This study is designed to fill in critical gaps in our understanding of primate-predator relationships. One of the most widely quoted reasons that primates live in groups is to reduce the risk of predation. However, some primates that suffer high predation rates nevertheless forage alone or in very small groups. This observation and the predictions of several theoretical models suggest that high predation risk can favor either very small or very large groups. Which outcome occurs depends on 1) the method and effectiveness of detection of the predator by the primate prey in groups of different sizes, and 2) the relative ability of prey and predator to detect each other. Despite many studies of vigilance behavior in primates, there is only fragmentary knowledge for any species on how effective vigilance is in detecting predators, at what average distance detection occurs, and the variables that affect the chance of detection. Even less is known about the hunting behavior and detection ability of the predators of primates.
To measure the important variables that explain group size in animals as a function of predation risk, five different-sized groups of wild brown capuchin monkeys in Argentina will have live (tethered) and model predators placed in their home ranges. Observers will follow individual monkeys and record a set of context variables (individual's sex and age, distance to the 'predator', monkey's height in the forest, direction the monkey is looking, etc.), along with whether or not the monkey detects the predator, as signaled by alarm calls or a sudden prolonged and fixed stare directly at the predator. Each live tethered bird of prey will be video-taped as a group of monkeys approaches; the distance at which it detects the group and whether the detection is by sight or sound will be determined from the video tape and associated verbal commentary. The detail and sample size of observations will improve greatly on prior studies because the use of model and captive predators permits greater control of the detection process, including systematic variation of detection conditions and recording observations both before and after detection occurs.
The results of this study will address an important question about the origins of social living in human ancestors, and thus may help to explain the form of current human societies. The project will also contribute to biodiversity conservation by: 1) continuing the long-term presence of researchers in the study site to discourage poachers, 2) contributing to the long-term database on group demography of this long-lived species, 3) continued monitoring of long-term changes in local ecology due to invasive species and climate change. The research will help train at least one U.S. graduate student, several college undergraduates, and one or more Argentine biologists. Data from this project will be contributed to a large-scale meta-database for animal behavior, to allow easy online access to current and future researchers. Genetic material sampled from the monkey population will be placed in a public biomaterials repository for future access.
