THE ACOUSTIC FEATURES OF GORILLA DOUBLE GRUNTS AND THEIR RELATION TO BEHAVIOR

Mountain gorillas (Gorilla gorilla beringei) give double-grunts to one another in a variety of situations, when feeding, resting, moving, or engaged in other kinds of social behavior. Some double-grunts elicit double-grunts in reply whereas others do not. Double-grunts are individually distinctive, and high-ranking animals give double-grunts at higher rates than others. There was no evidence, however, that the probability of eliciting a reply depended upon either the animals' behavior at the time a call was given or the social relationship between caller and respondent. The probability of eliciting a reply could be predicted from a double-grunt's acoustic features. Gorillas apparently produce at least two acoustically different subtypes of double-grunts, each of which conveys different information. Double-grunts with a low second formant (typically < 1600 Hz) are given by animals after a period of silence and frequently elicit vocal replies. Double-grunts with a high second formant (typically > 1600 Hz) are given by animals within 5 s of a call from another individual and rarely elicit replies. (C) 1994 Wiley-Liss, Inc.