FUNCTIONAL REFERENTS AND ACOUSTIC SIMILARITY - FIELD PLAYBACK EXPERIMENTS WITH RHESUS-MONKEYS

Rhesus monkeys, Macaca mulatta, on the island of Cayo Santiago, Puerto Rico produce one or more of five acoustically distinctive calls when they find food. Three of these calls ('warbles', 'harmonic arches' and 'chirps') are produced by individuals finding high-quality, rare food items, whereas the other two calls ('coos' and 'grunts') are produced upon encountering lower-quality, common food items, and in non-food c ontexts as well. To determine how rhesus classify such acoustic variat ion, I conducted habituation experiments using a subset of the five ca ll types. I designed experiments to reveal whether classification is b ased primarily on acoustic features or on the basis of a call's functi onal referent; caller identity was held constant within sessions. Habi tuation to 'warbles' transferred to 'harmonic arches', and vice versa. Thus, although these two calls are acoustically distinctive, they app eared to be perceptually clustered into one category based on referent ial similarities. In contrast, habituation to 'grunts' was followed by dishabituation to 'warbles' or 'harmonic arches', and habituation to 'warbles' or 'harmonic arches' was followed by dishabituation to 'grun ts'. Dishabituation could be due to acoustic or referential difference s. Significantly, the magnitude of the dishabituating response was asy mmetric and depended upon the call type used in the habituation series . Thus, when subjects were habituated to 'grunts', they responded sign ificantly more to tests of 'warbles' or 'harmonic arches' than when th e sequence was reversed. These results suggest that for rhesus monkey food-associated calls, referential differences carry more weight durin g perceptual classification than do acoustical differences. (C) 1998 T he Association for the Study of Animal Behaviour.