Factors influencing the prevalence and handedness for throwing in captive chimpanzees (Pan troglodytes)

Abstract

Humans throw right-handed, and it has been suggested that the neurophysiological demands of aimed throwing may have served as a precursor to the evolution of left hemisphere specialization for linguistic functions. Although there are descriptions of throwing by wild and captive chimpanzees (Pan troglodytes), systematic observations of aimed throwing and handedness have not been reported. In this article, evidence of population-level right-handedness for throwing is reported in 2 samples of captive chimpanzees. It is further reported that right-handed throwing is more pronounced than other measures of handedness in captive chimpanzees. The implications of these findings are discussed in the context of theories relating throwing to the evolution of lateralization for language functions.

Figure 1

Four sequential frames demonstrating a chimpanzee throwing a polyvinyl chloride (PVC) pipe toward a human in a tower above the subject. The chimpanzee stands bipedally, brings the PVC pipe back, and then throws the object. Note that the force of the chimpanzee’s throw causes him to leave the ground.

Figure 2

Funnel plot of the distribution of percentage of right-hand use (represented by black diamonds) for each subject in relation to the number of observed throwing events. The values represented by the white circles indicate the percentage of right- or left-hand use needed to be classified as left- or right-handed. Black diamonds falling in the middle of the funnel represent subjects with no hand preference. Black diamonds falling outside the funnel indicate subjects with significant left- or right-hand preference on the basis of z scores.

Figure 3

The observed and expected frequencies of concordance in hand preference between siblings.

Figure 4

Mean handedness indices for four different behavioral measures of handedness in the University of Texas M. D. Anderson Cancer Center and Yerkes chimpanzees. Handedness indices (HI) were calculated following the formula HI = [(#R − #L)/(#R + #L)], where #R and #L are the number of right-handed and left-handed responses, respectively.