The 'whinny' of spider monkeys: individual recognition before situational meaning

When spider monkeys (Ateles geoffroyi) are dispersed and moving through wooded areas in the dry forest of Santa Rosa National Park in Costa Pica, they give loud calls, whinnies, that can be heard over long distances and appear to be answered with the same call from other monkeys. We examined the circumstances in which this vocalization was emitted and the responses elicited from other group members. A total of 105 h of continuous recordings on emission of whinnies, 113 h of individual focal samples and 291 spectrograms were analyzed from a study group with 15 identified subjects. Whinnies emitted in different circumstances caused different reactions. Whinnies provoked by the sight of an observer were never responded to in any particular way, while those given during resting or feeding sometimes caused an active response of approach or calling. Most strikingly, whinnies given during group movement provoked hearers to approach or call significantly more often than either those given while feeding or resting. Playback experiments found one response, 'scan', to differ according to the original circumstances of emission of the broadcast call: monkeys scanned more in the direction of the sound when hearing feeding rather than group movement whinnies.

Acoustic analyses of individual differences in the whinny call showed that several features - maximum frequency, number of frequency modulations and frequency range - were available for spider monkeys to distinguish between the whinnies of different group members. No universal differences were found between whinnies from different contexts, but for each of three individuals examined in detail, group movement whinnies had a fundamental frequency with more arches or modulations than feeding or resting whinnies. That spider monkeys respond differently to playback of feeding whinnies suggests that they are capable of taking idiosyncratic individual variations in voice into account when extracting the call's message. The functional characteristics of the whinny are discussed in relation to the information the call may convey and its degree of referential specificity.