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.