Jacques Vauclair

Baboons communicate with their right hand.

Humans are mainly right-handed for many actions including gestures. This bias is strongly linked to a left cerebral hemispheric dominance for language functions. Whether similar lateralized systems for communicative behaviors are present in other animals is unclear. Here we report the first evidence of strong population-level right-handedness in 60 captive baboons for a species-specific communicative manual gesture. Our findings support the view that lateralization for language may have evolved from a gestural system of communication controlled by the left hemisphere.

Captive chimpanzees use their right hand to communicate with each other: Implications for the origin of the cerebral substrate for language

Whether precursors of the left-lateralization for human language can be found in the vocal and gestural communication systems of nonhuman primates remains a topic of intense research, particularly within theoretical discussions of the evolutionary origins of language. Although previous studies in chimpanzees have reported evidence of right-handedness for inter-species food-beg gestures produced exclusively toward humans, some might question the generality of these results to intra-species communicative signals. To address this issue, we recorded hand use in 70 captive chimpanzees for species-typical signals, that could be directed either toward conspecifics or humans. We found evidence of a predominance of right-handedness for species-typical gestures in captive chimpanzees when directed to both humans and conspecifics. Hand preferences during intra-species communication were significantly and positively correlated with gestures directed toward humans. By contrast, hand preferences for gestures did not significantly correlate with hand use for a non-communicative self-directed action. The collective results suggest (a) that evidence of predominance of right-handedness for human-directed gestures communication is not specific to this context and (b) the existence of a specific communicative system involving gestures constitutes an ideal prerequisite for the cerebral substrates of human language and its typical left-lateralization.

On the Origins of Human Handedness and Language: A Comparative Review of Hand Preferences for Bimanual Coordinated Actions and Gestural Communication in Nonhuman Primates

Within the evolutionary framework about the origin of human handedness and hemispheric specialization for language, the question of expression of population-level manual biases in nonhuman primates and their potential continuities with humans remains controversial. Nevertheless, there is a growing body of evidence showing consistent population-level handedness particularly for complex manual behaviors in both monkeys and apes. In the present article, within a large comparative approach among primates, we will review our contribution to the field and the handedness literature related to two particular sophisticated manual behaviors regarding their potential and specific implications for the origins of hemispheric specialization in humans: bimanual coordinated actions and gestural communication. Whereas bimanual coordinated actions seem to elicit predominance of left-handedness in arboreal primates and of right-handedness in terrestrial primates, all handedness studies that have investigated gestural communication in several primate species have reported stronger degree of population-level right-handedness compared to noncommunicative actions. Communicative gestures and bimanual actions seem to affect differently manual asymmetries in both human and nonhuman primates and to be related to different lateralized brain substrates. We will discuss (1) how the data of hand preferences for bimanual coordinated actions highlight the role of ecological factors in the evolution of handedness and provide additional support the postural origin theory of handedness proposed by MacNeilage [MacNeilage [2007]. Present status of the postural origins theory. In W. D. Hopkins (Ed.), The evolution of hemispheric specialization in primates (pp. 59-91). London: Elsevier/Academic Press] and (2) the hypothesis that the emergence of gestural communication might have affected lateralization in our ancestor and may constitute the precursors of the hemispheric specialization for language.

Judgment of conceptual identity in monkeys.

Baboons (Papio anubis) were tested on categorization tasks at two different conceptual levels. The monkeys showed their ability (1) to judge as identical or different the objects belonging to two categories, on a perceptual basis, and (2) to perform a judgment of conceptual identity—that is, to use the same/different relation between two previously learned categories. This latter experiment represents the first demonstration of judgment of conceptual identity in a monkey species.

Relationship between manual preferences for object manipulation and pointing gestures in infants and toddlers

The aim of this study was to measure the pattern of hand preferences for pointing gestures as a function of object-manipulation handedness in 123 infants and toddlers (10-40 months). The results showed that not only right-handers but also left-handers and ambidextrous participants tended to use their right hand for pointing. There was a significant correlation between manual preferences and pointing lateralization. Further analyses showed that the correlation between these two indexes was at its strongest during two key phases of language development (i.e. vocabulary spurt and syntax improvement) and weakened to become nonsignificant in the interim. These findings support the view that humans have a specialized area for communicative gestures and language in the left cerebral hemisphere that may be independent of the system controlling the purely motor functions of hand use.

Hand preference for pointing gestures and bimanual manipulation around the vocabulary spurt period

This study investigated the development of hand preference for bimanual manipulative activities and pointing gestures in toddlers observed longitudinally over a 5-month period, in relation to language acquisition. The lexical spurt was found to be accompanied by an increase in the right-sided bias for pointing but not for manipulation. Moreover, results revealed a significant correlation between hand preference for imperative pointing gestures and manipulative activities in children who did not experience the lexical spurt during the observational period. By contrast, measures of handedness for declarative pointing were never correlated with those of handedness for manipulation. This study illustrates the complex relationship between handedness and language development and emphasizes the need to take the different functions of pointing gestures into account.

Right hemisphere dominance for emotion processing in baboons.

Asymmetries of emotional facial expressions in humans offer reliable indexes to infer brain lateralization and mostly revealed right hemisphere dominance. Studies concerned with oro-facial asymmetries in nonhuman primates largely showed a left-sided asymmetry in chimpanzees, marmosets and macaques. The presence of asymmetrical oro-facial productions was assessed in Olive baboons in order to determine the functional cerebral asymmetries. Two affiliative behaviors (lipsmack, copulation call) and two agonistic ones (screeching, eyebrow-raising) were recorded. For screeching, a strong and significant left hemimouth bias was found, but no significant bias was observed for the other behaviors. These results are discussed in the light of the available literature concerning asymmetrical oro-facial productions in nonhuman primates. In addition, these findings suggest that human hemispheric specialization for emotions has precursors in primate evolution.

Infant Holding Preferences in Maternity Hospitals: Testing the Hypothesis of the Lateralized Perception of Emotions

Infant holding biases of 202 mothers were studied in four French maternity hospitals. The study collected laterality for holding in mother/child dyads as a means of testing the emotional hypothesis (Manning & Chamberlain, 1991). Maternal holding side preferences and handedness were collected through questionnaires. In addition, hemispheric specialization for perceiving visual and auditory emotional cues was examined using a chimeric figure and dichotic listening task. The mothers displayed a significant left holding bias as well as a general perceptual bias in favor of the left side/right hemisphere. However, no significant associations were found between holding biases and emotional perceptual asymmetry. The absence of significant relationships between hemispheric specialization and holding biases does not support directly the emotional hypothesis for infant holding but can be interpreted according to the nature of the holding relationship.

Sex difference in squirrel monkeys handedness for unimanual and bimanual coordinated tasks

Whether nonhuman primates exhibit population-level manual bias remains controversial. There is a growing body of evidence showing consistent individual lateralization and population-level handedness particularly for complex manual behaviours such as bimanual coordinated actions in both monkeys and apes. Whereas the few published studies on hand preferences in squirrel monkeys, Saimiri sciureus, have focused only on unimanual behaviours and generally failed to elicit population-level handedness, we report in this paper the first set of data on hand use for a bimanual coordinated behaviour elicited by the manipulation of an artificial tube in a sample of 37 captive-born squirrel monkeys. The task consisted of holding an opaque tube with one hand and removing the food inside the tube with the other hand. For comparison, hand use data were collected from 38 subjects for unimanual reaching for food from an experimenter when the squirrel monkeys were clinging vertically on the door of the cage. For bimanual coordinated actions, we found a nonsignificant trend towards left-handedness at a population level, especially in males, whereas unimanual reaching when clinging elicited a significant predominance of right-handedness, especially in females. These results are discussed within comparative approaches across primate species and within different theoretical frameworks about the determining factors of handedness in nonhuman primates.

Right-Handedness Predominance in 162 Baboons (Papio anubis) for Gestural Communication: Consistency Across Time and Groups

Whether nonhuman primates exhibit population level right manual bias remains a controversial topic. In captive baboons, population-level right-handedness has been reported for both coordinated bimanual actions and communicative gestures. However, some authors remain skeptical of these findings on both methodological and theoretical grounds. Here, we demonstrated the robustness and the consistency across time of the pattern of right-handedness for a species-specific communicative gesture in olive baboons (Papio anubis). First, we showed significant correlations in the 26 retested baboons for the measures of hand preferences between the first and the second session conducted 4 years later (2005-2009) by an observer blind to the previous handedness data. Second, the replication of the study in 96 novel individuals revealed a similar degree of population-level right-handedness than the one expressed in the first group of 66 subjects investigated in 2005. The implications of the findings are discussed within a theoretical framework about the origin of hemispheric specialization for language.