Hand Use and Gestural Communication in Chimpanzees (Pan troglodytes)

Contrary to the historical accounts (Warren, 1980), recent evidence suggests that hemispheric specialization is not unique to humans and can be found for a variety of tasks in numerous species (Bradshaw & Rogers, 1993; Fagot & Vauclair, 1991; Hopkins, 1996b; MacNeilage, Studdert-Kennedy, & Lindblom, 1987; Ward & Hopkins, 1993). One area of investigation in the study of hemispheric specialization has been communication. Specifically, whether animals exhibit similar forms of hemispheric specialization either in their natural communication or in the use of artificial communication systems has been a topic of interest since the earliest reports of laterality in humans (Harris, 1993). With the exception of data on bird song (Nottebohm, 1977), early studies in a variety of animal species failed to report any evidence of hemispheric specialization in communicative functions. In fact, the complete absence of evidence for hemispheric specialization of any form in animals led some to speculate that language and hemispheric specialization evolved de novo in humans (Corballis, 1991; Hamilton, 1977; Warren, 1980). Notwithstanding, whether communicative behaviors represent a unique class of lateralized behavior similar to that observed in humans remains relatively unstudied but warrants investigation given the inherent interest in linking the evolution of communication with organizational properties of the central nervous system (see Falk, 1987; Hewes, 1973; Kimura, 1993; Passingham, 1982). In female mice, Ehret (1987) found that the processing of ultrasonic pup sounds were better processed by the left than the fight hemisphere. In Japanese macaques, Peterson, Beecher, Zoloth, Moody, and Stebbins (1978) reported a right-ear advantage in the acquisition of a vocalization discrimination task. Using a more ethological approach, Hauser (1993) recorded the movements of the mouth and face when rhesus monkeys were vocalizing and reported that the left half of the mouth and face moved before the fight half. These results were interpreted as evidence of a right-hemisphere asymmetry in the production of facial expressions by rhesus monkeys. In a subsequent study, Hauser and Andersson (1994) reported a fight-sided sound localization bias in the perception of conspecific calls but not heterospecific calls (i.e., bird calls) in free-living rhesus monkeys. These findings were interpreted as evidence of a left-hemisphere asymmetry in the processing of species-specific vocalizations. In terms of artificial communication systems, both language-trained chimpanzees and dolphins have been reported to exhibit a left-hemisphere asymmetry in the processing of meaningful as opposed to nonmeaningful symbols or signs (Hopkins, Morris, & Savage-Rumbaugh, 1991; Morrel-Samuels, Herman, & Bever, 1989). Previous studies examining lateralization in natural communicative behaviors have emphasized either the perception of auditory stimuli or the production of species-specific vocalizations. As far as we know, there have been no reports of lateral bias in gestural rather than vocal communication in any nonhuman primate species with the exception of preliminary reports on captive bonobos (Hopkins & de Waal, 1995) and chimpanzees (Leavens, Hopkins, & Bard, 1996). Hopkins and de Waal reported that bonobos exhibited more species-typical gestures with their right than with their left hand. In the study by Leavens et al., 3 captive chimpanzees failed to show any strong evidence of laterality in gestures with the exception of those involving a single digit. On the basis of these preliminary results, bonobos appear to gesture with their fight hand whereas chimpanzees do not, suggesting an inherent species difference in communicative abilities between these two species, as has been suggested by some (Savage-Rumbaugh, 1984, 1991). However, the gestures observed by Hopkins and de Waal in bonobos were species typical, whereas the gestures studied by Leavens et al. were of a type seldom, if ever, observed in wild chimpanzees (i.e., pointing). The purpose of this study was to examine the occurrence and lateralization in hand use for gestural communication in captive chimpanzees. Of particular interest was the interaction between laterality and the expression of species-typical gestures. If laterality in hand use is contingent on the use of species-typical gestures, then chimpanzees exhibiting food begs should be more likely to use their fight hand than chimpanzees who exhibit gestures that are not species typical (i.e., pointing). Additionally, laterality in gestures in relation to rearing and subject variables was of interest.