Are Vigilance, Risk From Avian Predators and Group Size Consequences of Habitat Structure? A Comparison of Three Species of Squirrel Monkey (Saimiri oerstedii, S. boliviensis, and S. sciureus)

Abstract

We compared the interactions between vigilance, risk from avian predators, and typical group size for three species of squirrel monkey (Saimiri oerstedii, S. boliviensis, and S. sciureus). Field studies of each species in relatively undisturbed habitats provided data including behavioral interactions between avian predators and squirrel monkeys and the latter's vigilance response, habitat use, group size, population density and within-group food competition. Three hypotheses were examined. The first two identified aspects of the habitat structure exploited by squirrel monkeys and their avian predators as key parameters. (1) The overall success of avian attacks is a function of the density of understory vegetation present to hinder these birds from closely approaching individual troop members. (2) The extent of continuous canopy forest cover within habitats underlies which of two vigilance strategies are exhibited. Preemptive vigilance occurs when monkeys allocate time to visual scrutiny in a manner consistent with the intent of obtaining advance warning on the approach of an avian predator. During reactive vigilance, in contrast, monkeys are not overtly vigilant against an aerial attack until an attack or other stimulus strongly indicative of immediate risk occurs. Preemptive vigilance occurs in open habitats with little canopy, while reactive vigilance predominates when birds can make effective ambush attacks from forest canopy cover. Both hypotheses were supported. Group sizes vary significantly across the three squirrel monkey species with S. boliviensis having the largest and S. sciureus the smallest mean group size. Our third hypothesis examined the premise that group size in social animals reflects a tradeoff between predation risk (whether based on long-term observations or extrapolated from habitat structure) and within-group food competition. The power of this tradeoff mechanism to explain group-size variation was contrasted with a simpler, one parameter model, within-group food competition under an ideal free distribution. The ideal free distribution approach was consistent with the observed ranking of group sizes, whereas the tradeoff model provided mixed results. In retrospect, the weakness of the tradeoff model is not surprising. Changes in the risk incurred by individual members of a squirrel monkey troop as a consequence of variation in group size are likely of much smaller magnitude than the risk exposure resulting from local habitat structure.