Nancy G. Solomon

A Reexamination of Factors Influencing Philopatry in Rodents

Abstract The costs and benefits of philopatry (remaining at the natal nest) have been discussed in numerous papers. Nevertheless, there is still debate about the relative importance of factors that favor philopatry, which can result in the formation of social groups. The decision to remain at the natal nest can be examined by using models such as the delayed-dispersal threshold model, which takes into account risks of dispersal, probability of becoming established on a suitable territory, and probability of finding a mate. These factors, in turn, are influenced by ecological variables such as distribution of critical resources and population density. The often-cited conclusion from this and similar models is that ecological or social constraints promote philopatry whereas relaxation of the constraints result in dispersal. More recent theoretical approaches have included not only some of these ecological factors but also life-history traits (e.g., survival and age of maturation). Some of the latter models suggest that external constraints alone are inadequate to explain interspecific variation in group living. I review existing data to evaluate the relative importance of variables proposed to influence philopatry in rodents and argue that future studies may benefit from a broad approach that encompasses life-history and ecological factors, such as adult survival and territory quality.

An evolutionary framework for studying mechanisms of social behavior

Social interactions are central to most animals and have a fundamental impact upon the phenotype of an individual. Social behavior (social interactions among conspecifics) represents a central challenge to the integration of the functional and mechanistic bases of complex behavior. Traditionally, studies of proximate and ultimate elements of social behavior have been conducted by distinct groups of researchers, with little communication across perceived disciplinary boundaries. However, recent technological advances, coupled with increased recognition of the substantial variation in mechanisms underlying social interactions, should compel investigators from divergent disciplines to pursue more integrative analyses of social behavior. We propose an integrative conceptual framework intended to guide researchers towards a comprehensive understanding of the evolution and maintenance of mechanisms governing variation in sociality.

EFFECTS OF FOOD SUPPLEMENTATION ON THE SOCIAL ORGANIZATION OF PRAIRIE VOLES ( MICROTUS OCHROGASTER )

Abstract Mammalian social organization can vary over ecological time. We experimentally manipulated food resources in enclosed populations of prairie voles (Microtus ochrogaster) to test the hypothesis that food quality influences the potential for group formation. During each field season, populations were started by releasing 5 pairs of prairie voles into each of 8 0.1-ha enclosures. Populations were monitored for 18–19 weeks during each field season. One-half of the enclosures received supplemental food, and the other one-half were unsupplemented controls. Density of voles increased throughout each field season. There were significant increases through time in philopatry and number of groups in both the food-supplemented and unsupplemented treatments, but there were no differences between treatments. Groups formed early in the season, apparently before the need for thermoregulatory benefits. Results are consistent with the hypothesis that the social organization of prairie voles is not flexible in response to changes in food quality but that formation of groups might be a density-dependent response.

avpr1a length polymorphism is not associated with either social or genetic monogamy in free-living prairie voles

Recent discoveries of single-gene influences on social behaviour have generated a great deal of interest in the proximate mechanisms underlying the expression of complex behaviours. Length polymorphism in a microsatellite in the regulatory region of the gene encoding the vasopressin 1a receptor (avpr1a) has been associated with both inter- and intra-specific variation in socially monogamous behaviour in voles (genus Microtus) under laboratory conditions. Here, we evaluate the relationship between avpr1a length polymorphism and social associations, genetic monogamy, and reproductive success in free-living prairie vole (M. ochrogaster) populations. We found no evidence of a relationship between avpr1a microsatellite length and any of our correlates of either social or genetic monogamy in the field. Our results, especially when taken in conjunction with those of recent experimental studies in semi-natural enclosures, suggest that avpr1a polymorphism is unlikely to have been a major influence in the evolution or maintenance of social monogamy in prairie voles under natural conditions.

Female choice based on male spatial ability and aggressiveness among meadow voles

The way in which female mate choice, scramble competition and contest competition for mates interact to influence the evolution of male traits is poorly understood. We tested female preferences for males that varied in spatial ability and aggressiveness, traits likely to be selected by scramble and contest competition, respectively. Male meadow voles, Microtus pennsylvanicus, were tested for their spatial ability using a series of water maze tests, and their aggressiveness was determined using paired arena trials. Females were given a choice between two tethered males, and their preferences were scored based on relative frequency and duration of visits. Females preferred males with low aggressiveness and good spatial ability to males with high aggressiveness and poor spatial ability. Females showed no preference between males with high aggressiveness and good spatial ability and males with low aggressiveness and poor spatial ability. These results suggest that female choice and scramble competition will favour males with better spatial ability. In contrast, female choice does not seem to favour more aggressive males.

Behavioural suppression of female pine voles after replacement of the breeding male

Examination of the mechanism of reproductive suppression includes determining which cues are involved and the context in which they occur. We studied groups of pine voles, Microtus pinetorum, that were disrupted by the replacement of the breeding male and compared them with intact family groups. If reproductive suppression is mediated by chemical cues, then soiled bedding should be sufficient to prevent production of litters by daughters. If reproductive suppression involves a behavioural component, we should observe aggressive behaviours or those indicative of dominance interactions directed from the mother towards the daughter or the replacement male. If replacement of the breeding male leads to conflict between the breeding female and her daughter, then more aggression or dominance interactions would be expected in disrupted than in intact families. The presence of the mother decreased reproduction by daughters, but chemical cues alone were not sufficient to prevent the daughter from mating with the replacement male. Rather, this decrease in reproduction seemed to be mediated by behavioural interactions. We propose that the mothers tugging on the daughter may lead to subordination of the daughter. The mothers presence may also alter the behaviour patterns of the male and daughter, which could delay reproductive activation of the daughter, prevent the formation of pair bonds or inhibit sexual behaviour. These behavioural interactions appear to depend on the presence of an unfamiliar male, because tugging, for example, was less frequent in intact family groups. Copyright 1998 The Association for the Study of Animal Behaviour Copyright 1998 The Association for the Study of Animal Behaviour.

Costs and benefits of communal rearing to female prairie voles ( Microtus ochrogaster )

Communal rearing of offspring may help mothers maximize their investment in offspring at a reduced cost to their own bodily condition, thus maximizing their potential for reproductive success. The objective of this study was to quantify the costs and benefits of communal rearing to prairie vole (Microtus ochrogaster) pups and mothers. Mothers were assigned to one of three social units: solitary mothers, singularly breeding groups (i.e. one mother and one non-reproductive sister) and plurally breeding groups (i.e. two lactating sisters). For each type of social unit, some replicates were provided with food ad libitum, while others were provided with limited food. The body mass of focal mothers (i.e. the first mother to produce a litter) was a significant predictor of pup growth. Regardless of food availability, litters of focal mothers in plurally breeding groups gained more weight than litters reared by solitary mothers. Pups reared in singularly breeding groups were intermediate in weight gain, but did not gain significantly more weight than solitary offspring. There was no difference in the body mass of focal mothers from each type of social unit, regardless of food availability. Within plurally breeding groups, the weight gain of the two litters and body mass of focal and second mothers did not differ. However, focal mothers from plurally breeding groups nursed fewer pups than solitary mothers and also fewer pups than their nestmates when food was limited. Our results suggest that plural breeding results in greater fitness to mothers than solitary and singular breeding.

The Effects of Predation of Snakes on Social Organization of the Prairie Vole, Microtus ochrogaster

Seasonal differences in breeding units of Microtus ochrogaster were studied in E-central Illinois. Average breeding-unit size was 1.9 during spring-early autumn and 4.8 during late autumn and winter. Increased nest survival of juveniles (0.7 and 1.5/litter, spring-early autumn and late autumn-winter, respectively), combined with high philopatry at all seasons, appears to be the primary factor involved in formation of larger breeding units during late autumn and winter. Additions to the basic breeding unit included 71.1% young born into the family group, and 28.9% unrelated adults that joined the group. Large snakes are the only known predators on voles in this region that can enter undisturbed underground nests and are not active during late autumn-winter. Breeding unit size and composition in an exclosure from which snakes, but not other predators, were assumed to have been excluded were nearly identical in August and September to those of breeding units in the open population during late autumn and winter. We conclude that the basic breeding unit of M. ochrogaster is an extended-family, communal-nesting group which also includes 1-2 unrelated adults. Because of high juvenile nest mortality from snakes during spring-early autumn, such communal groups are observed primarily during late autumn and winter.

Intraspecific variability in the social and genetic mating systems of prairie voles, Microtus ochrogaster

Intraspecific variability in mating systems has been documented previously, but there are few studies where investigators have used intraspecific comparisons to investigate the influence of ecological or demographic factors on social and genetic mating systems. We studied two populations of prairie voles, Microtus ochrogaster, one near Lawrence, Kansas, U.S.A., and the other in Bloomington, Indiana, U.S.A. We examined differences in spatial structuring of vegetation between sites, which might contribute to intraspecific variation in social and genetic mating patterns. Since space use is often related to the mating system, we calculated home range size of a subset of adult males and females in both populations and used live trapping to examine the number of same- or opposite-sex conspecifics sharing capture locations. We applied social network analysis to live-trapping data to quantify the number and strength of social ties with opposite-sex conspecifics. Genetic parentage analysis provided an estimate of the number of mates per individual. We found that space use and overlap with members of the same and opposite sex were best explained by effects of site and sex. The relative strength of social associations with opposite-sex conspecifics was best predicted by site, density and sex. The genetic mating system was best explained by population density. Although we cannot disentangle population density from ecological variables related to site (e.g. distribution of vegetation) that may have affected social monogamy in our study populations, our study shows that extrinsic variables do affect differences in social and genetic mating systems between populations.

EFFECTS OF PATCH QUALITY ON DISPERSAL AND SOCIAL ORGANIZATION OF PRAIRIE VOLES: AN EXPERIMENTAL APPROACH

Abstract The quality of the habitat patch in which individuals reside may influence demographic processes, thus affecting social organization. We manipulated the risk of predation and food availability to test the hypothesis that high patch quality decreased the propensity to disperse, increased the likelihood of social units becoming groups, and increased overall group size in prairie voles (Microtus ochrogaster). Prairie voles are socially monogamous, at least in part of their range, and they display varied social organization within a population, including groups (containing a breeding pair and at least 1 additional adult of either sex), male–female pairs, and single females. Our results indicated that the likelihood of dispersal from high-quality patches was significantly less than from low-quality patches. Dispersers also were significantly more likely to settle in similar or higher quality patches than the ones in which they were originally released. These patterns were primarily due to the dispersal of young males. Although the proportion of social units that were groups appeared to decrease with lower patch quality, the difference in social organization among patch types was not statistically significant. The total number of founding voles and founding males per social unit residing in the highest quality patches at the end of the study were significantly greater than in the lowest quality patches. Thus, under the conditions of our experiment, patch quality affected dispersal and group size but not the tendency to form groups.