MANIPULATING FORAGING GROUP-SIZE - SPIDER MONKEY FOOD CALLS AT FRUITING TREES

The food calling behaviour of the Costa Rican spider monkey, Ateles geoffroyi, was studied to determine if food calls function as conditional signals designed to attract conspecifics and thereby manipulate the size of the feeding subgroups. First, individuals were shown to manipulate the number of conspecifics joining them by altering the frequency of calling. Then, based on the expected levels of feeding competition and male-female strategies, four predictions were made concerning the frequency with which individuals should call. As predicted, calls were given more often by subgroups containing dominant individuals than by subgroups with only subordinate individuals, in large trees more often than in small ones, and more frequently when food resources were abundant than when they were scarce. However, contrary to what was predicted, subgroups containing males did not call more than subgroups with only females. In general, the observations suggest that spider monkeys can conditionally broadcast information to manipulate their subgroup size and do so in ways that may decrease feeding competition. © 1990 The Association for the Study of Animal Behaviour.