Mountain gorillas maintain strong affiliative biases for maternal siblings despite high male reproductive skew and extensive exposure to paternal kin

Abstract

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 maternally and paternally related siblings, of the same and opposite sex, throughout their lives. Using nearly 40,000 hours of behavioral data collected over 14 years on 699 sibling and 1235 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. We observe a preference for maternal kin in a species with high reproductive skew (i.e., high relatedness certainty), even though low reproductive skew (i.e., low relatedness certainty) is believed to underlie such biases in other non-human primates. Our results call into question reasons for strong maternal kin biases when paternal kin are identifiable, familiar, and similarly likely to be long-term groupmates, and they may also suggest behavioral mismatches at play during a transitional period in mountain gorilla society.

Prosocial or photo preferences? Gorillas' prosocial choices using a touchscreen

“…Furthermore, the fact that these gorillas had opportunities to reciprocate in the task as well as in other contexts over time because they were housed together should increase the likelihood that they might behave prosocially. It should be noted, though, that previous research has suggested that gorillas (mountain gorillas in this case) prefer maternal over paternal siblings despite being able to recognize paternal kin (Grebe et al, 2022b).…”

Multiple mechanisms for inbreeding avoidance used simultaneously in a wild ape

“…We found strong support for familiarity- and age-based mechanisms of kin identification, but no support for mechanisms based on phenotypic matching used in inbreeding avoidance. Familiarity is likely to be a highly accurate mechanism for identifying maternal kin in mountain gorillas as offspring usually spend upwards of 6 years in close contact with their mothers and any close-in-age maternal siblings [ 53 , 55 ]. Familiarity is unlikely to be as reliable for identifying paternal kin, since females consistently mate with multiple partners [ 34 ], and rank rather than paternity predicts adult male–immature relationships [ 56 ].…”

“…Evidence from baboons demonstrates that inbreeding avoidance mechanisms can be present even when related individuals rarely have the opportunity to mate [ 16 ], suggesting that mate choice could predate changes in dispersal pattern. Kin recognition is common across primates, enabling a variety of benefits through kin-biased behaviours [ 81 ] and appears to be widespread across the gorilla genus [ 55 , 82 ]. In western lowland gorillas, which produce almost exclusively in single-male groups [ 32 ], familiarity-based kin recognition is likely to be highly accurate for both maternal and paternal kin.…”

“…Kin identification, a requisite for inbreeding avoidance via mate choice, primarily occurs through familiarity, age proximity (where close-in-age group members are more likely to have the same father) and/or phenotypic matching in primates [51,52]. In mountain gorillas, maternal kin are likely to be identified with relative ease through familiarity, due to the close social relationships maintained between mother and offspring, and between maternal siblings, throughout immaturity and often beyond [53][54][55]. However, identifying paternal kin is likely to pose difficulties in multimale groups, as females frequently mate with multiple males around conception [34] and male dominance rank is a better predictor of adult male-immature relationships than is paternity [56].…”

An early-life challenge: becoming an older sibling in wild mandrills

“…In addition, juveniles’ adjustment to the birth of a younger sibling could also be influenced by genetic relatedness between siblings, as predicted by the kin selection theory [ 2 , 64 ]. For instance, in species characterized by a high-reproductive skew and a long male tenure length, like in mountain gorillas ( Gorilla beringei beringei ), full siblings may be common, which could induce a smoother transition (but see [ 65 ] for the effect of relatedness on siblings’ relationships). In contrast, in species with lower relatedness between siblings, as in mandrills, where males’ reproductive skew is high but tenure length is relatively short [ 66 ], juveniles could experience a harsher transition after the birth of their sibling.…”