Female and male mammals have different behavioral strategies for maximizing their reproductive success. Pregnancy and lactation obligate female mammals to provide greater parental investment than males; thus, females compete with each other for food and space to rear their offspring, while male mammals compete with each other for female mates. Therefore, natural selection should favor any behavior among females that increases their access to food and space to rear successful offspring and any behavior among males that increases their access to females. In two enclosure experiments, the gray-tailed vole, Microtus canicaudus, was used as a model species to study (1) the relative influence of male competition versus access to females on space use by males and (2) facultative sex-ratio adjustment at the population level in response to low and high population densities, skewed adult sex ratios, and season. Home-range sizes of male voles did not appear to expand beyond an overlap with about five members of either sex. Intrasexual competition with 3-4 males and/or overlap with five females appear to set the upper limits to home-range size. Space use by males is influenced by intrasexual competition and by access to females with an upper limit of overlap with either sex. Additionally, facultative sex-ratio adjustment in response to changing population densities and skewing adult sex ratios did not occur, but a seasonal effect was detected with significantly more males produced in autumn than in spring. Under the conditions of this experimental study, I was not able to measure sex ratios of individual litters, but if any sex-ratio adjustment occurred in response to densities and adult sex ratios, it was not detectable at the population level. / Graduation date: 1999

The 3-5 year cyclical fluctuations in populations of many vole and lemming species have perplexed ecologists for many years. Numerous hypotheses have been proposed to explain microtine rodent cycles, including various aspects of social behavior. Microtine rodents commonly form kin groups composed of related females. Charnov and Finerty (1980) proposed that the formation and breakup of kin groups could, in part, explain the rates of population increase and decline associated with cycles. My experiment sought to determine if kin groups provided population-level benefits in gray-tailed voles, Microtus canicaudus. I compared unmanipulated populations with populations in which kin-structuring was experimentally disrupted to determine if kin groups affected population growth rates and size, reproduction, pregnancy and lactation rates, and recruitment, movement and survival of juveniles. I monitored demography and reproductive behavior in eight 0.2 ha experimental enclosures during a summer breeding season. I found no differences in demographic or female reproductive parameters between control and treatment enclosures, with the exception of a delayed time to first pregnancy for females introduced into the treatment enclosures. In addition, I found no differences in the time to sexual maturation or dispersal movements of juvenile males between control and treatment enclosures. I conclude that disrupting the formation of kin groups does not adversely affect demographic or reproductive parameters at the population-level in gray-tailed voles, and suggest that the contribution of kin groups to social behaviors that may affect population regulation is probably quite small. / Graduation date: 1998

Numerous laboratory studies with at least 12 species of rodents have reported that exposure of females to strange males results in pregnancy disruption or infanticide. The proximate causes and ultimate benefits of these behaviors have been proposed from an evolutionary perspective. To determine if exposure to strange males or females caused pregnancy disruption and (or) infanticide in a resident gray-tailed vole (Microtus canicaudus) population, pregnancy rate and juvenile recruitment were monitored in populations of 12 female and 12 male voles following introduction of unfamiliar adults. These experiments were conducted in 12 0.2 ha enclosures using three treatments and a control. Every 10 days 12 males, six males, or six females were removed and replaced in the three treatments, respectively, or the populations were left unmanipulated in the control (3 replicates/treatment). The time to first parturition, time between parturitions, number of juveniles recruited/parturition, and percent of births followed by lactation did not vary among the controls and three treatments. The only observable effects of treatment were a slight non-significant delay in time to first birth in the 12-male treatment and a slightly significant difference in the number of pregnancies per female. These results do not support previous laboratory studies indicating that exposure to strangers causes pregnancy disruption and (or) infanticide at high rates. Therefore, in field conditions, little evidence was found indicating that female gray-tailed voles' reproductive fitness declines after exposure to strangers. I propose that results from laboratory studies on behavioral aspects of mammals should be validated with field data prior to being extrapolated to natural populations and applied to evolutionary paradigms. / Graduation date: 1997

In an experimental field study, populations of gray-tailed voles (Microtus canicaudus) were exposed to a commercial formulation of vinclozolin, a fungicide effective for disease control on ornamental plants, turf grasses, and fruits and vegetables. Vinclozolin has been shown in laboratory experiments to behave as an androgen antagonist, impairing the reproductive development in males of several species of mammals when exposed in utero. However, when applied to grassland habitat containing populations of gray-tailed voles, no biologically significant impairment was observed in reproductive development of male voles whose mothers were exposed to the treatment while the young were in utero. Reproductive organs were sufficiently developed to result in high reproductive rates and juvenile recruitment in the field with no effects on population growth or demography. Under the conditions of this study, one standard application of Curalan[copyright] fungicide had no measurable reproductive or demographic consequences on gray-tailed voles. However, the results suggest that higher application rates or several successive applications may have negative impacts on reproductive development and demography of wild vole populations. / Graduation date: 1999

Considerable progress has been made in determining the factors in the environment which are responsible for the control of seasonal breeding in mammals and birds. There is less information available about how these factors produce their physiological effects. Light has been shown to be very important in the control of vole breeding seasons. Temperature may also have some effect. How these factors act is not known. As part of an attempt to understand how the breeding season of the vole is controlled, an investigation of some of its endocrine organs has been undertaken. Animals have been collected from the field in mid-summer (at the height of the breeding season) and in mid-winter (during the non-breeding period). A total of 174 adult field animals has been studied. As an approximate indication of the reproductive and endocrinological state of the animals, measurements have been made of the weights of their bodies, adrenals, thyroids, gonads and also certain accessory organs (the uteri of females in winter and the seminal vesicles and ventral prostates of males in both summer and winter). From the examination of this data it is clear that there are considerable endocrine changes with season. Furthermore, within both winter and summer populations reproductive sub-groups which show endocrine differences can be recognized. More detailed study of these endocrine differences has involved the use of cytological and histological methods. Because of the trophic control which the adenohypophysis exercises over the other endocrine organs, it may be presumed to be important in bringing about seasonal changes. It has, therefore, received particular study. The adenohypophyses of field animals have been examined by the methods of cytology, histochemistry and bioassay. In order to identify the cells responsible for the production of gonadotrophin and thyrotrophin, the pituitary function of laboratory bred animals has been altered by castration and the administration of goitrogens. In the adenohypophysis of the vole five cell types have been recognized. Oval basophils which are periodic acid-Schiff (PAS) positive and aldehyde fuchsin (AF) negative. They react to castration by degranulation and the subsequent formation of colloid filled castration cells. They are considered to secrete gonadotrophins. In field animals it has been found that the gonadotrophin producing cells (gonadotrophs) of winter animals are markedly different from those of summer animals. The gonadotrophin content of the pituitaries of male voles in winter and in summer has been studied by means of bioassay. In the summer the vole pituitary contains well granulated gonadotrophs and gonadotrophic hormone can be detected. In winter the gonadotrophs are vesiculated and no gonadotrophic hormone can be detected. This indicates that the granulated gonadotrophs contain hormone, but that the vesiculated cells are depleted of active gonadotrophic principles. Angular basophils which are PAS positive and AF positive. They react to the administration of goitrogen by the formation of colloid filled thyroidectomy cells. They are considered to secrete thyrotrophin. They show no marked changes with season, as would be anticipated from the lack of marked change in the thyroid itself. Round acidophils which show no marked change with season. This study provides no indication as to their function. A second type of acidophil which is numerous and well developed only in females which are pregnant or show signs of mammary development. It is suggested that they may be the source of prolactin. The adenohypophysis also contains a few large cells with poor staining qualities. They do not show any marked changes with season or in response to either castration or the administration of goitrogens. Their function is not known. The gonads were, in general, found to exhibit the expected seasonal change in activity. Sexually inactive males in winter may be divided into two groups on the basis of the structure of the tunica albuginea. It is suggested that these two groups represent, respectively, regressed males, which were sexually active in the preceding summer, and inhibited males, which were born late in the season and have never been sexually active. Similarly, on the basis of uterine weight, female voles in winter can be divided into two groups, parous and non-parous. The pituitary cytology of regressed and inhibited males, parous and non-parous females in winter is similar. In one winter collection the males show considerable evidence of being sexually active. This suggests the operation of a factor or factors other than light and temperature in the control of vole breeding seasons. These males were distinguished from the males in other winters by differences in pituitary cytology. The study of the pituitary suggests that gonadal changes at the end of the breeding season are secondary to changes in the pituitary. The alternative possibility, that the gonads are not competent to respond to pituitary hormones, was tested experimentally. Commercial gonadotrophins were injected into winter field animals and into laboratory bred animals whose sexual development had been inhibited by maintaining them on short days in the cold. The results suggest that the gonads of such animals are able to respond to gonadotrophins. The adrenal has been found to undergo marked changes in weight with season. In summer there is also a sex difference in adrenal weight. These weight changes can be correlated with striking histological differences in the inner regions of the adrenal cortex. In winter voles the adrenal cortex possesses a juxtamedullary zone. It is similar in cytological appearance to the X zone of mice and the two zones are seemingly homologous. The zone is present in all winter field voles, whether regressed or inhibited males, parous or non-parous females. The zone is small or absent in the adrenals of sexually active males. It must, therefore, be formed secondarily in the adrenals of regressed males. It also reappears in a similar, though not identical, form in males after castration. The zone can also disappear from females, but is present in an especially well developed form in all pregnant and lactating animals. Multipara and primipara differ in the details of the structure of the juxtamedullary zone. These changes are clearly related to sexual activity, but their significance is not known. The epithelium lining the ventral prostate is the site of marked stimulation in winter field males and in castrated laboratory animals. It is possible that the adrenal is the source of the hormone responsible for this stimulation. The zona glomerulosa and zona fasciculata of the adrenal cortex also show cytological and histological change with season. Their structure in the winter males which may have been sexually active suggests that these changes are not primarily related to sexual activity. The principal conclusions which can be drawn from this study are: That in the vole there is functional differentiation of adenohypophyseal cells. Two cell, types have been recognized which are clearly related to the production and secretion of gonadotrophic and thyrotrophic hormones, respectively. A third cell type is probably the source of prolactin. There is no evidence available on the function of the other two cell types which have been recognized. That the cessation of breeding in winter in the vole is apparently brought about by a cessation of both the synthesis and the secretion of gonadotrophin. That there are striking changes in the juxtamedullary region of the vole adrenal which appear to be related to sexual activity.

Apple drops were removed from a portion of an apple orchard in September and apples left available in the remainder of the orchard. Presence of apples had little effect on the immature:adult ratio or on body weight. However, adult males and juveniles of both sexes had higher values for reproductive characteristics in November and December in the area with apples available. In a laboratory experiment adult voles fed a rabbit chow diet supplemented with apple consumed less digestible energy but had higher body weights and more body fat, and males had heavier testes than those fed only chow. Adults on a 14L:10D photoperiod had higher values for reproductive characteristics than those on a declining photoperiod. In a third experiment juveniles fed a horse chow diet supplemented with apples had higher body weights and more body fat, initiated and completed molt to adult pelage at higher body weights and had heavier reproductive organs than those given only chow. In these experiments voles with access to apples dia not consume more digestible energy but they grew faster, were in better condition and were more likely to produce offspring. The consumption of apples possibly stimulated the hypothalamus anu;or pituitary gland, increasing synthesis or release of gonadotropins. Adult pine voles on a declining or short photoperiod may be inhibited reproductively because of antigonadotropic substances produced by the pineal gland. Consuming apples could affect the degree of inhibition. It was hypothesized pine voles use both photoperiod and nutrition as cues fox reproduction. / Master of Science