Natal philopatry, or delayed dispersal of sexually mature offspring, may be due to ecological constraints on dispersal. In this study, we manipulated the population density of prairie voles (Microtus ochrogaster) living in experimental outdoor enclosures to test a prediction from the habitat saturation hypothesis that philopatry and subsequent group formation in this cooperatively breeding mammal is affected by the availability of suitable territories. We detected a significant, positive relationship between the proportion of offspring remaining philopatric and density, with females being more philopatric than males at all densities. This increase in philopatry led to a significant increase in the proportion of social units that were groups as well as a significant increase in group size. These results provide the strongest evidence of a causal effect of density on dispersal and group formation in a mammal. Our findings suggest that habitat saturation is at least a partial explanation for philopatry in prairie voles. However, we cannot eliminate the possibility that other variables, such as benefits accrued from remaining philopatric, may also be factors contributing to philopatry. Nonetheless, these results show that changes in ecological conditions can influence social structure within a population, leading to group formation and a social milieu conducive to the evolution of cooperative breeding.
Most of our understanding of rodent sociality comes from studies of species from the northern temperate zone with a few exceptions. Caviomorphs are New World rodents, primarily found in the Neotropics, for which data on sociality is available from only about one-third of the approximately 250 extant species. Since the species of caviomorphs studied to date display a remarkable diversity in social organization, we need more data from caviomorph rodents to gain a better understanding of sociality in rodents, as well as mammals in general. We begin this chapter by discussing some problems with trying to define sociality, as well as some of the ultimate hypotheses proposed to explain sociality. We then summarize some of our knowledge of a few of the well-studied non-caviomorph rodents that provide examples of our understanding of sociality in rodents and are good taxa for comparative studies with caviomorphs. A few species of caviomorphs that have been well studied are selected for comparison and we review what we know about the fit between the currently proposed hypotheses and data from these species. We end by suggesting some future directions for research on caviomorphs to more fully understand the unity and diversity of sociality in rodents.
Philopatry can arise from lack of a suitable territory (ecological constraints hypothesis) or a potential mate (limited mate hypothesis) outside the natal area. We tested predictions from these hypotheses by creating territorial vacancies with and without potential mates for female prairie voles (Microtus ochrogaster). Standard mark-recapture techniques were used to determine group composition and residency before and after removals. Either all adults, all adult females, or no animals were removed from 1 group within each enclosure. Territorial vacancies stimulated rapid immigration of adult female prairie voles. The presence of adult male(s) on the territory resulted in an intermediate amount of immigration compared to vacant and control territories. Immigrants arrived from neighboring groups more often than expected, suggesting that proximity to territorial vacancies affected replacement. The patterns of immigration observed in this study were more consistent with the ecological constraint hypothesis than with the mate limitation hypothesis.
Examination of extensive data from field and laboratory studies indicates that prairie voles (Microtus ochrogaster) are socially monogamous and form pair-bonds. We used grid-trapping data from replicate populations of prairie voles living in 0.1-ha small mammal enclosures at 2 different initial densities to examine the extent to which opposite-sex adults captured together in the same live trap reflected social associations established from nest-use patterns and the probability that the pair produced offspring. Females classified as a resident at a nest that also included at least 1 resident male (male–female pairs or groups) were significantly more likely to be trapped with a male from the same social unit (64%) rather than a male from another social unit or a male wanderer (an animal not considered a resident at any nest site). On the other hand, females residing at a nest with no resident males were caught with wandering males significantly more often (84%) than with resident males from another nest. Finally, female wanderers were significantly more likely to be trapped with male wanderers (75%) than with resident males. A genetic analysis of parentage revealed that females were significantly more likely to have produced offspring with the male with which they were caught most frequently (80% of females) than to not breed with these males. None of these findings were influenced by density. Overall, the multiple-capture data were consistent with social monogamy and the relative frequency of male–female multiple captures were predictive of the likelihood of mating. However, although most females residing at nests with resident males bred with these males (84%), a similar percentage of these females also bred with at least 1 male that never resided in the same social unit as the female, suggesting that prairie voles are not genetically monogamous throughout their lives.
Abstract Individuals of either sex may display alternative behaviors to obtain copulations, but few studies have examined the breeding patterns of females and males in populations where individuals of both sexes exhibit alternative reproductive tactics (ARTs). In prairie voles (Microtus ochrogaster), most adults are territorial, residing at a single nest site either as male–female pairs or as solitary individuals. However, some adults adopt nonterritorial, wandering tactics. During two field seasons monitoring prairie vole populations maintained in seminatural enclosures, we found evidence that females exhibiting different ARTs bred differentially with resident and wandering males. Females residing at a nest with a male bred significantly more often with a paired resident male, primarily their social partner, and significantly less often with male wanderers compared to single resident females or wandering females. These patterns were not due to chance, because paired resident females produced offspring with paired resident males significantly more than expected based on the relative abundance of these males in the population, whereas single resident females produced offspring with male wanderers significantly more than expected based on the proportion of male wanderers in the population. We did not find any evidence that multiple paternity was greater in the litters of single resident females and wanderer females even though these females lacked a male social partner to limit mating access by multiple males. This suggests that mate guarding by a female’s male social partner was not the primary determinant of multiple paternity in the litters of females exhibiting different reproductive tactics. However, male ART did affect the likelihood of multiple paternity. Females that produced offspring with single resident or wanderer males had an increased likelihood of multiple paternity relative to females producing offspring with paired resident males. The results of this study show that female and male ARTs can affect breeding patterns.
