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Abstract Among human and nonhuman primates, mutual eye gaze (MEG) and gaze following are believed to be important for social cognition and communicative signaling. The goals of this study were to examine how early rearing experiences contribute to individual variation in MEG and to examine the potential role of genetic factors underlying this variation. Subjects included 93 female and 23 male baboons (Papio anubis) ranging from 3 to 20 years of age. Within the sample, there were 55 mother‐reared (MR) and 61 nursery‐reared (NR) baboons. MEG was assessed in four 60‐s test sessions. For each session, the duration, frequency, and bout length were recorded. Mean values were then calculated for each individual from the four sessions. A multivariate analysis of covariance revealed an overall significant main effect for rearing. Subsequent univariate analyses revealed significant rearing effects on mean bout length, but not mean duration or mean frequency, with MR baboons having longer bout lengths compared to NR baboons. Furthermore, mean bout length was found to be significantly heritable. These results indicate that rearing experiences, and to a small extent, genetic factors, affect patterns of mutual eye gaze ‐ in particular, bout length. These results differ from previous findings in MR and NR chimpanzees, further suggesting that rearing may impact MEG in a species‐specific manner that reflects the function of gaze in different primate species. 

ABSTRACT ObjectivesMost human brains exhibit left hemisphere asymmetry for planum temporale (PT) surface area and gray matter volume, which is interpreted as cerebral lateralization for language. Once considered a uniquely human feature, PT asymmetries have now been documented in chimpanzees and olive baboons. The goal of the current study was to further investigate the evolution of PT asymmetries in nonhuman primates. Materials and MethodsWe measured PT surface area in chimpanzees (Pan troglodytes,n = 90), bonobos (Pan paniscus,n = 21), gorillas (Gorilla gorilla,n = 34), orangutans (Pongospp.,n = 33), olive baboons (Papio anubis,n = 105), rhesus macaques (Macaca mulatta,n = 144), and tufted capuchins (Sapajus apella,n = 29) from magnetic resonance imaging scans. ResultsOur findings reveal significant leftward biases in PT surface area among chimpanzees, gorillas, olive baboons, rhesus macaques, and capuchins. We did not find significant population‐level asymmetries among orangutans and bonobos, which could be due, in part, to small sample sizes. We also detected significant age effects for rhesus macaques only, and no significant sex effects for any species. DiscussionThe observation of a population‐level leftward bias for PT surface area among not only hominids (chimpanzees and gorillas), but also two cercopithecoids (olive baboons and rhesus macaques) and one platyrrhine (tufted capuchins) suggests that PT lateralization was likely present in some early anthropoid primate ancestors and relatives. This provides further evidence that human brains have since undergone changes to the size and connectivity of the PT in response to selection for the cognitive processes needed to support the evolution of language and speech.