Sharing spaces: niche differentiation in diet and substrate use among wild capuchin monkeys

Understanding variation in social grouping patterns among animal taxa is an enduring goal of ethologists, who seek to evaluate the selective pressures shaping the evolution of sociality. Cohesive association with conspecifics increases intragroup feeding competition and may impose constraints on group size. Furthermore, in sexually dimorphic species, males and females may have different nutritional requirements, which can lead to suboptimal foraging in mixed-sex groups. How do animals living in permanent social groups mitigate these foraging costs? Niche differentiation is often hypothesized as a mechanism, but rigorous and detailed tests of the extent and context of differences in diet and habitat use, key tenets of this hypothesis, are rare. We investigated the potential for niche differentiation in foraging activity budget and environment use in a population of wild white-faced capuchin monkeys, Cebus imitator, in northwestern Costa Rica. Using a robust data set of 15 879 foraging scan samples collected from four groups over 13 months, we found that smaller individuals – e.g. juveniles and females – forage more often on smaller branches. We additionally found clear evidence of predator-sensitive foraging wherein the smallest individuals spent less time on the ground during invertebrate foraging. Importantly, niche differentiation was far more evident overall during invertebrate foraging, likely due to spatial constraints and environmental homogeneity imposed by fruit patches. In summary, we found considerable variation in habitat use across age and sex classes, likely attributable to differences in size and relative predation risk. These variables likely reduce intraspecific feeding competition by promoting differential diet and habitat use. Our results also provide insight into the limits of niche differentiation as a strategy for competition reduction and may shed light on the evolution of fission–fusion dynamics in highly frugivorous species.