Food-associated calls in rhesus macaques ( Macaca mulatta ): II. Costs and benefits of call production and suppression

In field experiments, free-ranging rhesus macaques (Macaca mulatto) gave food-associated calls in 45% of the trials in which they were presented with food, either monkey chow or coconut; they did not call in control trials when sticks were presented. Consistent with prior naturalistic observations, adult females called in a higher proportion of trials than adult males. Coconut, one of the most highly preferred food items in the diet, elicited different call types and a higher rate of calling from “discoverers” than did chow. The call types produced to coconut (warbles, harmonic arches, and chirps) were primarily those that, under nonexperimental conditions, were associated with relatively rare and preferred foods. In contrast, coos and grunts were primarily produced in response to chow. The relative hunger level of the discoverer had no significant effect on the call type produced but did affect the rate of call production; discoverers called at higher rates when they were hungry. Upon hearing food-associated calls, individuals within the vicinity of the discovery responded by rapidly approaching the caller. A larger number of individuals approached when discoverers called than when they did not. Discoverers who failed to call received significantly more aggression from group members and, in the case of females, actually consumed less food than discoverers who called. The probability of receiving aggression did not appear to be associated with the discoverer's dominance rank. Results suggest that food-associated calls are “honest” signals reflecting food possession. Those who fail to signal and are caught with food are apparently punished.