Reactive and pre-emptive spatial cohesion in a social primate

Spatial cohesion in group-living animals is assumed as a risk-sensitive characteristic. Few studies have explicitly investigated this assumption or asked whether risk-related changes in spatial cohesion operate over short-term or long-term scales. We explored whether two groups of wild samango monkeys, Cercopithecus albogularis schwarzi, would adjust cohesion in reaction to naturally occurring risk from eagles and intergroup encounters using the number of conspecific neighbours as our response. Data on these directly observed encounters were used to assess reactive responses to immediate events. GPS-recorded locations of these encounters allowed us to create relative risk landscapes to investigate whether these groups might pre-emptively increase cohesion in high risk locations, in the absence of a direct threat. Multimodel inference was used to compare support for candidate models representing biological hypotheses. We found support for changes in cohesion in reaction to immediate intergroup conflict in both study groups. In contrast, only eagle risk apparently elicited a pre-emptive response. These results suggest that spatial cohesion is risk sensitive, but that responses differ between types of risk and between groups.