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The literature on manual laterality in nonhuman primates provides inconsistent and inconclusive findings and is plagued by methodological issues (e.g., small samples, inconsistency in methods, inappropriate measures) and gaps. Few data are available on bonobos and these are only from small samples and for relatively simple tasks. We examined laterality in a large sample of bonobos for a complex task. We tested 48 bonobos from Lola Ya Bonobo sanctuary (DR Congo) in an extension of our previous study of 29 bonobos from 3 European zoos. We assessed hand preferences using the tube task, which involves bimanual coordination: one hand extracts food from a tube that is held by the other hand. This task is a good measure of laterality and it has been used in other studies. We recorded events (frequency) and independent bouts of food extraction. We found significant manual laterality, which was not influenced by the settings or rearing history. We observed little effect of sex and found an influence of age, with greater right hand use in adults. The laterality was marked, with strong preferences and most individuals being lateralized (when analyzing frequency). We found individual preferences, with no group-level bias, even when we combined the data from the sanctuary and the zoos to enlarge the sample to 77. These first data, for a complex task and based on a large sample, are consistent with previous findings in bonobos and in other nonhuman primate species for a variety of tasks. They suggest that, despite particular features in terms of proximity to humans, language and bipedalism, bonobos do not display a laterality that is more marked or more similar to human handedness compared to that of other nonhuman primate species.

Grooming is a complex set of motor actions, common in highly social primates. We tested for asymmetries in hand use during unimanual and bimanual allogrooming in 215 captive chimpanzees. In addition to hand use, we coded in the ethogram whether the manual grooming action co-occurred with the use of the mouth. Overall, grooming did not elicit strong handedness at the individual level, but there is a small yet significant population-level right-hand bias for bimanual grooming. Mouth use during grooming had no influence on hand use. A comparison of the findings with previously published data on handedness for grooming in wild chimpanzees suggests that wild apes are more right-handed than captive individuals are for allogrooming. Collectively, the results suggest that role differentiation of the hands is an important factor in the assessment of handedness for grooming, and perhaps additional manual actions of chimpanzees and other primates.

We investigated laterality in nipple preference in 16 mother-infant dyads, 8 bonobos, and 8 chimpanzees via an event sampling procedure in infants 5-55 mo of age. Both bonobo and chimpanzee infants preferred the left nipple, and the results were stable over time. Maternal age, experience, and rearing history did not influence laterality in offspring nipple preferences. We discuss the results in the context of the role of early mother-infant interactions on the development of laterality in primates.

Left-sided maternal cradling has been widely reported in human populations. In this paper, I review the evidence of laterality in maternal cradling and infant positional biases in non-human primates. The review revealed some evidence of population-left sided cradling in great apes but little consistency in bias was found among Old and New World monkeys. Very little data have been reported in prosimians. I further describe how asymmetries in either maternal cradling or infant positional biases may explain individual and species differences in hand preference.

The purpose of this study was to evaluate the reliability of previously published findings on hand preferences in chimpanzees by evaluating hand use in a second colony of captive chimpanzees. We assessed hand preferences for a coordinated bimanual task in 116 chimpanzees (Pan troglodytes) at the University of Texas M. D. Anderson Cancer Center and compared them to previously published findings in captive chimpanzees at the Yerkes National Primate Research Center. The new sample showed significant population-level right handedness, which is consistent with previously published findings in the Yerkes chimpanzees. Combined data on the 2 chimpanzee colonies, revealed a significant effect of rearing history on hand preference, with wild-caught chimpanzees showing less right-handedness than captive-born mother-reared chimpanzees. We discuss the results in terms of the role of early environment on the development of laterality.

We tested whether chimpanzee handedness could be characterized as either unidimensional or multidimensional when considered across multiple measures of hand use. We determined for each of 6 different tasks in a sample of 105 captive chimpanzees hand preferences, and subjected the individual hand preference scores to a factor analysis. Five of the 6 tasks loaded on two separate factors that accounted for 54% of the variance. To assess population-level handedness, we calculated handedness indices for the loadings on each factor, for the item loadings across all factors, and for all tasks including ones that did not load on any factor. There is significant population-level right handedness for all 4 indices, which suggests that chimpanzee handedness is multidimensional and not task specific.

I describe methodological and statistical issues in the assessment of hand preference in nonhuman primates and discuss them in the context of a recent paper by McGrew and Marchant (1997) in which they conclude that there is no convincing evidence of population-level hand preferences in nonhuman primates. The criteria used by them to evaluate individual and population-level hand preferences are flawed, which results in an oversimplification of findings in nonhuman primates. I further argue that the classification schema used by McGrew and Marchant (1997) to compare hand preference distributions between species is theoretically weak and does not offer a meaningful way to compare human and nonhuman primate handedness.

We tested the hand preferences of 20 chimpanzees (Pan troglodytes) for a haptic task requiring individuals to search for grapes in an opaque bucket filled with water. We compared these data to the hand preferences displayed by the same chimpanzees during reaching and bimanual feeding tasks. The chimpanzees displayed no significant hand preference for the reaching or bimanual feeding tasks, but exhibited a right-hand preference while performing the haptic task. In contrast, New and Old World monkeys display left-hand preferences for similar tasks. We discuss the relevance of these findings for the evolution of handedness in primates.

We assessed laterality for leading limb in crawling and leading limb in stepping in 13 chimpanzees (Pan troglodytes) during the first 3 months of life. Overall, there is a significant populational right-side bias for crawling but not for stepping. There is a significant negative correlation between laterality in stepping and crawling. The females are more right-sided in lateral bias than the males across both measures. These data suggest that asymmetries in postural organization are present early in life and are specific to the inherent locomotor behavior of a given species.

We report observational data on behavioral laterality in 10 captive bonobos (Pan paniscus)at the San Diego Zoo. The unimanual measures include carrying, leading limb in locomotion, self-touching, face-touching, reaching, and gestures. We also recorded bimanual feeding in these subjects. A significant population level left-hand bias exists for carrying. Right-hand biases occur for leading limb in locomotion and gestures. During bimanual feeding, the bonobos hold food items with the left hand while feeding with the right hand. Overall, bonobos exhibit behavioral asymmetries that are similar to previous findings in other pongid ape species. The asymmetries in gestures and bimanual feeding represent novel findings with theoretical implications for the origins of tool use and language.

We present data on hand preference in great apes and discuss them in the context of theoretical models of hand preference in nonhuman primates presented by MacNeilageet al. (1987) and by Fagot and Vauclair (1991). We also discuss several methodological and statistical issues as they pertain to the assessment of hand preference in great apes and other primate species. Finally, we present a comparative framework for the study of hand preference, emphasizing the importance of studies with great apes in developing evolutionary models of hemispheric specialization.

There is little evidence of vocal learning in nonhuman primates despite the well-documented abilities found in avian species. We describe the vocal repertoire of five bonobos (Pan paniscus), four of which live in a seminatural environment. The fifth subject, Kanzi, has been reared with humans during the course of language training. The data indicated that the four bonobos living in a seminatural environment exhibit a variety of species-typical vocalizations. In addition to producing all the species-typical vocalization, Kanzi produced four structurally unique vocalizations that were not heard among the other subjects. These data suggest that Kanzi has learned vocalizations that are novel due to his unique rearing experience. Discussion is focused on the flexibility of vocal communication and vocal comprehension inPan paniscus.

The purpose of this study was to evaluate the reliability of previously published findings on hand preferences in chimpanzees by evaluating hand use in a second colony of captive chimpanzees. We assessed hand preferences for a coordinated bimanual task in 116 chimpanzees (Pan troglodytes) at the University of Texas M. D. Anderson Cancer Center and compared them to previously published findings in captive chimpanzees at the Yerkes National Primate Research Center. The new sample showed significant population-level right handedness, which is consistent with previously published findings in the Yerkes chimpanzees. Combined data on the 2 chimpanzee colonies, revealed a significant effect of rearing history on hand preference, with wild-caught chimpanzees showing less right-handedness than captive-born mother-reared chimpanzees. We discuss the results in terms of the role of early environment on the development of laterality.