Social organization of a species may impact behavior, reproductive ecology, mating system, population genetic structure and overall fitness. A spectrum of sociality exists from solitary individuals to aggregations to integrated, highly related groups. A large body of knowledge exists for sociality and life-history characteristics of ground-dwelling sciurids, including several overarching models to explain the evolution of sociality. These models predict round-tailed ground squirrels (Xerospermophilus tereticaudus) to be solitary based on small body size (~125g), relatively long period of activity (January-June) and a short period of adult-juvenile overlap. However, previous behavioral observations suggest round-tailed ground squirrels have a clustered matrilineal structure with a suite of social behaviors, suggesting that they may represent a unique outlier in ground squirrel sociality models. Within the population of round-tailed ground squirrels at the Casa Grande Ruins National Monument in Coolidge, Arizona, USA, rates of amicable and agonistic interactions between adults were low, with no relationship between relatedness of individuals and rates of social interactions. No population substructure was evident with Bayesian analyses, global or pairwise F(ST) values, and average relatedness among females did not differ from males. Contrary to previous behavioral studies, round-tailed ground squirrels did not have high levels of social behavior, nor did they form significant genetic subpopulation structuring. The active season of round-tailed ground squirrels closely followed patterns of precipitation and peak resource availability. Body mass differed between males and females, across years, and within seasons. Males were heavier than females at emergence, prior to mating and pre-hibernation, but not when females began gestation. Emergence of litters and litter size are related to amount and timing of winter rainfall. Foraging and vigilance behaviors compose 64-66% of the activity budget, but differ in that males spend a greater proportion of time foraging, whereas females spend a greater proportion of time vigilant. Round-tailed ground-squirrels have a polygynandrous mating system. Polygyny was evident in 2004, 2005, and 2006, and multiple paternity occurred in the majority of litters with 2.5 sires/litter; litter size was positively correlated with the number of sires. These findings support predictions generated by sociality models for ground-dwelling squirrels.

Studies of mating systems and social organisation have been central to understanding of the evolution of social behaviour. The European badger Meles meles is a good species in which to study these processes, as its complex social system provides an opportunity to investigate how both natural and kin selection shape the evolution of mating systems and social structure. In this thesis, I use behavioural and genetic data to describe the mating system and social organisation of a high-density badger population and examine the occurrence of cooperative breeding. I genotyped 915 (85%) badgers trapped in Wytham Woods (1987–2005), 630 of which were cubs, and assigned both parents to 331 cubs with 95% confidence. This revealed a polygynandrous mating system, with up to five mothers and five fathers per social group. Mounting behaviour was also polygynandrous and I show the strongest evidence to date for multiple-paternity litters. I demonstrate, for the first time, that groups consisted of close and distant kin: approximately one third of group members were first-order kin, and overall group members had slightly lower relatedness levels than half-siblings. Within groups, adult and yearling females had higher pairwise relatedness than males, and neighbouring groups contained relatives. These findings result from the high level (42%) of extra-group paternities, 86% of which were assigned to neighbouring males. For the first time I show that females avoided inbreeding by mating with extra-group males; however, incestuous matings did occur. Promiscuous and repeated mountings were observed, which may reduce male–male aggression and infanticide, but may also promote sperm competition, genetic diversity, and / or genetic compatibility. Just under a third of adult males and females were assigned parentage each year and I quantify, for the first time, reproductive skew within badger groups. Correlations between relatedness, group productivity, and reproductive skew were not consistent with the predictions of incomplete-control models; rather, resource availability may play a role. Older and younger badgers displayed reduced annual breeding success, with male success increasing initially with experience. The Restraint, Constraint, and Selection Hypotheses did not explain the age-related breeding pattern in females. Variance in lifetime breeding success (LBS) was greater for males. Males that only bred within or only outside of their groups had half the LBS of males that did both. Females that were assigned maternity probably bred cooperatively and allonursed non-offspring, which has not been demonstrated previously. No benefit was established, however, in terms of litter size, probability of offspring breeding, or offspring lifetime breeding success, with more mothers in a group. In conclusion, badger social groups are fostered through kinship ties. Polygynandry and repeated mounting may have evolved originally to reduce male–male aggression and infanticide by males, through paternity masking. Although plural breeding occurs, group living appears to be costly. Motivation to disperse may be reduced through high-levels of extra-group paternities, which may also reduce inbreeding. Cooperative breeding among mothers may represent a low-cost behaviour with indirect benefits due to high levels of relatedness between female group-members. Badger sociality therefore represents an early stage in the evolution of social behaviour.