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Analysis of Handedness and Laterality Through Observation of Bipedal Feeding in Captive Chimpanzees

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by Blair Quinius

The human brain is an intricate system of highly specialized areas that coordinate our daily functions. Hemispheric specialization is a unique evolutionary characteristic, allowing the brain to be divided into separate halves, each performing specific activities. This conserves volume without decreasing the capacity to execute new functions. Hemispheric specialization and laterality are areas of interest that have been studied for many years in humans, however, not much is known about these topics in other species. Recently, laterality in chimpanzees has become a topic of growing interest in comparative psychology due to the close evolutionary relationship between humans and chimpanzees. Highly illustrative ways of studying hemispheric specialization in chimpanzees include analyses of handedness, grip type, and fingerprint patterns.

Handedness is an indicator of laterality and hemispheric specialization because the use of a dominant hand suggests that one specific hemisphere is in control of producing certain behaviors. A large number of handedness tasks need to be assessed in chimpanzees to obtain a full understanding of laterality and hemispheric specialization. This experiment looked at a specific aspect of handedness in feeding that has never been studied before, bipedal feeding tasks. Bipedal feeding means the chimpanzees are observed while standing on only their feet as they reach for a suspended food item with one or both hands. This feeding task mimics situations in the wild in which a chimpanzee must reach for food and still have posture control. Among the broad array of handedness-related behaviors, this is a more complex feeding task that may provide unique insights into the specializations required for behaviors such as feeding, foraging, and gestural communication.

The methods of the experiment were fairly straightforward. The experiment consisted of suspending food, connected to fishing line, from the roof directly above the chimpanzees. The food hung almost out of the chimpanzee¹s reach causing them to stand up on both feet to obtain the food. Data was collected from 50 chimpanzees (a minimum of 30 responses per individual) over a period of 15-20 weeks, 1-2 days per week. The hand and grip type used to procure the food were recorded and their relationships were analyzed.

This experiment revealed many interesting results. The results suggest and provide stronger evidence than before that there are individual hand preferences among chimpanzees. However, no population-level preference was shown for the right hand. The high percentage of individual-level hand preference could provide support for the theory that population-level handedness developed as a result of bipedalism. This would explain why population-level hand preference is evident in humans, who are bipedal, and only sometimes in chimpanzees, who act in a bipedal manner for only certain tasks. Hand preference also appears to have relationships with grip type. Chimpanzees with a right-hand preference were twice as likely to use a more complex grip type (thumb-index) than left-handed chimpanzees. These results provide evidence in support of hemispheric specialization of motor skills in chimpanzees.

Many handedness tasks need to be studied, but bipedal feeding is an innovative way of observing handedness in chimpanzees. This novel technique for determining handedness provided a broader range of data from which we can determine relationships of laterality and brain specialization. These findings could provide many answers to questions related to the human brain and its development. Since chimpanzees are the closest living relatives of humans, innovative research, such as this, should provide valuable data for understanding the evolution of the unique cognitive, intellectual, and linguistic capabilities of humans and Great Apes.