Investigating the relationship between sociality and reproductive success in wild female crested macaques, Macaca nigra

Studying the link between sociality and fitness is valuable to understand the costs and benefits of sociality. In many species, sociality is positively linked to fitness: having more, stronger, more equitable or predictable affiliative relationships leads to higher reproductive success, greater survival, or longevity, at least in females. We tested this sociality-fitness link in wild, female, crested macaques, Macaca nigra, in Tangkoko, North Sulawesi, Indonesia. Over 15 years, we studied six groups and collected behavioral, ecological, and demographic data on 140 females. We modeled the annual probability that females gave birth as a function of grooming rate with other females, adjusting for a number of control variables. We found no evidence for an overall association between grooming and the probability of giving birth. Further exploration of the results revealed large uncertainties in the model estimates and substantial idiosyncratic variation within and between females, and to a lesser extent, years and groups. This may have to do with constraints imposed on grooming, on reproduction, or on both. Further investigations into the significance and sources of this variation will help to disentangle the pathways by which social interactions with other group members relate to fitness outcomes.