Group size but not dominance rank predicts the probability of conception in a frugivorous primate

In social mammals, within-group competition for food can drive variation in female fitness. Frugivores may face particularly strong competition because they use patchily distributed usurpable resources. Dominance rank and group size influence how a female experiences within-group competition. Both are predicted to affect access to food and, thus, reproductive success. We used 15 years of behavioral, demographic, and reproductive data from wild frugivorous blue monkeys to examine effects of rank and group size on the probability that a female conceived. We used generalized linear mixed models, controlling for potentially confounding maternal and environmental factors. Blue monkey females compete aggressively and disproportionately for fruits and exhibit linear dominance hierarchies, but neither rank index we tested significantly predicted the probability of conception. Although earlier studies found that group size effects on activity budgets were minimal, we found that group size had a quadratic relationship with the probability of conception, which peaked at around 31 members. The lack of a rank effect may reflect behavioral strategies (e.g., switching resources, spreading out during feeding, using cheek pouches) that minimize the strength of within-group competition, thus facilitating group-level cooperation in between-group contests. The significant quadratic effect of group size on reproduction may occur if individuals in small groups do not obtain the full benefits of group living (e.g., predator avoidance, increased foraging success, communal care for offspring) and those in large groups experience a lower quality diet or constrained feeding time. Ultimately, measures of reproduction are preferable to behavioral proxies for accurately assessing within-group competition.