Hand preferences of captive chimpanzees (Pan troglodytes) in simple reaching for food
Rikako Tonooka, Tetsuro Matsuzawa
We examined chimpanzee hand preference in simple reaching for food, with special reference to manipulative patterns and the developmental shift. We observed 80 captive chimpanzees, ranging from 1 to 25 years old. We also studied the manipulative patterns (grip-types) of 70 individuals as they reached for raisins scattered randomly on the floor. We employed LQ score as a measure of hand preference and designated the subjects right-handers (or left-handers) if they used their right hands (left hands) above chance level. Although the numbers of right-handers and left-handers are almost equal the distribution of the strength is not symmetrical in both groups. Strong preference was exhibited by more left-handers than right-handers. Subjects >9 years old exhibited greater hand preference, whereas subjects <9 years old were ambidextrous. We classified manipulative patterns for reaching into five basic grip-types and analyzed them vis-a-vis age. There is no significant correlation between preferred hand and manipulative patterns. However, adult subjects tended to use an index- and -middle-finger grip with the left hand and to use imprecise grips with the right hand more often than other patterns regardless which hand they preferred. These data demonstrate a developmental shift in hand preference and manipulative patterns and also reveal functional asymmetries between the right and the left hand in Pan troglodytes.