continuously for 100 yards or more. Because each individual vole +normally confines its activity to a small area, only a fraction of +an acre, it is evident that individuals living at different places +overlap in their home ranges, and also in the trailways followed in +foraging. A high degree of tolerance is indicated. Where population +is so sparse that the systems of surface runways comprise separate +and isolated units, trapping experience has shown that one such +system may harbor several or many individuals. + +As direct observations on voles under natural conditions are rarely +feasible, because of the animals' timidity, their utilization of +concealing cover, and tendency to crepuscular habits, best evidence +of social habits and underground life is based upon behavior of +captive individuals. Many voles were kept in confinement for varying +lengths of times, either singly or in association with others. Under +such conditions there was sometimes sporadic fighting, but it was +mainly defensive and serious injuries were rare. Two or more voles +caught at a given spot regardless of whether they were found in the +same trap simultaneously, or trapped separately within a short time, +usually were completely tolerant of each other. When at rest in +their container, such voles would huddle together in a corner or in +a nest, if materials were provided, so that collectively they +presented the minimum exposed surface. The intimacy and lack of +antagonism displayed on such occasions, suggested that the voles +were accustomed to living together amicably in the same nest +chamber. In live-trapping, "double" captures in a single trap often +involved the same two individuals. Such trap-mates were often male +and female, and in many instances the female was not in breeding +condition. That the voles are not monogamous in habits was +demonstrated when the same female was often trapped in association +with either of two males. Other trap associates taken together +repeatedly often were two males, or two females. Voles that are nest +mates or "neighbors" may tend to move about together in their +foraging, or one confined in a trap may attract the other +sufficiently to cause it to force an entrance by lifting the heavy +door of a trap. + +When a new vole, caught at a different location, is added to a +container in which one or more are already confined, there is mutual +circumspection between the original occupants and the newcomer. At +first, each vole is intimidated by movements of the other, and as a +result, the original occupants huddle in their established corner +while the newcomer cowers in the most remote part of the container. +Gradually the voles become less timid and one may approach another +slowly and cautiously, to sniff at it. The vole approached may react +with a show of hostility which is largely defensive. In the +characteristic posture of threat for defense, the vole crouches, or +rears back on its haunches, with snout elevated and incisors +prominently displayed. If the warning posture is unheeded, or if the +vole is made unusually aggressive by having young to defend, or for +some other reason, it attacks with a sudden forward lunge, striking +the adversary simultaneously with both forefeet and with the +incisors. The lunge is so rapid that when I have observed it, I have +been unable to discern whether the attacker bit its opponent. The +attack serves to force back the other animal, throwing it off +balance and intimidating it. The attacked animal may dodge nimbly to +avoid the lunge, but whether or not it is actually struck, it +usually retreats, avoiding or postponing further hostilities. Voles +that have been kept in containers for periods of hours or days tend +to be more hostile and aggressive toward a newcomer than are those +newly introduced. After series of meetings resulting from the +exploratory behavior of the newcomer and the curiosity or normal +activity of those longer confined, hostility gradually subsides. +Within a few hours a newcomer is usually accepted, and thenceforth +he huddles with other members of the group when at rest, and +hostility is rarely evident. + +This ready acceptance on short acquaintance of strange voles into +the family or social group suggests that lack of territoriality +extends even to the use of the nest burrows, and that groups of +voles may share the same nest, huddling together and deriving mutual +benefit from the association, such as warmth in cold weather. +Schmidt (1931: 113), studying this vole in Clark County, Wisconsin, +noted its colonial habits. He found isolated small mounds that were +riddled with burrows, and little sign in intervening areas. At one +mound he trapped two adult males, one adult female, and two young; +at another mound, two adult males, two adult females, and four young +were trapped. My individuals that were released from live-traps were +on many occasions trailed by means of a stiff wire collar with spool +of thread attached, to holes that presumably were their home +burrows. Data obtained in this manner indicated that ordinarily +several or many individuals use the same burrow system. The +histories of individual voles on the study area at the Reservation +indicate shift of home base from time to time, usually for short +distances within the area already included in the home range, but +occasionally to new areas relatively remote from the original home +range. + +Severe fighting between adult prairie voles occurs at times. +Occasionally, sharp squeaks accompanied by brisk rustling in the +grass suggesting pursuit or conflict, are heard in their habitat. An +unusually large adult male, long resident on a study area, suddenly +lost weight and deteriorated in condition over a period of several +days, then was found dead in a nest-box attached to a trap. +Dissection revealed numerous punctures in the skin and flesh of the +neck and back, probably made by the incisors of another vole. +Extensive hemorrhage and swelling had occurred, and obviously these +injuries were the cause of death. + +Although it was not feasible to study the home life of the voles +underground, clues were gained from those uncovered in runways and +nests beneath large boards and strips of tarpaper, previously +distributed for this purpose. Nests were constructed by the voles +beneath several such pieces of tarpaper and runways appeared beneath +all the pieces that were placed in habitat favorable to the voles. +In summer, however, the high daytime temperatures beneath these +shelters made them uninhabitable to the voles, and they were used +mainly in spring. From February 15 to May 1, 1953, 14 voles were +caught 19 times beneath five of the tarpaper strips, and many other +voles that were seen beneath them escaped. Upon turning one of the +strips I often discovered voles in close proximity. Sometimes two or +more darted from the same nest. The disturbance of repeatedly +raising the strips and exposing the voles' shelters soon caused them +to desert the sites; consequently the information obtained by this +means was limited. + + + + +SEXUAL BEHAVIOR + + +There is sexual activity in every month of the year, but its +incidence varies greatly from one season to another. As has been +indicated by various authors, male voles reach sexual maturity later +than females. It seems that ordinarily the availability of sexually +active males is not a limiting factor, however. While males that are +still well below average adult size produce mature spermatozoa, and +are probably capable of breeding (Jameson, 1947: 145), certain +large old males may sire a disproportionately large percentage of the +litters produced. Observations on males in confinement indicated +that sexual activity tended to be directly proportional to the size +of the testes. Occasional individuals, having much enlarged scrotal +testes were more readily stimulated to sexual activity and more +aggressive toward females than were those in which the testes were +of more nearly typical size or abdominal or were smaller than +normal. The combination of factors controlling size of testes is not +well understood, but males having unusually large testes were caught +most often when food supply was optimum, for instance after a period +of heavy precipitation when an abundant supply of new grass provided +succulent and nutritious food. + +In confinement sexual activity was largely inhibited and attempts to +establish a laboratory colony met with failure. Sexual activity was +observed mainly in recently captured males, and their interest was +aroused chiefly by females that had given birth to litters within a +few hours previously. Oestrus is known to follow closely after +parturition. Females found in live-traps with newborn young often +were brought to the laboratory for observation. An apparent instance +of hostility between rival males competing for an oestrus female was +observed on September 2, 1950. The female was found in a trap with +four newborn young, and since the young had not yet attached to her +teats, she was temporarily returned to the trap after recording, to +prevent desertion of the litter. Returning twenty minutes later I +found another adult vole at this trap. It would suddenly emerge from +dense grass nearby, and would move over the trap or around it, with +jerky, halting movements, then would dart back under cover. The +female emerged from the nest box into the trap runway, and sniffed +at the other, and both pressed against the intervening wire barrier. +There was gnawing on the wire by one or both. A third adult vole +appeared. As it moved toward the trap, all three suddenly took alarm +and darted back under cover, the female hiding in the trap nest box. +In a few seconds they again appeared. The two outsiders, presumably +both males, were not individually recognizable, but several times +one was seen to dart at the other, chasing it away momentarily. They +were seldom both in sight at once. + +Males confined with post-partum females usually evinced sexual +interest, following them about persistently and nuzzling their +genitalia. The females, however, were often unreceptive perhaps +because they were disturbed by strange surroundings and by the +presence of their litters, so that they usually attempted to escape, +or to rebuff the male's attention. At first the female might flee, +squeaking in protest at the male's pursuit. If he still continued to +follow, she would turn on him, rearing back in the characteristic +threatening pose, and would lunge at him, striking him sharply or +driving him back. After such rebuff, males were usually intimidated +or discouraged so that they temporarily or permanently abandoned +their advances, and small males were more easily rebuffed than were +larger individuals. On several occasions large males having enlarged +testes were not readily rebuffed by females but continued to follow +them. When the female turned upon him, such a male might lunge +against her, throwing her off balance, and causing her to attempt to +escape, and then continuing the pursuit until it ended in copulation +or in more severe fighting. Although not accepted sexually, a +rebuffed male might be readily accepted as a nest-mate, huddling +along with the female and perhaps other individuals of both sexes. +In huddling voles, the most frequently observed type of social +behavior was grooming; one individual would slide its chin or muzzle +through the other's fur with a stroking movement consisting of a +series of rapid forward jerks and the stroking movements might +continue for periods of minutes. The recipient of the grooming +usually made no evident response indicative of either pleasure or +displeasure. Often it seemed to be sleeping while the grooming was +performed. Individuals of both sexes performed this grooming and the +recipient might be of either the same sex or the opposite sex. This +grooming may have some significance as a search for ectoparasites +such as fleas, or mites that often infest the voles. However, after +prolonged grooming by a companion, a vole's fur was of mussed and +disarranged appearance. Although the grooming that occurs between +voles that are resting in nests seems to have no direct significance +as sexual behavior, somewhat similar actions constitute part of the +mating pattern. A sexually aroused male overtaking a receptive +female, slides his chin forward along her back with jerky, stroking +movements. In some observed instances this behavior continued +intermittently for several minutes before actual copulation. In some +other instances it was almost lacking. + + + + +CHANGES IN FEMALE GENITALIA + + +In female voles that are sexually quiescent, both those that have +not yet attained breeding maturity, and those that have undergone +regression after attainment of sexual maturity, the vaginal orifice +is not evident. The canal is sealed externally by a membranous +layer of epithelium. Presence of a vaginal orifice indicates that +the individual is in some active stage of the breeding cycle. The +appearance of the orifice varies between different females, and it +changes in the same female from day to day or even from hour to +hour. Presumably these changes in the vaginal orifice are cyclical +and are closely correlated with oestrus, but attempts to trace them +were unsuccessful largely because the normal cycle was rapidly +suppressed in captive voles, which soon became sexually quiescent. +Individual voles living under natural conditions were not trapped +with sufficient regularity to permit tracing the details of changes +in their genitalia. + +In those females having the vaginal orifice most developed, the +margins are turgid and slightly inflamed. The circular opening gapes +1.0 to 1.5 mm. in diameter when the tail is raised. A female may +remain in this condition for two days or more. Vaginal smears at +this stage often showed nucleated cells characteristic of oestrus. +Subsequently the margins of the orifice become less prominent and +the opening becomes smaller. The dorsal and ventral walls adhere +until an opening is no longer evident unless the adjacent skin is +stretched. + +In pregnancy the orifice is occasionally sealed, but usually is +evident. It is, however, less prominent than in oestrus, and does +not gape. The margins are less turgid than in oestrus, and the +opening is in the form of a transverse slit through which the +purplish epithelial lining of the dorsal wall of the vagina can be +seen. After parturition, placentae and bloody discharge often are in +evidence in the vaginal canal. Females that have not given birth to +young recently may also have bloody mucous discharge. Its +significance has not been determined. In females that are undergoing +sexual regression, the margins of the vaginal orifice become +shrunken and pale, and the orifice becomes partly or wholly sealed. + +Bodenheimer and Sulman (1946:255) concluded from their study of +_Microtus guentheri_ that in this species, as in "the cat," "the +rabbit," "the ferret," and a few other mammals, ovulation is induced +by copulation, and that there is no regular vaginal cycle. Hoyte +(1955:412) disagreed with these conclusions for other species of +_Microtus_, as he trapped individuals of _M. oeconomus_ that had +recently ovulated without copulation (at least no sperm were found +in the genital tracts). In _M. ochrogaster_ oestrus seems to be +controlled largely by the food supply, at least the incidence of +perforate females was found to fluctuate irregularly tending to +follow the trend of rainfall, and, probably in more direct +correlation, the amount of new grass present (see Table 1, and +Martin, 1956:383-384). It therefore seems unlikely that in this +species ovulation is dependent on copulation. + +In females that have not yet produced young the teats are minute and +well concealed in the fur, so that they are difficult to find, but +in lactation they become conspicuous. In early lactation the teats +are typically about 1 mm. in diameter and 2.5 mm. in length. As +lactation progresses, they become thickened to nearly twice the +original diameter. After lactation, as inversion occurs, they shrink +to scabrous low prominences, 2 mm. to 3 mm. in diameter, surrounded +by bare skin. There are three pairs of mammae, one pair pectoral and +the other two abdominal. As mentioned by Jameson (1947:146), the +pectoral mammae show little evidence of use in lactating prairie +voles. Probably they are not used at all except in females with more +than the four young in a litter accommodated by the abdominal +mammae. As in various other rodents, the suckling young may cling to +the female's teats and may be dragged over the ground as she moves +about. When the female forages near the nest, she may drag the young +with her instead of leaving them, but she can detach them instantly +if she so desires. On many occasions females found in live-traps had +young that were several days old clinging to their teats. In some +instances young that had their eyes open may have followed the +female into the trap and attached afterward. + + + + +SEASONAL INCIDENCE OF BREEDING + + +In the region of my study the prairie vole breeds the year round, but +the rate of breeding changes continually. There is no regularity in +the trend of the breeding season from year to year. It is obvious that +the species is responsive to environmental changes and is so well +attuned that its breeding is speedily initiated or inhibited by +changes to favorable or unfavorable weather. The incidence of breeding +is highest when temperature is moderate and both water and foods of +preferred sorts are plentiful. + +Tables 1 and 2 and Fig. 1, based on 11,109 records representing each +month over a four-year period, show the changing trends from month to +month. The perforate condition recorded in Table 1 may represent any +of several stages in oestrus or pregnancy, but is regarded as a crude +index of rate of breeding, since voles in the anoestrus stage lack the +vaginal orifice. Highest percentages of perforate females occurred in +the months of February, March, April, May, and June, while by far the +lowest percentages were recorded in the drought summers of 1952 and +1953. Even in mid-winter a substantial proportion of the females +trapped were perforate. + + [Illustration: FIG. 1. Average catch per day in a + three-acre field, in a grid of 100 live-traps, over a four-year + period. For each year, solid line represents total and dashed + line represents number of young up to 30 grams in weight. Numbers + caught are roughly indicative of population density, but many + variables distort this relationship. Young are never represented + in the catch in their true ratio to adults, since on the average + they are less vagile and less attracted to traps.] + + + Table 1. Percentages of Adult Females Recorded as Perforate in the + Monthly Samples From 1950 Through 1953. + + =======+======+======+======+====== + | 1950 | 1951 | 1952 | 1953 + -------+------+------+------+------ + Jan. | .... | 27.3 | 41.7 | 33.3 + Feb. | .... | 47.7 | 53.1 | 72.9 + Mar. | 40.6 | 38.5 | 77 | 50 + Apr. | 76 | 41.9 | 51.9 | 73 + May | 84 | 40 | 52 | 58.2 + June | 67.7 | 41.5 | 19.3 | 16.6 + July | 57.3 | 45.5 | 12.7 | 15.4 + Aug. | 43.1 | 52.2 | 5.4 | 31.3 + Sept. | 47 | 56.5 | 51.6 | 56.2 + Oct. | 44.8 | 48.9 | 43.4 | 60 + Nov. | 24.4 | 45 | 24.1 | 61.5 + Dec. | 31.1 | 45 | 37.5 | 41.6 + + Table 2. Percentages of Adult Females Recorded to Be in Late + Pregnancy in the Monthly Samples From 1950 Through 1953. + + =======+======+======+======+====== + | 1950 | 1951 | 1952 | 1953 + -------+------+------+------+------ + Jan. | .... | 2.3 | 0 | 0 + Feb. | .... | 0 | 10.4 | 9.1 + Mar. | 5.8 | 0 | 22.6 | 13.3 + Apr. | 8 | 19.4 | 22.6 | 27.5 + May | 21 | 37.1 | 29.5 | 39.4 + June | 13.3 | 14.9 | 16.5 | 5.5 + July | 57.3 | 6.7 | 7.9 | 3.8 + Aug. | 43.8 | 15.2 | 10.8 | 12.5 + Sept. | 40.4 | 15 | 20.3 | 6.2 + Oct. | 45.2 | 21.9 | 18.9 | 10 + Nov. | 7 | 8.9 | 3.3 | 23 + Dec. | 0 | 0 | 0 | 8.3 + +Usually pregnancy can be recognized only in the last week before birth +of the litter, when the female's abdomen is noticeably distended by +the enlarged fetuses. Palpating to detect embryos was not attempted +because of the danger of injuring them or the female. Because +gestation is of approximately three weeks duration, the figures in +Table 2 represent roughly perhaps one-third, or a little less, of the +adult females actually pregnant. At most times of year a substantial +proportion of adult females (sometimes nearly all) are pregnant. Only +in the winter (including March in 1951) were samples taken in which no +recognizably pregnant females were found. Incidence of pregnancy was +notably high in July, August, September, and October of 1950, May, +1951, May, 1952, and April and May, 1953. A high rate of breeding was +not necessarily followed by an increase in the population. A +relatively low rate of breeding was adequate to maintain the +population level, provided that environmental factors remained +favorable. Fig. 1 shows the average catch per day (with approximately +100 live-traps) over the four-year period, 1950 through 1953. The +young (including all those weighing 30 grams or less, and +corresponding roughly with the part of the population less than two +months old) are shown separately. It is noteworthy that throughout +the entire period the ratio of young to adults tended to be fairly +stable--usually fluctuating between ten and thirty per cent of the +total catch. Ratios of young to adults were notably high in March and +May, 1950; April, June and July, 1952; and April, May and June, 1953. +Ratios of young were notably low in June and December, 1950; January, +February, March, and June through October, 1951; January, February, +and March, 1952; and November, 1953. + +In Fig. 1 the catch per day of voles, varying from month to month, +reflects chiefly the changing population density. However, other +factors also have important effects on the catch. For example, bait +acceptance is better in the winter when natural foods, especially +greens, are scarce, with the result that a higher catch can be made +with the same population density. Interference with the trap line by +other animals also affected the catch of voles. In warm weather the +traps were checked in both morning and evening, and the catch was +correspondingly greater than it was in cool weather when the traps +were checked only once daily. The ratios obtained of young to adult +voles cannot be accepted at face value as the true ratios in the +population, either. For the first several days of each trapping +period, the voles caught were mostly adults previously marked and, +presumably, conditioned to the grain bait. Later, young voles not +previously recorded, came to the traps in increasing numbers. The +young, being at first not conditioned to the bait, and also having +relatively small home ranges, would generally be less well represented +in the catch than would the adults. + + + + +GESTATION + + +In other species of _Microtus_, so far as known, a 21-day gestation +period seems to be the rule (Bailey, 1924:528; Hamilton, 1941:13; +Hatfield, 1935:264). _M. ochrogaster_ seems to conform to this +pattern, but the data obtained were meager, because breeding activity +was usually inhibited in voles kept in confinement. + +A female live-trapped on July 23, 1951, appeared to be in breeding +condition. When trapped two days later, she had a copulatory plug, and +21 days after this she was found with a newborn litter in a trap. A +female thought to have given birth to a litter between successive +captures on July 20, and July 21, 1951 (on the basis of appearance of +genitalia, and reduction in weight from 53 to 46 grams), appeared to +have just completed parturition when she was examined on August 10. A +female that gave birth to a litter in confinement on May 18, 1954, +bred and was released the same day. She was recorded as pregnant in +the first week of June, but on June 7 was no longer pregnant. If this +pregnancy terminated normally, a gestation of 20 days or less is +indicated. + +Greenwald (1956:221) suggested that in _M. californicus_, oestrus +might occur in the period of lactation, because he found recently +formed corpora lutea in lactating females. In the course of my field +work on _M. ochrogaster_, I obtained precise or approximate dates of +successive litters born at intervals of somewhat more than 21 days +apart. In different females, intervals of 23, 23, 24, 26, and +approximately 27 (between 26 and 28) days were recorded between +successive litters. In four other females intervals between litters +were known only approximately because one of two records was based on +a capture in late pregnancy judged to be within two or three days of +parturition. For these females, intervals of 23, 24, 24, and 26 days +were recorded. From the trend of these records, it seems that females +often became pregnant within a few days after birth of a litter. +Pregnancy from post-partum oestrus would seem to be less frequent than +pregnancies beginning a few days after birth of the previous litter, +and within the period of lactation. + + + + +NUMBER OF YOUNG PER LITTER + + +Jameson (1947:146) found an average of 3.4 young per litter in 58 +litters of _M. ochrogaster_ from northeastern Kansas, mostly from +Douglas County. Martin (1956:386) recorded a somewhat lower mean of +3.18 +- 0.24 in 65 litters on the Reservation in 1950, 1951, and 1952. +For a total of 82 litters recorded from 1950 through 1956, inclusive, +I obtained an average of 3.37 +- .075 young per litter. Several litters +that were recorded were excluded from this computation as in each +instance there was reason to suspect that they were incomplete. These +included instances of females found in traps with young several days +old, females that may not have completed parturition when they were +released with newborn young, and those litters that might have +sustained losses through cannibalism by the mother or her trap-mates. + +Mean numbers of young per litter were found to vary from year to year +and from month to month, as shown by the following lists: 1950, 3.0 +(13 litters); 1951, 3.5 (23 litters); 1952, 3.5 (11 litters); 1953, +3.4 (5 litters); 1954, 3.4 (15 litters); 1955, 4.1 (7 litters); 1956, +3.8 (5 litters); January 2.0 (1 litter); February 3.5 (4 litters); +March 4.5 (4 litters); April 3.9 (12 litters); May 3.3 (25 litters); +June 3.0 (9 litters); July 2.7 (4 litters); August 2.9 (7 litters); +September 2.8 (6 litters); October 3.4 (7 litters); November 5.0 (2 +litters); December 4.0 (1 litter). + +These differences can be logically explained on the basis of changes +in the average age of the breeding females in the population. On the +average, with greater length, weight and age, females produced +progressively larger litters, although individuals did not necessarily +conform to this general trend. For 24 females recorded in 1954-1956 +and measured within a few days of birth of their litters, average +length was correlated with number of young as follows: 6 young, 163.5 +mm.; 5 young, 158.0 mm.; 4 young, 157.7 mm.; 3 young, 154.6 mm.; 2 +young, 160.5 mm. + +For 48 other females, recorded in 1950-1953, that were not measured, +but that were mostly assignable to broad age groups on the basis of +their individual histories in the trapping records, the following well +defined trend was demonstrated. + + Table 3. Number of Young per Litter Correlated with Age or Size + of Female. + + =====================================+============+================== + | Number | Average + Age or Size Group of Female | of females | number of + | in sample | young per litter + -------------------------------------+------------+------------------ + More than one year old | 4 | 4.25 + 6 to 12 months old | 16 | 3.50 + Large (age indeterminate) | 9 | 3.44 + 2 to 5 months old | 9 | 2.90 + Small and medium (age indeterminate) | 10 | 2.80 + -------------------------------------+------------+------------------ + +It seems that the exceptionally high average numbers of young per +litter in March and April result from the breeding females in those +months being nearly all fully mature survivors of the previous year. +In summer, when many females that are only a few weeks old become +pregnant, the average litter declines to less than three young. The +small average litter of 3.0 young for 1950 probably resulted from the +fact that the population on the Reservation was then expanding rapidly +in the newly favorable habitat created by one year's crop of +vegetation after discontinuance of grazing, and had an unusually high +percentage of breeding females that were not fully adult. + + + + +SIZE AT BIRTH + + +In four newborn young, total lengths, in mm., were 47, 45, 45, and 42. +From the length-weight relationships shown in Fig. 2, it seems that a +length of approximately 47 mm. is typical of newborn young of average +weight. Martin (1956:388) found a mean weight of 2.8 +- 0.36 grams in +sixteen newborn prairie voles from the Reservation. For a series of 67 +other newborn voles representing 27 different litters in seven +different years, I found an average of 2.9 +- .05 grams. Young ranged +in weight from 3.8 to 2.0 grams. Weights of the newborn voles could +not be correlated with season, size, age of females, or food +conditions. However, a distinct trend toward larger size in those +litters that contained fewer young was evident, as shown in Table 4. + + Table 4. Weight of Newborn Young, Correlated with Number of Young + per Litter. + + ==============+============+============+=========== + | Mean | Number | Number + Known Young | weight | of litters | of young + Per Litter | in grams | in sample | in sample + --------------+------------+------------+----------- + 2 | 3.1 +- .09 | 7 | 13 + 3 | 3.0 +- .17 | 11 | 28 + 4 | 2.7 +- .22 | 6 | 17 + 5 | 2.6 +- .42 | 3 | 9 + --------------+------------+------------+----------- + + + + +EARLY GROWTH + + +Voles less than 100 mm. in total length were seldom captured, +because those less than this size are dependent on the female, and +rarely venture far enough from the nest to be caught in a trap. A +further difficulty in obtaining growth records on the smallest young +is that of making accurate measurements. During their first few days +they partially retain the fetal posture, usually lying on one side, +with the head, body and tail flexed in an arc almost completed by +the tail approximating the muzzle. Straightening the animal by +stretching it and holding it with sufficient firmness to obtain a +measurement might have involved injury to it. Therefore, in most +instances the newborn voles examined were merely weighed or an +approximate measurement was estimated without stretching the young +to its full length. + +Newborn voles were obtained when females that were caught in +live-traps produced their litters before they were found and +released. In some instances, females caught while in late pregnancy +were retained in the laboratory for a day or more until parturition +occurred. Many of the newborn voles were marked by toe-clipping, +according to the same system used for adults. Early growth was +measured in some instances by keeping the female with her litter in +confinement, measuring and weighing the young at intervals. In most +instances, the female was released at the point of capture +(presumably near her nest burrow) with the young clinging to her +teats. For the young so released, the incidence of recovery was +remarkably low, seeming to indicate that they were subject to +decimating losses. Perhaps such losses are normal, at least on the +study area where voles are live-trapped regularly. Holding of adults +and partly grown young in live-traps ordinarily has no harmful +effects on them, but the resultant separation of females from newly +born litters may often result in death of the young either from +hunger and exposure, or from attack by other voles and natural +enemies. + +During the first ten days the increase in length from an original 47 +mm. is from three to four mm. per day. Figs. 2, 5, and 8 show length +and weights of voles whose ages in days were definitely known +because they were born in the laboratory, or in a live-trap after +the female was caught there. Young voles marked at birth and +released with the female were rarely recovered in the period of +suckling, as they ordinarily remain in the nest burrow when the +female ventures out to forage. Litters retained in the laboratory +therefore have provided most of the records of growth in suckling +young. Growth varied greatly between litters. It was not clearly +correlated with size of female, size of young at birth, or number of +young in litter, but probably was influenced by attentiveness of the +female, her adjustment to captivity, and her productivity of milk. +Within each litter there were usually persistent differences in +development, but these were minor (except for those of occasional +runts) compared with the differences between litters. In several +litters of five young, one was usually smaller than the others at +birth and therefore could not compete successfully with its litter +mates, so that it never gained possession of a teat other than one +of the pectoral pair, and always succumbed within a few days, after +failing to gain weight as its litter mates did. The relatively few +voles marked at birth and recovered after developing under natural +conditions, did not deviate from the trend of those in confinement. + + + + +CARE OF YOUNG + + +Females in confinement were attentive to young, and, soon after +parturition, licked them clean and huddled over them protectively. +Ordinarily, the newborn young soon attached to a teat, and spent a +large part of its time attached during its early development. +Females found in live-traps with their litters of young less than a +day old, often had some or all of the young clinging to their teats. +Females with newborn litters, when released from live-traps, always +left without attempting to retrieve any young that were unattached. +Such young usually were permanently deserted, but in some instances +disappeared within an hour or less, perhaps rescued by the female +returning for them. + +Females with newborn young were made far more aggressive than most +other voles by their tendency to protect their young from possible +danger. In captivity such females usually took the offensive in +attacking or rebuffing any other voles confined with them. +Post-partum females obviously in oestrus were prevented from being +fully receptive by their hostility toward males whose presence might +endanger the young. Such a female has been seen to turn on a +pursuing male and attack him viciously, several times within a few +minutes, before copulation occurred. In captivity, at least, such +attacks would soon discourage a male so that unless he was +exceptionally active sexually, mating was prevented. + +Cannibalism, involving destruction of the newborn, is probably an +important factor in the population dynamics of the prairie vole. +Only a small percentage of the young known to have been born on an +area ever survived to be live-trapped; this small percentage was +indirect evidence of decimating losses in the young. Under +unfavorable conditions each of several females killed and ate her +own litter, but the degree of provocation varied greatly among +individuals. Females that gave birth to young in live-traps +occasionally ate one or more of their newborn young, as evidenced by +discarded remnants. Perhaps other instances passed unnoticed because +no remnants were found. That need for food or moisture as well as +psychological stress often motivated such cannibalism was suggested +by the fact that surviving litter mates might be accepted and cared +for by a female that had already eaten one or more of her young. +Although cannibalism is most likely to occur within a few hours +after birth of the young, they may be killed and eaten at any stage +of development. One female that had probably eaten one or more of +her litter, soon after parturition, nursed the two survivors. When +these were two weeks old, all were "pastured out" in a wire mesh +cage in tall brome grass. When the supply of grass had become scarce +(though some was still available), the female killed and partly ate +both her remaining young. + +One female was captured with three young attached that were several +days old. The young were detached from the female's teats with great +difficulty. When these young were returned to the female a few +minutes later, after they had been measured, weighed and marked, she +attacked them viciously, and within a few seconds had killed all of +them by biting their heads. In this instance the dead young were not +eaten, although they were temporarily left with the female. + +Females with young have ample cause for their circumspective +demeanor toward adult males, which are especially inclined to eat +the newborn. A male engaged in sexual pursuit has been observed to +grasp a young dangling behind the female, pull it from her teat, and +pausing momentarily, nibble its head off, before continuing to +follow the female. Like the genitalia of the post-partum female, the +newborn young seem to have an odor that attracts and excites the +male. + +To a lesser degree, adult females also display marked interest in +the newborn young of other individuals, which is liable to result in +cannibalism. The incidence of cannibalism is affected by the +condition, collectively, of the population of voles, and the +availability of nutritious food and moisture. In periods of summer +drought the grass becomes coarse and fibrous, and its protein +content declines. Under such conditions many voles appear to be +undernourished, and some are actually emaciated. Dehydration may be +an important factor at times when dew is unavailable for drinking +and the green vegetation remaining is exceptionally low in moisture +content. Voles caught at such times and brought to the laboratory, +drank avidly, and gained several grams soon after being offered +water or succulence. Cannibalism by adults on newborn young in times +of drought may be motivated by the acute need for moisture and +nutritious food. In times of drought the birth rate is at low ebb. + +Adult males have never been observed to display paternal solicitude +toward young, but some individuals, kept with females and their +litters, did not molest the young and were accepted by the females +as members of the family group. + +Other things being equal, cannibalism involving the young might be +expected to be greater at times of high population density. Then, +young left in the nest by a female in the course of her foraging +would more often encounter adults and partly grown young, both those +that lived in the same burrow system and exploring intruders from +other areas. + +The eyes open at an age of nine or ten days. Then the young enter +upon an exploratory period, when each wanders out of the nest, +emerges from the burrow, and wanders through the adjacent surface +runways in frequent short forays, sometimes following the female and +sometimes alone. Such forays usually cover only a few inches at +first, but as the young vole grows, becomes familiar with its +surroundings, and takes more plant food, its sphere of activity +gradually widens, and family ties are dissolved. Voles reared to an +age of three weeks in the laboratory and then released, survived +just as well if the female was not released with them demonstrating +that they were fully capable of shifting for themselves at this age. +In confinement, however, young voles of greater age continued to +suckle and remained closely associated with the female. Females in +confinement evinced much uneasiness because of their inability to +evade the young when the latter were old enough to walk. The young +then followed the female continually and suckled whenever she +stopped or even while she moved about, unless she paused to remove +them from her teats, but they would not remain detached for more +than a few seconds. When a young followed the female away from the +nest and then attached to a teat, the female after pulling the young +from her teat, would usually carry it, grasped between her incisors, +back to the nest and deposit it there. On one occasion a young vole +caught in a live-trap was partly plucked and eventually killed by +the female on the outside trying to pull it through the wire mesh. + +On several occasions, young were successfully transferred from the +mother to another lactating female in confinement, which accepted +them as part of her own litter. Young, up to the time of weaning, +appeared not to differentiate between the mother and other adult +voles. They would follow any larger individual indiscriminately, and +would huddle against it or nuzzle its undersurface searching for a +teat. + + + + +EARLY DEVELOPMENT OF YOUNG + + +The following notes are based upon many different litters, and give +some idea of the sequence of events in their early development. + +Newborn: The skin is pinkish gray dorsally and pink ventrally. In +profile, sparse and exceedingly fine hairs less than 1 mm. in length +are discernible. The vibrissae are approximately 2 mm. long. The +skin is thin and partly transparent, much wrinkled, with some deeper +folds, notably one between the knee and the heel. The young lie on +their sides making violent convulsive respiratory movements. When +not attached to the female's teats, they may make faint squeaking +sounds. + +One day old: Little changed in appearance or behavior except that +the dorsal surface has become darker because of growth of hair. + +Two days old: Covering of fine brown hair readily discernible on +dorsal surface; lower incisors protruding about .5 mm. from the gum; +upper incisors have barely pierced the gum. + +Four days old: Pale brown hair averaging about 1 mm. in length over +the dorsal surface gives the young a sleek, seallike appearance. The +young have gained greatly in muscular co-ordination. Part of the +time they may still lie on their sides, but they are able also to +gain an upright sprawling posture. In crawling, they are unsteady +and often topple over on their sides after taking a few halting +steps. They make frequent jerky lateral flexions of the body, +probably to search for a teat. Their eyes and ears still are sealed +shut. + +Five days old: Young have changed but little in appearance since the +preceding day, but they have become notably more active, with +movements better co-ordinated. When placed on a level surface they +can crawl briskly. + +Eight days old: Young are able to stand erect, with bodies held +clear of the ground, and they can even run, but the gait is slow and +clumsy, and the forequarters and hind quarters are poorly +co-ordinated, so that the voles tend to fall on their sides. The fur +averages approximately 3 mm. in length. + +Nine days old: At this stage all young have their eyes open or +beginning to open. + +Ten days old: All young of this age have their eyes open, but not to +their fullest extent, and the eyes are still slitlike in appearance. +The young have become rather gopherlike in appearance and gait. They +walk briskly but unsteadily, with bodies held high off the ground. +When handled, they struggle vigorously, and try to bite. These young +are similar in size and appearance to the smallest voles caught in +live-traps apart from their mothers. + +Thirteen days old: Hair on back has grown to an average length of 8 +mm. (shorter on ventral surface, head, and limbs). + +Seventeen days old: The young have become alert, and almost as quick +in their movements as adults. They have molariform teeth, and are +taking plant food. When a family group was examined, the young +instantly detached from the female's teats and scattered. The hair +on the back averages 10 mm. long and the vibrissae average 20 mm. +long. + +There is intense competition among the young of a litter, especially +if the litter has more than the average number of young. In litters +with more than four young, there is competition for the inguinal +teats, since, in most females at least, the pectoral teats seem to +have an inadequate milk supply. As a result, it is doubtful whether +more than four young to a litter are ever able to survive. From the +time their eyes open, the young compete actively. When litters in +confinement were fed with fresh greens, there was nearly always +quarrelsome squeaking and scuffling, as the young competed for food. +At such times, they have been seen to chase and attack each other. + + + + +GROWTH FROM WEANING TO MATURITY + + +No individual vole was recaptured with sufficient regularity, from +birth to maturity, to provide a complete growth curve. The curve in +Fig. 7 is a composite based on all available records of voles that +were recorded as making growth in length and were recaptured before +they were fully grown, so that growth rates could be computed. The +figure shows that growth is extremely rapid for the first three +weeks, and thereafter slows gradually but steadily, until in +individuals of adult size, the increment per day is much less than +that in the small young. + +Since rate of growth changes rapidly, with a slowing trend, only +those young voles that were recaptured within a few weeks showed the +approximate growth rate for any specific portion of the ontogenetic +curve. Table 5 summarizes the records of 98 such young sorted into +size groups representative of several stages in development. The +slowing trend of growth in voles that are nearing subadult size is +well shown by these records. Throughout the greater part of the +growth curve no difference could be found in rate between the sexes. +It is only after sexual maturity has been attained and growth has +become relatively slow that males become noticeably larger than +females. This tendency for continued growth in the adult males +results in a much more marked disparity in size between the sexes in +the oldest voles, as evident in Fig. 2. + + [Illustration: FIG. 2. Size distribution of prairie voles + in a year-around sample, including all the measurements of voles + taken over a three-year period. Young are not represented in + their actual ratio to the total population in this sample, + because they are less attracted to the bait, and range less + widely than adults. The higher ratios of males than of females + in the three largest size groups is well shown, as is the higher + ratio of females among those voles of small adult size.] + + + Table 5. Average Growth (in Over-all Length) in Young Voles of + Several Sizes. ([M] = Male; [F] = Female) + + ==================+==============+===========+=========================== + Average lengths | Average | Average | + in mm. at | length, | increment | Total, and + beginning and | in days, | per day | number of each + end of growth | of growth | in mm. | sex in sample + period | periods | | + ------------------+--------------+-----------+--------------------------- + 97.0 to 126.6 | in 16.8 | 1.76 | 5 (1 [M], 4 [F] [F]) + 103.3 to 127.3 | in 14.9 | 1.61 | 9 (3 [M] [M], 6 [F] [F]) + 107.5 to 123.4 | in 11.0 | 1.44 | 8 (5 [M] [M], 3 [F] [F]) + 114.0 to 132.3 | in 17.5 | 1.05 | 6 (5 [M] [M], 1 [F]) + 118.5 to 136.0 | in 19.7 | .88 | 6 (3 [M] [M], 3 [F] [F]) + 122.1 to 135.8 | in 16.2 | .85 | 15 (5 [M] [M], 10 [F] [F]) + 129.3 to 145.5 | in 22.8 | .71 | 4 (all [M] [M]) + 130.6 to 146.1 | in 19.8 | .78 | 12 (all [F] [F]) + 139.8 to 147.5 | in 29.5 | .26 | 10 (all [M] [M]) + 141.2 to 148.8 | in 26.2 | .29 | 23 (all [F] [F]) + ------------------+--------------+-----------+--------------------------- + + [Illustration: FIG. 3. Changing numbers and composition + (according to size of individual) in a population of voles on + an area of approximately one half an acre that was intensively + sampled with live-traps over periods of months. The population + as a whole and the ratio of young to adults tended to be higher + in spring and summer, but with little regularity from one year + to the next. Weather was far more important than season in + determining the population trend. Many of the voles recorded + on the half-acre area ranged more or less beyond its boundaries.] + + [Illustration: FIG. 4. Weight in free-living prairie voles + in a year-around sample from juveniles to large adults (grouped + in length-classes of 6 mm. range, separate for each sex). In each + sample mean, standard error, standard deviation, and extremes are + shown. Note that mean weight is proportional to length, that in + each size class females average heavier (because of pregnancy + in some) and have a much wider range of variation in weight.] + +Martin (1956:389) stated that growth in young prairie voles was, in +general, most rapid in the period April-May-June and least rapid in +mid-winter. However, his data were based entirely on weights. The +high incidence of pregnancy in the larger young females in spring +and early summer may have caused the trend. Measurements taken by me +of lengths do not bear out the idea of more rapid growth in the +spring and summer, but, indeed, show the opposite. In most +instances, voles of comparable sizes made significantly more rapid +growth in the colder half of the year (mid-October to mid-March) +than in the warmer half. Dividing the young voles in eight size +groups and separating each group into comparable summer and winter +samples, I found more rapid average growth in the summer sample in +only two instances. These deviations from the general trend +probably resulted from inadequately small sizes of some samples. On +the average, the growth rate in summer was 92 per cent of that in +winter. + + [Illustration: FIG. 5. Over-all length plotted against + weight in young prairie voles, from newborn to the minimum + size at breeding maturity. The range of variation increases + as development proceeds, especially after the age of weaning + is attained.] + + + + +SIZE AND AGE AT SEXUAL MATURITY + + +Greenwald (1956: 220) found that in females of _Microtus +californicus_ some individuals are extremely precocious sexually, +and might, at an age of as little as two weeks, produce corpora +lutea and have sperm in the uterus. Greenwald mentioned one +perforate female which weighed only 10 grams, but most reached a +weight of at least 30 grams before their first pregnancies. The +sterile cycles passed through earlier seemed to represent a +"tuning-up" stage before establishment of the pituitary-gonad +relationship. + + [Illustration: FIG. 6. Weight plotted against age in young + voles, from birth up to 25 days. The range is wide at the start + and increases as development proceeds.] + +Although females of _M. ochrogaster_ are much less precocious in +their manifestations of puberty, they may become perforate well +before impregnation can occur, and seem to pass through sterile +cycles before becoming pregnant. The 18 smallest females recognized +as being pregnant were of the following over-all lengths, in mm.: +149, 149, 149, 148, 148, 148, 147, 146, 145, 145, 144, 144, 143, +143, 143, 142, 135, and 134. As pregnancy is ordinarily recognized +only in the last four days the females must have been impregnated +from 20 to 17 days earlier--when they were in most instances 7 to 11 +weeks old and 135 to 145 mm. in length. The two smallest +individuals, recorded as pregnant at 135 and 134 mm., must, if they +were of typical size for their age, have become pregnant at an age +of approximately one month, when they were only 119 and 122 mm. in +length. The smallest lactating females (some of them pregnant also) +were recorded at lengths of 149, 148, 148, 147, 147, 146, 144, 144, +143, 143, and 142 mm. Occasionally females of less than 120 mm. were +found to be perforate, and seemingly had begun oestral cycles. +Records of a female of definitely known age, typical of many of the +same size in her development, are cited below: + +March 19, 1956 Born in captivity. + +April 7, 1956 (19 days old) Released on study area at site of mother's +capture; length 102 mm., weight 11.1 gms. + +April 15, 1956 (27 days old) Recaptured; perforate with a copulatory +plug; length 113 mm., weight 13.4 gms. + +April 27, 1956 (39 days old) Recaptured; imperforate; length 131 mm., +weight 24.3 gms. + +May 12, 1956 (54 days old) Recaptured; perforate and in late pregnancy; +length 146 mm. + +May 25, 1956 (67 days old) Recaptured; imperforate, in an advanced +state of lactation; length 150 mm., weight 33 gms. + + [Illustration: FIG. 7. Growth curve in the prairie vole; + dots are based on means of series of definitely known age (born + in captivity); circles are based on mean lengths of recaptured + marked young whose ages were not precisely known.] + + [Illustration: FIG. 8. Over-all length in young prairie + voles of definitely known ages, up to 40 days. All were born + in captivity. Some were released with the female and developed + under natural conditions, but their growth rate did not differ + discernibly from that of those kept in the laboratory. Dots + indicate individual records; circles are means for ages at + which four or more records were obtained.] + +When captured on May 12, at an age of 54 days, this female appeared +to be within two or three days of parturition, and hence must have +become pregnant at an age of approximately 35 or 36 days. Pregnancy +in the more precocious females probably occurs at a length of +approximately 130 mm. and an age of a little less than 40 days. Such +females are still growing so rapidly that by the time their litters +are born, they have grown to more than 140 mm. + + + + +GROWTH IN SUBADULTS AND ADULTS + + +Table 6 is a summarization of 73 records of individuals that made +substantial growth as adults, after they were marked and measured. +These records show the slowing trend of growth with advanced age. +Also, they show the wide range of individual variation in growth +rate, and difference between the sexes. With advanced age, growth in +females lags behind that in males to an increasing extent. +Exceptionally large individuals, of either sex, are many months old, +but some individuals live to be a year old or more without growing +much beyond average adult size. The average growth rate of more than +1 mm. per day in young has slowed to less than .1 mm. per day, on +the average, in adults exceeding 160 mm., and has slowed to less +than .05 mm. per day, on the average, in those exceeding 165 mm. + + Table 6. Size Groups (Over-all Length) in Recaptured Voles That + Were Marked Before Maturity and Therefore Were of Approximately + Known Ages. ([M] = Male; [F] = Female) + + ===============+======================================+============ + | Estimated age, in days | + Size Group +------------+------------+------------+ Number + Length in mm. | Average | Maximum | Minimum | in sample + ---------------+------------+------------+------------+------------ + 171 to 175 | [M] 435 | ..... | ..... | 1 + | [F] 324 | 338 | 310 | 2 + | All 361 | 435 | 310 | 3 + | | | | + 166 to 170 | [M] 304 | 523 | 179 | 9 + | [F] 398 | 597 | 158 | 6 + | All 346 | 597 | 158 | 15 + | | | | + 161 to 165 | [M] 227 | 465 | 104 | 15 + | [F] 257 | 394 | 134 | 18 + | All 243 | 465 | 104 | 33 + | | | | + 156 to 160 | [M] 188 | 349 | 107 | 12 + | [F] 187 | 284 | 93 | 11 + | All 188 | 349 | 93 | 23 + ---------------+------------+------------+------------+------------ + + + + +SUMMARY + + +The prairie vole is non-territorial and somewhat social. Several or +many individuals of both sexes and various sizes may use the same +system of surface runways and burrows and even the same nest. In +general, members of such a group are mutually tolerant. A strange +vole may provoke some hostility at first, but may soon be accepted +as a member of a new group. Consequently, there are frequent shifts +from one home base to another. Sexual relations are probably more or +less promiscuous, although a male and female may rest and travel +together in a semi-permanent association. In confinement only those +males having markedly enlarged scrotal testes showed interest in +females that were in oestrus. Post-partum females especially were +eagerly pursued by such males. Anoestrus females are imperforate, +and a vaginal orifice is present only during an active oestral cycle +or in pregnancy. The perforate condition therefore, is a crude index +of breeding activity in the population. In adult females the ratio +of those that were perforate usually fluctuated between one-fourth +and three-fourths of the total. Only in severe summer drought did +the numbers decline below 24 per cent. Normally, breeding continues +the year around, but it is temporarily inhibited in unusually cold +weather or drought. The highest incidence of pregnancy normally is +in late spring and early summer. The ratio of juveniles in the +population from month to month and year to year is far more stable +than the actual population density. + +Gestation is 21 days or a little less. The mean litter is 3.37 ++- .075 young. Three is the most frequent number per litter, with +four, two, and five in that order of frequency. Larger and older +females have more young per litter, on the average. Average size +is greater in those litters having fewer young. At birth, young +are between 40 and 50 mm. in length (typically, 47 mm.), and weigh +2.9 +- .05 grams. + +At an age of nine days the young have their eyes open, and they may +be weaned at an age of approximately three weeks. Young suckle +chiefly from the four abdominal teats. The pectoral mammae seem to +be inadequately developed, with the result that in exceptionally +large litters of five, six or seven young, usually no more than four +survive. Until weaning the young spend much of their time attached +to the female's teats. She may even drag them behind as she forages. +Females that have suckling young become much less tolerant of other +voles. Attacks on young, and cannibalism, are common. Adult males, +especially, are liable to eat the newborn young. The acquisition of +cannibalistic habits by individuals, and seasonal lack of adequately +nutritious plant foods may result in the killing off of young in +such numbers that the population level is held down. + +In young females sterile oestral cycles often begin at about the +time of weaning. Earliest pregnancies occur when females are +approximately one month old, but most are several weeks older before +they become pregnant. Rate of growth declines steadily from a length +increment of approximately 2 mm. per day in voles less than two +weeks old to an increment of approximately one-fourth mm. per day in +subadults. Growth rate is highly variable among individuals at all +stages, and especially in those that have attained adult size. Even +adults tend to gain in length, slowly, as well as in weight, and the +largest individuals are all many months old. + + + + +LITERATURE CITED + + +BAILEY, V. + + 1924. Breeding, feeding and other life habits of meadow mice. + Jour. Agric. Res., 27: 523-536. + + +BODENHEIMER, F. S., and F. SULMAN. + + 1946. The estrous cycle of _Microtus guentheri_ D. and A. and + its ecological implications. Ecol., 27: 255-256. + + +GREENWALD, G. S. + + 1956. The reproductive cycle of the field mouse, _Microtus + californicus_. Jour. Mamm., 37: 213-222, 2 figs., 1 pl. + + +_Hamilton, W. J., Jr._ + + 1941. The reproduction of the field mouse, _Microtus + pennsylvanicus_. Cornell Univ. Agric. Exp. Sta. Mem., + 237: 1-23. + + +HATFIELD, D. M. + + 1935. A natural history of _Microtus californicus_. Jour. Mamm., + 16: 261-271. + + +HOYTE, H. M. D. + + 1955. Observations on some small mammals of Arctic Norway. Jour. + Animal Ecology, 24: 412-425. + + +JAMESON, E. W. + + 1947. Natural history of the prairie vole. Univ. Kansas Mus. + Nat. Hist. Publ., 1: 125-151. + + +MARTIN, E. P. + + 1956. A population study of the prairie vole (_Microtus + ochrogaster_) in northeastern Kansas. Univ. Kansas Mus. + Nat. Hist. Publ., 8: 361-416. + + +SCHMIDT, F. J. W. + + 1931. Mammals of western Clark County, Wisconsin. Jour. Mamm., + 12: 99-117. + + + + + + [] + 26-7561 + + + + +UNIVERSITY OF KANSAS PUBLICATIONS +MUSEUM OF NATURAL HISTORY + + +Institutional libraries interested in publications exchange may +obtain this series by addressing the Exchange Librarian, University +of Kansas Library, Lawrence, Kansas. Copies for individuals, persons +working in a particular field of study, may be obtained by +addressing instead the Museum of Natural History, University of +Kansas, Lawrence, Kansas. There is no provision for sale of this +series by the University Library which meets institutional requests, +or by the Museum of Natural History which meets the requests of +individuals. However, when individuals request copies from the +Museum, 25 cents should be included, for each separate number that +is 100 pages or more in length, for the purpose of defraying the +costs of wrapping and mailing. + + * An asterisk designates those numbers of which the Museum's + supply (not the Library's supply) is exhausted. Numbers + published to date, in this series, are as follows: + + Vol. 1. Nos. 1-26 and index. Pp. 1-638, 1946-1950. + + *Vol. 2. (Complete) Mammals of Washington. By Walter W. Dalquest. + Pp. 1-444, 140 figures in text. April 9, 1948. + + Vol. 3. *1. The avifauna of Micronesia, its origin, evolution, + and distribution. By Rollin H. Baker. Pp. 1-359, + 16 figures in text. June 12, 1951. + + *2. A quantitative study of the nocturnal migration of + birds. By George H. Lowery, Jr. Pp. 361-472, 47 + figures in text. June 29, 1951. + + 3. Phylogeny of the waxwings and allied birds. By M. Dale + Arvey. Pp. 473-530, 49 figures in text, 13 tables. + October 10, 1951. + + 4. Birds from the state of Veracruz, Mexico. By George H. + Lowery, Jr. and Walter W. Dalquest. Pp. 531-649, + 7 figures in text, 2 tables. October 10, 1951. + + Index. Pp. 651-681. + + *Vol. 4. (Complete) American weasels. By E. Raymond Hall. Pp. 1-466, + 41 plates, 31 figures in text. December 27, 1951. + + Vol. 5. 1. Preliminary survey of a Paleocene faunule from the + Angels Peak area, New Mexico. By Robert W. Wilson. + Pp. 1-11, 1 figure in text. February 24, 1951. + + 2. Two new moles (Genus Scalopus) from Mexico and Texas. + By Rollin H. Baker. Pp. 17-24. February 28, 1951. + + 3. Two new pocket gophers from Wyoming and Colorado. By + E. Raymond Hall and H. Gordon Montague. Pp. 25-32. + February 28, 1951. + + 4. Mammals obtained by Dr. Curt von Wedel from the barrier + beach of Tamaulipas, Mexico. By E. Raymond Hall. Pp. + 33-47, 1 figure in text. October 1, 1951. + + 5. Comments on the taxonomy and geographic distribution of + some North American rabbits. By E. Raymond Hall and Keith + R. Kelson. Pp. 49-58, October 1, 1951. + + 6. Two new subspecies of Thomomys bottae from New Mexico + and Colorado. By Keith R. Kelson. Pp. 59-71, 1 figure in + text. October 1, 1951. + + 7. A new subspecies of Microtus montanus from Montana and + comments on Microtus canicaudus Miller. By E. Raymond + Hall and Keith R. Kelson. Pp. 73-79. October 1, 1951. + + 8. A new pocket gopher (Genus Thomomys) from eastern + Colorado. By E. Raymond Hall. Pp. 81-85. October 1, 1951. + + 9. Mammals taken along the Alaskan Highway. By Rollin H. + Baker. Pp. 87-117, 1 figure in text. November 28, 1951. + + *10. A synopsis of the North American Lagomorpha. By E. + Raymond Hall. Pp. 119-202, 68 figures in text. December + 15, 1951. + + 11. A new pocket mouse (Genus Perognathus) from Kansas. By E. + Lendell Cockrum. Pp. 203-206. December 15, 1951. + + 12. Mammals from Tamaulipas, Mexico. By Rollin H. Baker. Pp. + 207-218. December 15, 1951. + + 13. A new pocket gopher (Genus Thomomys) from Wyoming and + Colorado. By E. Raymond Hall. Pp. 219-222. December 15, + 1951. + + 14. A new name for the Mexican red bat. By E. Raymond Hall. + Pp. 223-226. December 15, 1951. + + 15. Taxonomic notes on Mexican bats of the Genus Rhogeessa. + By E. Raymond Hall. Pp. 227-232. April 10, 1952. + + 16. Comments on the taxonomy and geographic distribution of + some North American woodrats (Genus Neotoma). By Keith R. + Kelson. Pp. 233-242. April 10, 1952. + + 17. The subspecies of the Mexican red-bellied squirrel, + Sciurus aureogaster. By Keith R. Kelson. Pp. 243-250, + 1 figure in text. April 10, 1952. + + 18. Geographic range of Peromyscus melanophrys, with + description of new subspecies. By Rollin H. Baker. + Pp. 251-258, 1 figure in text. May 10, 1952. + + + 19. A new chipmunk (Genus Eutamias) from the Black Hills. + By John A. White. Pp. 259-262. April 10, 1952. + + 20. A new pinon mouse (Peromyscus truei) from Durango, Mexico. + By Robert B. Finley, Jr. Pp. 263-267. May 23, 1952. + + 21. An annotated checklist of Nebraskan bats. By Olin L. Webb + and J. Knox Jones, Jr. Pp. 269-279. May 31, 1952. + + 22. Geographic variation in red-backed mice (Genus + Clethrionomys) of the southern Rocky Mountain region. By + E. Lendell Cockrum and Kenneth L. Fitch. Pp. 281-292, + 1 figure in text. November 15, 1952. + + 23. Comments on the taxonomy and geographic distribution of + North American microtines. By E. Raymond Hall and E. + Lendell Cockrum. Pp. 293-312. November 17, 1952. + + 24. The subspecific status of two Central American sloths. By + E. Raymond Hall and Keith R. Kelson. Pp. 313-337. November + 21, 1952. + + 25. Comments on the taxonomy and geographic distribution of + some North American marsupials, insectivores, and + carnivores. By E. Raymond Hall and Keith R. Kelson. Pp. + 319-341. December 5, 1952. + + 26. Comments on the taxonomy and geographic distribution of + some North American rodents. By E. Raymond Hall and Keith + R. Kelson. Pp. 343-371. December 15, 1952. + + 27. A synopsis of the North American microtine rodents. By E. + Raymond Hall and E. Lendell Cockrum. Pp. 373-498, 149 + figures in text. January 15, 1953. + + 28. The pocket gophers (Genus Thomomys) of Coahuila, Mexico. + By Rollin H. Baker. Pp. 499-514, 1 figure in text. + June 1, 1953. + + 29. Geographic distribution of the pocket mouse, Perognathus + fasciatus. By J. Knox Jones, Jr. Pp. 515-526, 7 figures + in text. August 1, 1953. + + 30. A new subspecies of wood rat (Neotoma mexicana) from + Colorado. By Robert B. Finley, Jr. Pp. 527-534, 2 figures + in text. August 15, 1953. + + 31. Four new pocket gophers of the genus Cratogeomys from + Jalisco, Mexico. By Robert J. Russell. Pp. 535-542. + October 15, 1953. + + 32. Genera and subgenera of chipmunks. By John A. White. + Pp. 543-561, 12 figures in text. December 1, 1953. + + 33. Taxonomy of the chipmunks, Eutamias quadrivittatus and + Eutamias umbrinus. By John A. White. Pp. 563-582, + 6 figures in text. December 1, 1953. + + 34. Geographic distribution and taxonomy of the chipmunks of + Wyoming. By John A. White. Pp. 584-610, 3 figures in + text. December 1, 1953. + + 35. The baculum of the chipmunks of western North America. + By John A. White. Pp. 611-631, 19 figures in text. + December 1, 1953. + + 36. Pleistocene Soricidae from San Josecito Cave, Nuevo Leon, + Mexico. By James S. Findley. Pp. 633-639. December 1, 1953. + + 37. Seventeen species of bats recorded from Barro Colorado + Island, Panama Canal Zone. By E. Raymond Hall and William + B. Jackson. Pp. 641-646. December 1, 1953. + + Index. Pp. 647-676. + + *Vol. 6. (Complete) Mammals of Utah, _taxonomy and distribution_. By + Stephen D. Durrant. Pp. 1-549, 91 figures in text, 30 tables. + August 10, 1952. + + Vol. 7. *1. Mammals of Kansas. By E. Lendell Cockrum. Pp. 1-303, + 73 figures in text, 37 tables. August 25, 1952. + + 2. Ecology of the opossum on a natural area in northeastern + Kansas. By Henry S. Fitch and Lewis L. Sandidge. Pp. + 305-338, 5 figures in text. August 24, 1953. + + 3. The silky pocket mice (Perognathus flavus) of Mexico. By + Rollin H. Baker. Pp. 339-347, 1 figure in text. February + 15, 1954. + + 4. North American jumping mice (Genus Zapus). By Philip H. + Krutzsch. Pp. 349-472, 47 figures in text, 4 tables. + April 21, 1954. + + 5. Mammals from Southeastern Alaska. By Rollin H. Baker and + James S. Findley. Pp. 473-477. April 21, 1954. + + 6. Distribution of Some Nebraskan Mammals. By J. Knox Jones, + Jr. Pp. 479-487. April 21, 1954. + + 7. Subspeciation in the montane meadow mouse, Microtus + montanus, in Wyoming and Colorado. By Sydney Anderson. + Pp. 489-506, 2 figures in text. July 23, 1954. + + 8. A new subspecies of bat (Myotis velifer) from southeastern + California and Arizona. By Terry A. Vaughn. Pp. 507-512. + July 23, 1954. + + 9. Mammals of the San Gabriel mountains of California. By + Terry A. Vaughn. Pp. 513-582, 1 figure in text, 12 tables. + November 15, 1954. + + 10. A new bat (Genus Pipistrellus) from northeastern Mexico. + By Rollin H. Baker. Pp. 583-586. November 15, 1954. + + 11. A new subspecies of pocket mouse from Kansas. By E. + Raymond Hall. Pp. 587-590. November 15, 1954. + + 12. Geographic variation in the pocket gopher, Cratogeomys + castanops, in Coahuila, Mexico. By Robert J. Russell and + Rollin H. Baker. Pp. 591-608. March 15, 1955. + + 13. A new cottontail (Sylvilagus floridanus) from northeastern + Mexico. By Rollin H. Baker. Pp. 609-612. April 8, 1955. + + 14. Taxonomy and distribution of some American shrews. + By James S. Findley. Pp. 613-618. June 10, 1955. + + 15. The pigmy woodrat, Neotoma goldmani, its distribution and + systematic position. By Dennis G. Rainey and Rollin H. + Baker. Pp. 619-624, 2 figs. in text. June 10, 1955. + + Index. Pp. 625-651. + + Vol. 8. 1. Life history and ecology of the five-lined skink, Eumeces + fasciatus. By Henry S. Fitch. Pp. 1-156, 26 figs. in text. + September 1, 1954. + + 2. Myology and serology of the Avian Family Fringillidae, a + taxonomic study. By William B. Stallcup. Pp. 157-211, 23 + figures in text, 4 tables. November 15, 1954. + + 3. An ecological study of the collared lizard (Crotaphytus + collaris). By Henry S. Fitch. Pp. 213-274, 10 figures in + text. February 10, 1956. + + 4. A field study of the Kansas ant-eating frog, Gastrophryne + olivacea. By Henry S. Fitch. Pp. 275-306, 9 figures in + text. February 10, 1956. + + 5. Check-list of the birds of Kansas. By Harrison B. Tordoff. + Pp. 307-359, 1 figure in text. March 10, 1956. + + 6. A population study of the prairie vole (Microtus + ochrogaster) in northeastern Kansas. By Edwin P. Martin. + Pp. 361-416, 19 figures in text. April 2, 1956. + + 7. Temperature responses in free-living amphibians and reptiles + of northeastern Kansas. By Henry S. Fitch. Pp. 417-476, 10 + figures in text, 6 tables. June 1, 1956. + + 8. Food of the crow, Corvus brachyrhynchos Brehm, in + south-central Kansas. By Dwight Platt. Pp. 477-498, + 4 tables. June 8, 1956. + + 9. Ecological observations on the woodrat, Neotoma floridana. + By Henry S. Fitch and Dennis G. Rainey. Pp. 499-533, + 3 figures in text. June 12, 1956. + + 10. Eastern woodrat, Neotoma floridana: Life history and + ecology. By Dennis G. Rainey. Pp. 535-646, 12 plates, + 13 figures in text. August 15, 1956. + + Index. Pp. 647-675. + + Vol. 9. 1. Speciation of the wandering shrew. By James S. Findley. + Pp. 1-68, 18 figures in text. December 10, 1955. + + 2. Additional records and extensions of ranges of mammals from + Utah. By Stephen D. Durrant, M. Raymond Lee, and Richard M. + Hansen. Pp. 69-80. December 10, 1955. + + 3. A new long-eared myotis (Myotis evotis) from northeastern + Mexico. By Rollin H. Baker and Howard J. Stains. Pp. 81-84. + December 10, 1955. + + 4. Subspeciation in the meadow mouse, Microtus pennsylvanicus, + in Wyoming. By Sydney Anderson. Pp. 85-104, 2 figures in + text. May 10, 1956. + + 5. The condylarth genus Ellipsodon. By Robert W. Wilson. + Pp. 105-116, 6 figures in text. May 19, 1956. + + 6. Additional remains of the multituberculate genus + Eucosmodon. By Robert W. Wilson. Pp. 117-123, 10 figures + in text. May 19, 1956. + + 7. Mammals of Coahuila, Mexico. By Rollin H. Baker. Pp. + 125-335, 75 figures in text. June 15, 1956. + + 8. Comments on the taxonomic status of Apodemus peninsulae, + with description of a new subspecies from North China. + By J. Knox Jones, Jr. Pp. 337-346, 1 figure in text, + 1 table. August 15, 1956. + + 9. Extensions of known ranges of Mexican bats. By Sydney + Anderson, Pp. 347-351. August 15, 1956. + + 10. A new bat (Genus Leptonycteris) from Coahuila. By Howard + J. Stains. Pp. 353-356. January 21, 1957. + + 11. A new species of pocket gopher (Genus Pappogeomys) from + Jalisco, Mexico. By Robert J. Russell. Pp. 357-361. + January 21, 1957. + + More numbers will appear in volume 9. + + Vol. 10. 1. Studies of birds killed in nocturnal migration. By + Harrison B. Tordoff and Robert M. Mengel. Pp. 1-44, 6 + figures in text, 2 tables. September 12, 1956. + + 2. Comparative breeding behavior of Ammospiza caudacuta and + A. maritima. By Glen E. Woolfenden. Pp. 45-75, 6 plates, + 1 figure. December 20, 1956. + + 3. The forest habitat of the University of Kansas Natural + History Reservation. By Henry S. Fitch and Ronald R. + McGregor. Pp. 77-127, 2 plates, 7 figures in text, + 4 tables. December 31, 1956. + + 4. Aspects of reproduction and development in the prairie vole + (Microtus ochrogaster). By Henry S. Fitch. Pp. 129-161, + 8 figures in text, 6 tables. December 19, 1957. + + More numbers will appear in volume 10. + + + + +Transcriber's Notes + +With the exception of the list of typographical corrections below and +minor corrections not noted here, the text presented here is the same +as in the original printed version. Tables 1 and 2 were rotated so that +the width would be less than the limit of 75 characters wide. In the +tables, the title and some column headers were printed in small caps. +Those are usually displayed as ALL CAPS; but for ease of reading, they +were left as mixed case. The list of UKMNH publications was compiled +at the end of the article's text. + + +Typographical Corrections + + Page(s) Correction + ============ ======================================= + 129, 130, ii This publication: 4 tables => 6 tables + 138 cyle => cycle + +Text Emphasis + + _Text_ - Italic + + + + + +End of the Project Gutenberg EBook of Aspects of Reproduction and +Development in the Prairie Vole (Microtus ochrogaster), by Henry S. 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