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Abstract

All else being equal, cooperating with kin results in overall larger inclusive fitness benefits than with non-kin. Animals can live with kin, non-kin or both, and this kinship composition can thus influence the benefits and costs of group-living and the evolution of within-group cooperation. Yet, the kinship composition of mammalian groups remains uncharacterised. We characterised the taxonomic representation and evolutionary trajectory of kinship composition in mammals using pedigree data from the literature. We found that the ancestors shared by the 18 species in our sample likely lived with kin for most of their evolutionary history. However, we found that only roughly half of the 18 species lived in groups where all same-sex individuals were related, whereas the other half lived in groups where same-sex individuals featured kin and non-kin. Because it is not obvious why individuals might live with non-kin, these results spurred two questions: 1) When might group-members benefit from living with non-kin? 2) How might groups that feature non-kin arise from groups that only feature related individuals? To answer the first question, we used an analytical model to calculate the theoretical optimal kin to non-kin ratio for a group according to the benefits that non-kin provide. Overall, we found that living with some non-kin is beneficial when non-kin provide benefits that are larger than the benefits from inclusive fitness of the potential kin they displace. For the second question, we used agent-based models to simulate a scenario in which group-members might need to increase group size and we explored under what conditions recruiting immigrant non-kin is beneficial. Overall, our results indicated that recruiting unrelated immigrants is beneficial when distance to optimal group size is large and most potential immigrants are unrelated to group-members. Our findings indicate that living with a mixture of kin and non-kin is not rare in mammals and that non-kin can be valuable group-members, further highlighting the importance of considering both indirect and direct fitness benefits as co-drivers of the evolution of sociality.