Protection service of a leading silverback male from external threats in wild western gorillas.

Primate males normally protect reproductive females, genetic offspring, and other relatives from external threats. Nevertheless, male protection of group members other than the above individuals is widely reported. Here, we show qualitative data on a silverback's charging behaviors toward human observers (predator surrogates) to protect group members having various age-sex and kinship traits in a group of wild western gorillas containing one reproductive male. We observed 106 and 33 charging behaviors by the leading silverback in two separate study periods. Two natal infants were often involved in his protective charging. Further, the silverback provided protection services to reproductive females. Surprisingly, immigrant individuals (i.e., unrelated to the silverback), including a wide range of age-sex classes, were also protected multiple times. His protection services for natal infants and adult females can be interpreted as a form of parenting effort and mating effort, respectively. Further, those for some immigrant immatures accompanied by their mothers can be considered part of mating effort, advertising his quality as a mate to the mothers. Finally, his charging behaviors to protect immigrant young males, who could be reproductive threats to him, may be due to group augmentation benefits. That is, the recruitment of additional males in exchange for protection services would improve the ability of group defense. Protection services of the leading silverback in the one-male group of western gorillas, in which members of various age-sex classes and kinship traits coexist, could be interpreted by some existing functional explanations.

Does kinship with the silverback matter? Intragroup social relationships of immature wild western lowland gorillas after social upheaval.

In primates living in one-male groups, the sole resident male is often an important social partner for group immatures. For such groups, however, replacement of the male and subsequent disruptions of their relationships are almost inevitable. Here, we described social relationships of immature wild western lowland gorillas within a habituated group, where two natal and eight immigrant immatures lived with the resident silverback. We recorded 5 m proximities among group members as an indicator of social closeness. We found that natal immatures spent more time within 5 m of the silverback than immigrant ones. The social closeness between the silverback and the younger immigrant immatures sharply increased after 1 year, but these values were still below those of the natal immatures. Regarding the development of independence from the mother, we found no significant difference between natal and immigrant immatures. The socially preferred nonmother mature for natal immatures was the silverback, whereas many immigrant immatures preferred a paternal adult sister who had previously co-resided with them in a previous group. Our results suggest that familiarity may be an important determinant of the social closeness between the silverback and immatures, but 1 year of co-residence might be too short to construct sufficient familiarity. The paternal sister may have played a pivotal role in the assimilation of immigrant immatures into the non-natal group. Nonetheless, it is not negligible that the silverback and immigrant immatures formed day-to-day close proximities. His tolerance toward co-residence with immigrant immatures can be considered a reproductive tactic.