Neither kin selection nor familiarity explain affiliative biases towards maternal siblings in wild mountain gorillas

Evolutionary theories predict that sibling relationships will reflect a complex balance of cooperative and competitive dynamics. In most mammals, dispersal and death patterns mean that sibling relationships occur in a relatively narrow window during development, and/or only with same-sex individuals. Besides humans, one notable exception are mountain gorillas, in which non-sex biased dispersal, relatively stable group composition, and the long reproductive tenures of alpha males mean that animals routinely reside with both same and opposite-sex, and full and half siblings, throughout their lives. Using nearly 40,000 hours of observation data collected over 14 years on 699 sibling and 1258 non-sibling pairs of wild mountain gorillas, we demonstrate that individuals have strong affiliative preferences for full and maternal siblings over paternal siblings or unrelated animals, consistent with an inability to discriminate paternal kin. Intriguingly, however, aggression data imply the opposite. Aggression rates were statistically indistinguishable among all types of dyads except one: in mixed-sex dyads, non-siblings engaged in substantially more aggression than siblings of any type. This pattern suggests mountain gorillas may be capable of distinguishing paternal kin, but nonetheless choose not to affiliate with them over non-kin. A preference for maternal kin occurs despite gorillas not possessing low male reproductive skew, the key characteristic believed to underlie such biases. These results call into question reasons for strong maternal kin biases when paternal kin are identifiable, familiar, and similarly likely to be long-term social partners, and suggest behavioral mismatches at play during a transitional period in mountain gorilla society.