Multiple expressions of hemispheric asymmetry in captive chimpanzees

Abstract

The degree to which non-human primate behaviour is lateralized, at individual or population levels, remains controversial and over the last century, the issue of brain lateralization in primates has been extensively researched and debated, yet no previous study has reported eye preference or head turning in great apes. This thesis examines three different expressions of hemispheric asymmetry in lateralized behaviours: hand preference for bipedal tool use, eye preference, and auditory laterality. It is reported that bipedalism induced the subjects to become more lateralized, but not in any particular direction. Instead, it appeared that subtle pre-existing lateral biases, to the right or left, were emphasized with increasing postural demands. Eye preference was assessed when animals looked through a hole, using one eye, at an empty box, a mirror, a picture of a dog, a rubber snake, food biscuits, bananas, a rubber duck and a video camera. Main effects of stimulus type were reported for direction of eye preference, number of looks, and looking duration, but not for strength of eye preference. A left-eye bias was found for viewing the rubber snake and a right eye bias was found for viewing the bananas. In addition, a significant shift in eye preference took place from the initial look to subsequent looks when viewing the snake. The results reported are not consistent with the literature for other primate studies. Lastly, auditory laterality was assessed using the Hauser and Andersson (1994) head turning paradigm. Chimpanzee and American crow calls were broadcast to subjects from 180° behind them and directional head turning was recorded. No difference in turning direction or latency was found. This lack of result was attributed to the methodology and underlying assumption that head turning is directly related to hemispheric asymmetries and not influenced by any other processes.