When zoo-living gorillas perform communicative physical motions that seem to resemble those of signing gorillas, what does it mean? What are the processes involved in developing these gestures? What communicative behavior is universal for the species and what is individually learned? Though some gestures are shared by all gorillas and others are unique to individuals, such a simple dichotomy does not really tell the whole story. We have found that, in the gorilla group at the San Francisco Zoo, “species typical” expressions such as slapping, clapping, pounding, and chest beating develop quite differently in each individual. Individuals also create gestures, some of which have not been described for other gorillas. Other gestures are shared by a few, but not all, individuals in a group. Gestural repertoires of different individuals at the same ages vary both in type and quantity. There is variation in individual gorillas' usage of gestures over time and in accord with changing social conditions. The purpose of the present research is to describe this variation, explore the diverse physical and functional properties of these gestures, and learn why and how they have developed in this particular captive group of gorillas.

Before beginning the observations at the San Francisco Zoo that are described here, the first author (JT) had worked for 8 years with the signing gorillas Koko and Michael of the Gorilla Foundation, developing a particular interest in untaught signs and “natural” gestures of sign language tutored gorillas (Patterson, Tanner, & Mayer, 1988; Patterson & Tanner, 1988).