Part 1 (2/2)

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.

s.e.xUAL BEHAVIOR

There is s.e.xual 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 s.e.xual maturity later than females. It seems that ordinarily the availability of s.e.xually 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 s.e.xual activity tended to be directly proportional to the size of the testes. Occasional individuals, having much enlarged scrotal testes were more readily stimulated to s.e.xual 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 gra.s.s provided succulent and nutritious food.

In confinement s.e.xual activity was largely inhibited and attempts to establish a laboratory colony met with failure. s.e.xual 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 gra.s.s 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 s.e.xual 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 s.e.xually, a rebuffed male might be readily accepted as a nest-mate, huddling along with the female and perhaps other individuals of both s.e.xes.

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 s.e.xes performed this grooming and the recipient might be of either the same s.e.x or the opposite s.e.x. 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 s.e.xual behavior, somewhat similar actions const.i.tute part of the mating pattern. A s.e.xually 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 s.e.xually quiescent, both those that have not yet attained breeding maturity, and those that have undergone regression after attainment of s.e.xual maturity, the v.a.g.i.n.al orifice is not evident. The ca.n.a.l is sealed externally by a membranous layer of epithelium. Presence of a v.a.g.i.n.al 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 v.a.g.i.n.al 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 s.e.xually 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 v.a.g.i.n.al 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. v.a.g.i.n.al 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 v.a.g.i.n.a can be seen. After parturition, placentae and b.l.o.o.d.y discharge often are in evidence in the v.a.g.i.n.al ca.n.a.l. Females that have not given birth to young recently may also have b.l.o.o.d.y mucous discharge. Its significance has not been determined. In females that are undergoing s.e.xual regression, the margins of the v.a.g.i.n.al 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 v.a.g.i.n.al 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 gra.s.s 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 v.a.g.i.n.al 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.

[Ill.u.s.tration: 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 relations.h.i.+p. 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.

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