12/16/2023 0 Comments Chimpanzee hand top![]() In addition to the data on the distribution of hand preferences in wild chimpanzees, we also report evidence that hand preferences run in families of wild chimpanzees, at least in terms of the association between offspring and their mothers' and siblings' hand preferences. In the case of wild chimpanzees ( 28), significant sex differences were found for bimanual feeding, with males showing left-handedness and females showing right-handedness therefore, characterizing population-level handedness was sex dependent. ![]() In fact, the most compelling evidence of population-level handedness in wild apes has been for measures of bimanual feeding in wild gorillas and chimpanzees ( 27, 28). For example, in studies with captive chimpanzees, the importance of measuring coordinated bimanual actions has been emphasized by a number of investigators ( 16, 17, 27), and rarely have coordinated bimanual actions been studied in wild apes. Moreover, the types of handedness measured in captive and wild chimpanzees vary dramatically and make comparison of the findings in these two settings very difficult. Studies of captive chimpanzees have significantly more statistical power and therefore are more sensitive to detecting effects compared with studies of wild chimpanzees. For example, nearly all of the studies in wild chimpanzees have relatively small sample sizes compared with reports about captive apes ( 11, 38). In contrast, others have suggested that the absence of population-level handedness in wild chimpanzees reflects a lack of sophistication of the behaviors measured and limited sample sizes within a given study ( 11). The discrepancy in findings between captive and wild apes, notably chimpanzees, has prompted some to argue that the findings from captive apes are due to the individuals being reared in a human, right-handed environment and that population-level asymmetry in hand use is not a species-typical trait ( 10, 12). In contrast to findings in captive great apes, evidence of population-level handedness in wild apes is extremely rare ( 27- 29) or virtually absent in the case of chimpanzees ( 30- 37). These findings are consistent with reports in other captive great apes including gorillas (refs. Recent studies in captive chimpanzees have reported evidence of population-level right-handedness for several measures, including simple reaching ( 13, 14), bimanual feeding ( 15), coordinated bimanual actions ( 16, 17), throwing ( 18), and manual gestures ( 19). Notwithstanding the positive findings in lower vertebrates, one area that continues to be a source of considerable empirical and theoretical debate is whether nonhuman primates, particularly great apes, show population-level handedness ( 9- 12). Collectively, these findings raise questions about the assumption that language is a necessary condition for the expression of laterality. Other studies have shown that frogs show right paw preferences for certain motor actions such as removing substrates from their body ( 7), and fish show preferential looking biases with one eye or the other when viewing a predator ( 8). For example, studies in chickens and pigeons have shown different abilities of the left and right hemispheres for different visual discrimination tasks ( 6). However, recent studies in a host of vertebrate species have demonstrated evidence of population-level behavioral asymmetries ( 5). ![]() This argument rests primarily on the lack of evidence for population-level handedness in nonhuman primates, particularly in our closest living relative, the chimpanzee. Right-handedness is a universal trait of humans ( 1), and some have argued that population-level handedness is unique to hominid evolution ( 2- 4). Contrary to previous claims, our results demonstrate that populationlevel handedness is evident in wild chimpanzees and suggest that the antecedents of lateralization of function associated with hand use were present at least 5 million years ago, before the Pan- Homo split. Last, when combing all of the published data on tool use in wild chimpanzees, we show that hand preferences are heritable. Here, we report evidence of population-level handedness in wild chimpanzees for a tool-use task known as “termite-fishing.” We subsequently compared the handedness for termite-fishing with other published reports on handedness for nut-cracking and wadge-dipping and found task-specific differences in handedness. One continued subject of discussion is whether evidence of population-level handedness in primates is confined to studies in captive animals or whether it is in both captive and wild subjects. Whether nonhuman primates exhibit population-level handedness remains a topic of considerabe theoretical and empirical debate.
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