Maternal conflict intervention is more frequent in chimpanzee compared to bonobo development
Rachna B. Reddy, Liran Samuni, Veronika Städele, Linda Vigilant, Martin Surbeck
One way mammalian mothers support offspring is by intervening on their behalf when they receive conspecific aggression. Maternal intervention protects offspring and facilitates mother–offspring rank correlation in several female-philopatric species. We tested the hypothesis that maternal intervention during development similarly facilitates mother–offspring rank correlation in one of our two male-philopatric closest living relatives: bonobos, Pan paniscus, for whom male ranks have been described as matrifocal but mechanisms of status transmission are unclear. We predicted mothers would intervene at higher rates in bonobos compared to chimpanzees, Pan troglodytes, where sons earn adult ranks independently. We expected this difference would be especially pronounced for sons as they reached reproductive age (∼8 years) and when aggressors were adult males, which female bonobos, but not female chimpanzees, often dominate. However, for both sons and daughters of all ages (0.5–16 years) and against aggressors of varying age and sex, bonobo mothers in Kokolopori, DRC (N = 22 pairs, 210 conflicts) intervened less frequently than did chimpanzee mothers at Ngogo, Uganda (N = 66 pairs, 221 conflicts), doing so in 8% versus 49% of all conflicts. These differences persisted regardless of the severity of aggression offspring received and the distress they demonstrated. Our results provide no support for our hypothesis that conflict intervention facilitates mother–son rank correlation in bonobos. Nor are patterns explained by additional alternative hypotheses related to species variation in costs of retaliation, quality of female–female relationships or infanticide risk. We introduce two new hypotheses: (1) that success in aggressive competition is less important for male bonobos than for male chimpanzees and (2) proclivities for support are heightened in chimpanzees compared to bonobos because of increased ingroup protectiveness in chimpanzees. The latter hypothesis arises from our unexpected, preliminary observation that nonmother bonobo bystanders intervened less often than nonmother chimpanzee bystanders during these events.
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