Evidence of Language Prerequisites in Pan: Orofacial-Motor and Breath Control in Chimpanzees and Bonobos
Human spoken language requires the concomitant utilization of numerous cognitive and motor skills. Two particularly relevant skills are orofacial-motor control (OFM), the ability to purposefully move one’s facial muscles, and breath control (BC), subglottal air pressure that fuels sound production, as both are necessary in the voluntary production of speech. Many have claimed these competencies are uniquely human qualities without great ape antecedents. However, here we describe both skills in genus Pan, which contains our closest extant relatives: chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). We hypothesized that OFM and BC would be present in both species of Pan and that bonobos would demonstrate increased OFM and BC due to their stronger reliance on vocal communication compared to chimpanzees. To test this hypothesis, different groups of a total of forty-three apes (24 chimpanzees and 19 bonobos) were trained to protrude their lower lip and tongue, inhale to retrieve a food item, and exhale to elevate a ball to a certain height in a clear cylinder. Apes underwent 50 OFM trials per condition, and the number of times the requested action was completed successfully was recorded. No significant differences were found between species for lower lip protrusions (t(42)=0.59, p=0.55); however, bonobos were significantly better at tongue protrusions (t(36)=4.46, p< 0.001). Diffusion tensor images were available for a subset of the chimpanzee sample (n=17). These subjects were then divided into high and low orofacial-motor performers and differences in intrahemispheric connectivity and FA values of the left and right inferior of the precentral gyri (IPrCG; the area responsible for mouth movements in chimpanzee brains) were examined with no significant differences between groups (left IPrCG: t(15)=.58, p=0.57; right IPrCG: t(15)=- 1.03, p=0.32; right and left IPrCG: t(15)=-0.244, p=0.81). For BC, apes underwent 40 trials for both inhalation test trials and exhalation test trials; success and time it took to succeed were recorded. Significantly more chimpanzees reached inhale training criterion (Z(33)=-2.0737, p=0.03), and chimpanzees were more often successful in both conditions of inhale test trials (5inch tube: (t(33)=-2.27049, p=0.03); 9.5inch tube: (t(33)=-3.14644, p=0.003)). There were no significant differences between species in reaching exhale training criterion (Z(33)=-1.5958, p=0.11). These data suggest that these two language prerequisites existed at a rudimentary level prior to the Pan/Homo split and, thus, evolved outside of the hominid lineage.
Language Evolution, Chimpanzees, Bonobos, Orofacial-Motor Control, Breath Control, Biology, Integrative Biology
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