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Determining the impact that the KIAA0319 gene has on primate brain morphology can provide insight into the evolution of human cognition and language systems. Here, we tested whether polymorphisms in KIAA0319 in chimpanzees account for gray matter volumetric variation in brain regions implicated in language and communication (particularly within the posterior superior temporal gyrus and inferior frontal gyrus). First, we identified the nature and frequencies of single nucleotide variants (SNVs) in KIAA0319 in a sample of unrelated chimpanzees (Pan troglodytes spp.). Next, we genotyped a subset of SNVs (those important for gene regulation or likely to alter protein structure/function) in a sample of chimpanzees for which in vivo T1-structural magnetic resonance imaging scans had been obtained. We then used source-based morphometry (SBM) to test for whole-brain gray matter covariation differences between chimpanzees with different KIAA0319 alleles. Finally, using histologic sections of 15 postmortem chimpanzee brains, we analyzed microstructural variation related to KIAA0319 polymorphisms in the posterior superior temporal cortex. We found that the SNVs were associated with variation in gray matter within several brain regions, including the posterior superior temporal gyrus (a region associated with language comprehension and production in humans). The microstructure analysis further revealed hemispheric differences in neuropil fraction, indicating that KIAA0319 expression may be involved in regulation of processes related to the formation and maintenance of synapses, dendrites, or axons within regions associated with communication. Significance Statement Studying the impact of language-related genes on primate brain morphology can provide insight into the evolution of human cognition and language systems. Here, we show that two KIAA0319 variants in chimpanzees are linked to differences in gray matter volume of the posterior superior temporal gyrus, a region associated with language comprehension and production in humans. Examination of the microstructure of this region showed that one KIAA0319 variant is associated with hemispheric differences in neuropil fraction, indicating that KIAA0319 expression may be involved in the formation and maintenance of synapses, dendrites, or axons within a region directly involved in communication. Together, these findings suggest that KIAA0319 variants may underlie individual variation in auditory processing and associated brain regions in nonhuman primates.

Although previous studies have shown that many species follow gaze, few have directly compared closely related species, and thus its cross-species variation remains largely unclear. In this study, we compared three great ape species (bonobos, Pan paniscus, chimpanzees, Pan troglodytes, orang-utans, Pongo abelii) and humans (12-month-olds and adults) in their gaze-following responses to the videos of conspecific and allospecific models. In the video, the model turned his head repeatedly to one of two identical objects. We used a noninvasive eye-tracking technique to measure participants' eye movements, and used both conspecific and allospecific models as stimuli to examine their potential preference in following conspecific rather than allospecific gaze. Experiment 1 presented to great apes the videos of conspecific and human models. We found that all species followed the conspecific gaze. Chimpanzees did not follow the human gaze, whereas bonobos did. Bonobos reacted overall more sensitively than chimpanzees to both conspecific and human gaze. Experiment 2 presented to human infants and adults the videos of human, chimpanzee and orang-utan models. Both infants and adults followed the human gaze. Unlike adults, infants did not follow the ape gaze. Experiment 3 presented to great apes the videos of allospecific ape models. Consistent with experiment 1, chimpanzees did not follow the allospecific ape gaze, whereas bonobos and orang-utans did. Importantly, preferential following of conspecific gaze by chimpanzees (experiment 1) and human infants (experiment 2) was mainly explained by their prolonged viewing of the conspecific face and thus seems to reflect their motivation to attend selectively to the conspecific models. Taken together, we conclude that gaze following is modulated by both subject species and model species in great apes and humans, presumably a reflection of the subjects' intrinsic sensitivity to gaze and also their selective interest in particular models. (C) 2014 The Association for the Study of Animal Behaviour. Published by Elsevier Ltd. All rights reserved. Postprint

Non-human primates use various communicative means in interactions with others. While primate gestures are commonly considered to be intentionally and flexibly used signals, facial expressions are often referred to as inflexible, automatic expressions of affective internal states. To explore whether and how non-human primates use facial expressions in specific communicative interactions, we studied five species of small apes (gibbons) by employing a newly established Facial Action Coding System for hylobatid species (Gibbon-FACS). We found that, despite individuals often being in close proximity to each other, in social (as opposed to non-social contexts) the duration of facial expressions was significantly longer when gibbons were facing another individual compared to non-facing situations. Social contexts included grooming, agonistic interactions and play, whereas non-social contexts included resting and self-grooming. Additionally, gibbons used facial expressions while facing another individual more often in social contexts than non-social contexts where facial expressions were produced regardless of the attentional state of the partner. Also, facial expressions were more likely 'responded to' by the partner's facial expressions when facing another individual than non-facing. Taken together, our results indicate that gibbons use their facial expressions differentially depending on the social context and are able to use them in a directed way in communicative interactions with other conspecifics.

Non-human primates use various communicative means in interactions with others. While primate gestures are commonly considered to be intentionally and flexibly used signals, facial expressions are often referred to as inflexible, automatic expressions of affective internal states. To explore whether and how non-human primates use facial expressions in specific communicative interactions, we studied five species of small apes (gibbons) by employing a newly established Facial Action Coding System for hylobatid species (Gibbon-FACS). We found that, despite individuals often being in close proximity to each other, in social (as opposed to non-social contexts) the duration of facial expressions was significantly longer when gibbons were facing another individual compared to non-facing situations. Social contexts included grooming, agonistic interactions and play, whereas non-social contexts included resting and self-grooming. Additionally, gibbons used facial expressions while facing another individual more often in social contexts than non-social contexts where facial expressions were produced regardless of the attentional state of the partner. Also, facial expressions were more likely 'responded to' by the partner's facial expressions when facing another individual than non-facing. Taken together, our results indicate that gibbons use their facial expressions differentially depending on the social context and are able to use them in a directed way in communicative interactions with other conspecifics.

Non-human primates use various communicative means in interactions with others. While primate gestures are commonly considered to be intentionally and flexibly used signals, facial expressions are often referred to as inflexible, automatic expressions of affective internal states. To explore whether and how non-human primates use facial expressions in specific communicative interactions, we studied five species of small apes (gibbons) by employing a newly established Facial Action Coding System for hylobatid species (Gibbon- FACS). We found that, despite individuals often being in close proximity to each other, in social (as opposed to non-social contexts) the duration of facial expressions was significantly longer when gibbons were facing another individual compared to non-facing situations. Social contexts included grooming, agonistic interactions and play, whereas nonsocial contexts included resting and self-grooming. Additionally, gibbons used facial expressions while facing another individual more often in social contexts than non-social contexts where facial expressions were produced regardless of the attentional state of the partner. Also, facial expressions were more likely ‘responded to’ by the partner’s facial expressions when facing another individual than non- facing. Taken together, our results indicate that gibbons use their facial expressions differentially depending on the social context and are able to use them in a directed way in communicative interactions with other conspecifics.