Testing the Predation Hypothesis for Vertebrate Sociality: Prospects and Pitfalls

Primates are in some ways excellent subjects for studying the impact of predation on prey. They are generally easy to watch and identify as individuals, so that long-term tracking of both death rates and anti-predator behaviors is possible, as amply shown by many of the studies in this volume. On the flip side, their low predation rates and large group sizes require very large total sample sizes for statistically powerful tests of the direct effects of sociality on predation rates. To study the indirect effects of predation On primate behavior requires defining the intrinsic predation risk they experience, that is the expected rate of predation they would suffer under standardized levels of anti-predator behavior (possibly none — see Hill & Dunbar, this volume). This abstract variable can be assessed qualitatively across different conditions by reference to modeling or common sense, or quantitatively by analyzing the hunting success of the predator independent of the prey's behavior (Cowlishaw, 1997). Great care must be taken in interpreting the behavioral responses of animals to different levels of predation risk when a given behavior can serve multiple functions, such as is the case with vigilance. Furthermore, most anti-predator behaviors carry fitness costs, not only from the lost opportunity to perform other fitness-enhancing activities, but even in terms of predation itself — apparently some primate species benefit from living in small groups which are very difficult for predators to detect instead of using a large-group early-warning defense as postulated in many theoretical models. Such costs will limit the extent to which primates are able to reduce their intrinsic predation risk (Fig. 1).