Adrien Meguerditchian

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.

Gray Matter Asymmetries in Chimpanzees as Revealed by Voxel-Based Morphometry

Determination of whether nonhuman primates exhibit neuroanatomical asymmetries would inform our understanding of the evolution of traits in humans that show functional hemispheric dominance, including language and handedness. Here we report the first evidence of population-level asymmetries in the chimpanzee neocortex using voxel-based morphometry (VBM). MRI scans of the brain were collected in a sample of 31 chimpanzees including 9 males and 22 females, and the resulting images were segmented into gray matter, white matter and CSF. Gray matter images were then co-registered to a template and these normally oriented volumes were flipped on the left-right axis to create mirror volumes. In total, significant asymmetries were found in 13 regions including several that have been described previously in great apes using traditional region-of-interest approaches. The results from this VBM analysis support previous reports of hemispheric lateralization in chimpanzees and reinforce the view that asymmetries in the central nervous system are not uniquely human.

Brief communication: Captive gorillas are right-handed for bimanual feeding.

Predominance of right-handedness has historically been considered as a hallmark of human evolution. Whether nonhuman primates exhibit population-level manual bias remains a controversial topic. Here, we investigated the hypothesis that bimanual coordinated activities may be a key-behavior in our ancestors for the emergence and evolution of human population-level right-handedness. To this end, we collected data on hand preferences in 35 captive gorillas (Gorilla gorilla) during simple unimanual reaching and for bimanual coordinated feeding. Unimanual reaching consisted of grasping food on the ground, while bimanual feeding consisted of using one hand for holding a food and processing the food item by the opposite hand. No population-level manual bias was found for unimanual actions but, in contrast, gorillas exhibited a significant population-level right-handedness for the bimanual actions. Moreover, the degree of right-handedness for bimanual feeding exceeds any other known reports of hand use in primates, suggesting that lateralization for bimanual feeding is robust in captive gorillas. The collective evidence is discussed in the context of potential continuity of handedness between human and nonhuman primates.

The sound of one-hand clapping: handedness and perisylvian neural correlates of a communicative gesture in chimpanzees

Whether lateralization of communicative signalling in non-human primates might constitute prerequisites of hemispheric specialization for language is unclear. In the present study, we examined (i) hand preference for a communicative gesture (clapping in 94 captive chimpanzees from two research facilities) and (ii) the in vivo magnetic resonance imaging brain scans of 40 of these individuals. The preferred hand for clapping was defined as the one in the upper position when the two hands came together. Using computer manual tracing of regions of interest, we measured the neuroanatomical asymmetries for the homologues of key language areas, including the inferior frontal gyrus (IFG) and planum temporale (PT). When considering the entire sample, there was a predominance of right-handedness for clapping and the distribution of right- and left-handed individuals did not differ between the two facilities. The direction of hand preference (right- versus left-handed subjects) for clapping explained a significant portion of variability in asymmetries of the PT and IFG. The results are consistent with the view that gestural communication in the common ancestor may have been a precursor of language and its cerebral substrates in modern humans.

Left Hemisphere Specialization for Oro-Facial Movements of Learned Vocal Signals by Captive Chimpanzees

Background The left hemisphere of the human brain is dominant in the production of speech and signed language. Whether similar lateralization of function for communicative signal production is present in other primates remains a topic of considerable debate. In the current study, we examined whether oro-facial movements associated with the production of learned attention-getting sounds are differentially lateralized compared to facial expressions associated with the production of species-typical emotional vocalizations in chimpanzees. Methodology/ Principal Findings Still images captured from digital video were used to quantify oro-facial asymmetries in the production of two attention-getting sounds and two species-typical vocalizations in a sample of captive chimpanzees. Comparisons of mouth asymmetries during production of these sounds revealed significant rightward biased asymmetries for the attention-getting sounds and significant leftward biased asymmetries for the species-typical sounds. Conclusions/Significance These results suggest that the motor control of oro-facial movements associated with the production of learned sounds is lateralized to the left hemisphere in chimpanzees. Furthermore, the findings suggest that the antecedents for lateralization of human speech may have been present in the common ancestor of chimpanzees and humans ∼5 mya and are not unique to the human lineage.

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.

Termite fishing laterality in the fongoli savanna chimpanzees (Pan troglodytes verus): Further evidence of a left hand preference

Whether nonhuman primates show population-level handedness is a topic of much scientific debate. A previous study of handedness for termite fishing reported population-level left handedness in the chimpanzees from Gombe National Park, Tanzania. In the current study, we examined whether similar hand preferences were evident in a savanna-dwelling chimpanzee population with regards to termite fishing. Hand preference data were collected for 27 chimpanzees from February 2007 through July 2008 and November 2011 through January 2012 in southeastern Senegal. Overall, the Fongoli chimpanzees demonstrate a trend toward population-level handedness, though the results did not reach conventional levels of statistical significance likely due to the limited sample size. Fongoli chimpanzees showed the same pattern of left hand preference as reported at Gombe and the two populations did not differ significantly. When the data were combined across all studies, wild chimpanzees showed a population-level left hand preference for termite fishing.

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.