Foot use and hand preference during feeding in captive black-and-white snub-nosed monkeys (Rhinopithecus bieti)

Postural origin theory predicts that body postures are related to hand preference in nonhuman primates due to hemispheric specialization. Foot preference, especially in manipulating objects, is also a good predictor of hemispheric specialization in humans. We studied limb (hand and foot) preferences in 11 captive adult black-and-white snub-nosed monkeys (Rhinopithecus bieti) to see how limb preference is influenced by postures and foot manipulation. Hand preference was significantly different for this group between quadrupedal standing and clinging postures, and sitting and clinging postures, but not between bipedal standing and clinging postures. Individuals were significantly more likely to use the right hand in the clinging posture than in quadrupedal standing or sitting postures. In the sitting posture, individuals maintained their respective hand preference even when the food was on the other side of the body. There was a gender difference in the sitting posture, where females preferred their right hand but males preferred their left. Individuals who did not routinely use their feet to manipulate objects, compared to those who did, shifted to greater right hand use from the clinging posture to the bipedal posture. One male individual and his offspring were more likely to use their feet to manipulate objects than the rest of the monkeys. In the present study, we reveal the first evidence of a postural effect on hand preference in R. bieti as well as a foot preference in this species. Our results mostly agree with the postural origin theory and hemispheric specialization.

Comparing human and nonhuman primate handedness: challenges and a modest proposal for consensus

In the past 20-25 years, there have been a number of studies published on handedness in nonhuman primates. The goal of these studies has been to evaluate whether monkeys and apes show patterns of hand preference that resemble the right-handedness found in the human species. The extant findings on handedness in nonhuman primates have revealed inconsistent evidence for population-level handedness within and between species. In this article, I discuss some of the methodological and statistical challenges to comparative studies of handedness in human and nonhuman primates. I further offer a framework for developing some consensus on evaluating the validity of different handedness measures and the characterization of individual hand preferences.

Comparative and familial analysis of handedness in great apes

Historically, population-level handedness has been considered a hallmark of human evolution. Whether nonhuman primates exhibit population-level handedness remains a topic of considerable debate. This paper summarizes published data on handedness in great apes. Comparative analysis indicated that chimpanzees and bonobos show population-level right handedness, whereas gorillas and orangutans do not. All ape species showed evidence of population-level handedness when considering specific tasks. Familial analyses in chimpanzees indicated that offspring and maternal (but not paternal) handedness was significantly positively correlated, but this finding was contingent upon the classification criteria used to evaluate hand preference. Overall, the proportion of right handedness is lower in great apes compared with humans, and various methodological and theoretical explanations for this discrepancy are discussed.

Wild chimpanzees show population-level handedness for tool use

Whether nonhuman primates exhibit population-level handedness remains a topic of considerable theoretical and empirical debate. 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. 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. Last, when combing all of the published data on tool use in wild chimpanzees, we show that hand preferences are heritable. Contrary to previous claims, our results demonstrate that population-level 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.

A Meta-Analysis of Primate Hand Preferences, Particularly for Reaching

P. F. MacNeilage, M. G. Studdert-Kennedy, and B. Lindblom (1987) proposed a progression for handedness in primates that was supposed to account for the evolution of a right bias in human handedness. To test this proposal, the authors performed meta-analyses on 62 studies that provided individual data (representing 31 species: 9 prosimians, 6 New World monkeys, 10 Old World monkeys, 2 lesser apes, and 4 greater apes), of the 118 studies of primate handedness published since 1987. Although evidence of a population-level left-handed bias for prosimians and Old World monkeys supports P. F. MacNeilage et al., the data from apes, New World monkeys, and individual species of prosimians and New World monkeys do not. Something other than primate handedness may have been the evolutionary precursor of the right bias in hand-use distribution among hominids.

Comparative assessment of handedness for a coordinated bimanual task in chimpanzees (Pan troglodytes), gorillas (Gorilla gorilla) and orangutans (Pongo pygmaeus)

Hand preferences for a coordinated bimanual task were assessed in a sample of 31 captive gorillas (Gorilla gorilla) and 19 captive orangutans (Pongo pygmaeus) and were compared with chimpanzee (Pan troglodytes) hand preferences in subjects that were matched on the basis of age, sex, and rearing history. The task required that the apes remove food from the inside edges of a symmetrical polyvinyl chloride pipe presented to them in their home cages. The results indicate significant species differences with chimpanzees showing population-level right-handedness and orangutans showing population-level left-handedness. The gorillas showed a nonsignificant trend toward right-handedness. The results are discussed in terms of possible ecological or biomechanical factors that may influence hand preferences in different ape species.

"Right-Trunkers" and "Left-Trunkers": Side Preferences of Trunk Movements in Wild Asian Elephants (Elephas maximus)

In this article, the side preferences of feeding-related trunk movements of free-ranging Asian elephants (Elephas maximus) were investigated for the first time. It is hypothesized that a functional asymmetry of the trunk is necessary to perform skillful feeding movements more efficiently. This might be connected with a corresponding hemispheric specialization. Video recordings of 41 wild elephants provided frequencies and durations of the following trunk-movement categories: object contact, retrieval, and reaching. In each category, individual side preferences were found. The strength of side preferences varied between the trunk-movement categories and the sexes. Mean durations of retrieval and reaching correlated negatively with the strength of side biases. Comparing the side preferences in the unpaired trunk with analogous phenomena in other unpaired grasping organs and in primate handedness. the authors discuss possible explanations for the evolution of asymmetries in unpaired grasping organs.