Ambush predators may select for small group size in rainforest primate prey

Classic ecological models of social groups suggest that increasing group size typically leads to a decrease in both individual predation risk and net food intake, with optimal group size being a compromise between these benefits and costs. Among rainforest primates, the main antipreda-tor benefits of sociality are thought to result from the dilution effect and collective detection of predators. However, recent research suggests that vegetation density in rainforest habitats limits the benefits of collective detection against ambush predators such as raptors, felids and snakes. Further, while larger groups are acknowledged to be more conspicuous to predators, it is widely assumed that this cost is unlikely to outweigh dilution benefits. Here we show in a simple model that per-individual rates of both encounters with predators and successful ambush attacks per encounter can increase with group size, under conditions likely to hold for many primate groups (when increases in group size lead to increases in group spread, conspicuousness and daily trav-el distance). Consequently, individual risk against ambush predators that employ a sit-and-wait strategy to search for prey, such as many snakes and some raptors, is lowest in small to medium-sized groups. In contrast, individuals in relatively large groups are favoured against ambush pred-ators like felids that employ a cruising strategy to search for prey, although even in this case in-creasing group size above some threshold increases individual risk. These results suggest that maximum group size among primates can be limited by increasing predation risk. Research fo-cused on primate predators is needed to determine the extent to which the model accurately re-flects their behaviour.