Henry S. Fitch

“ 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 ”

“ 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. ”

“ 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.
[Pg 149] 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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 [Pg 136] large old males may sire a disproportionately large percentage of the litters produced. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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. ”

“ 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.
[Pg 152] ”