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

When spider monkeys (Ateles geoffroyi) are dispersed and moving through woo ded 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 appea r to be answered with the same call from other monkeys. We examined the cir cumstances in which this vocalization was emitted and the responses elicite d from other group members. A total of 105 h of continuous recordings on em ission of whinnies, 113 h of individual focal samples and 291 spectrograms were analyzed from a study group with 15 identified subjects. Whinnies emit ted in different circumstances caused different reactions. Whinnies provoke d 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 re sponse of approach or calling. Most strikingly, whinnies given during group movement provoked hearers to approach or call significantly more often tha n either those given while feeding or resting. Playback experiments found o ne response, 'scan', to differ according to the original circumstances of e mission 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 f requency range - were available for spider monkeys to distinguish between t he whinnies of different group members. No universal differences were found between whinnies from different contexts, but for each of three individual s examined in detail, group movement whinnies had a fundamental frequency w ith more arches or modulations than feeding or resting whinnies. That spide r monkeys respond differently to playback of feeding whinnies suggests that they are capable of taking idiosyncratic individual variations in voice in to account when extracting the call's message. The functional characteristi cs of the whinny are discussed in relation to the information the call may convey and its degree of referential specificity.