Kinship composition in mammals

Abstract

Understanding the evolution of group-living and cooperation requires information on who animals live and cooperate with. Animals can live with kin, non-kin or both, and kinship structure can influence the benefits and costs of group-living and the evolution of within-group cooperation. One aspect of kinship structure is kinship composition, i.e. a group-level attribute of the presence of kin and/or non-kin dyads in groups. Despite its putative importance, the kinship composition of mammalian groups has yet to be characterized. Here, we use the published literature to build an initial kinship composition dataset in mammals, laying the groundwork for future work in the field. In roughly half of the 18 species in our sample, individuals lived solely with same-sex kin, and, in the other half, individuals lived with related and unrelated individuals of the same sex. These initial results suggest that it is not rare for social mammals to live with unrelated individuals of the same sex, highlighting the importance of considering indirect and direct fitness benefits as co-drivers of the evolution of sociality. We hope that our initial dataset and insights will spur the study of kinship structure and sociality towards new exciting avenues.

The impact of kinship composition on social structure

“…Furthermore, the probability that kin and non-kin dyads had a relationship and the mean strength of that relationship decreased as the proportion of non-kin dyads increased (Figure S6), possibly because individuals had to distribute their social availability through more partners (since groups with more non-kin dyads are larger). Overall, these observations further corroborate the relationship between group size and kinship composition (Pereira et al 2023) and suggest that group size might have explained some important variation in social structure, reducing our capacity to detect a relationship between kinship composition and social structure. Finally, it is also possible that the empirical results corresponded to a true null result.…”

“…The study groups vary in size and number of matrilines, but they share the same overall social organisation, i.e., they are large, female philopatric groups that feature clusters of close kin (Berman 2016; Pfefferle et al 2016; Widdig, Kessler, et al 2016), which might limit how much the groups vary in kinship composition. Kinship composition might thus be more likely to differ between groups of species that exhibit markedly different organisations (Pereira et al 2023). For example, cooperative breeding groups that arise from offspring retention by a single breeding pair might have different kinship composition and kin biases compared to groups that arise from the gathering of independent families.…”

“…Mean relatedness is the average relatedness across all dyads in a group (Lukas et al 2005; Briga et al 2012; Lukas and Clutton-Brock 2018; Dyble and Clutton-Brock 2020). Kinship composition characterises the to what extent groups consist of kin and/or non-kin, thus complementing mean relatedness with explicit information on the dyadic kinship categories that make up groups (Pereira et al 2023). So far, research into how kinship structure modulates social structure has focused on mean relatedness (Smith 2014), and has shown, for example that groups with higher mean relatedness have denser networks (Smith et al 2010) and that members of groups with lower mean relatedness are more likely to form coalitions and to reciprocate grooming symmetrically (Lukas and Clutton-Brock 2018).…”

Explaining variation in the kinship composition of mammal groups

“…Empirical studies have shown that average coefficients of relatedness ( r ) between group members vary widely between animal species and, in mammals, range from close to zero to values exceeding r = 0.5, the expected level of relatedness between full siblings ( Briga et al 2012 ; Dyble and Clutton-Brock 2020 ; Clutton-Brock 2021 ; Pereira et al 2023 ). Comparative analyses have revealed that these interspecific contrasts in kinship are consistently associated with interspecific differences in social behavior ( Griffin and West 2003 ; Lukas and Clutton-Brock 2018 ; West et al 2021 ) and that, as would be predicted by inclusive fitness theory ( Hamilton 1964 ; Gardner et al 2011 ), species exhibiting the most highly developed forms of alloparental care tend to live in social groups within which the average degree of genetic relatedness between groupmates is high ( Briga et al 2012 ; Lukas and Clutton-Brock 2018 ).…”

Multiple mechanisms for inbreeding avoidance used simultaneously in a wild ape

“…Strong and stable social relationships have been linked with a range of fitness benefits across social mammals [1][2][3][4][5], and relationships with kin can provide even greater benefits [6][7][8][9]. Remaining in the same social group throughout life and interacting regularly with close kin could therefore increase fitness.…”