Differences in the mutual eye gaze of bonobos (Pan paniscus) and chimpanzees (Pan troglodytes).

Eye gaze is widespread in nonhuman primate taxa and important for social cognition and communicative signaling. Bonobos and chimpanzees, two closely related primate species, differ in social organization, behavior, and cognition. Chimpanzees' eye gaze and gaze following has been studied extensively, whereas less is known about bonobos' eye gaze. To examine species differences using a more ecologically relevant measure than videos or pictures, the current study compared bonobo and chimpanzee mutual eye gaze with a human observer. A multivariate analysis of variance revealed significant species differences in frequency and total duration, but not bout length, of mutual eye gaze (p < .001). Specifically, bonobos engage in mutual eye gaze more frequently and for longer total duration than chimpanzees. These results are likely related to species differences in social behavior and temperament and are consistent with eye-tracking studies in which bonobos looked at the eye region of conspecifics (in pictures and videos) longer than chimpanzees. Future research should examine the relationship between mutual eye gaze and gaze following, as well as examine its genetic and neurological correlates. (PsycInfo Database Record (c) 2020 APA, all rights reserved).

Mirror self-recognition and its relationship to social cognition in chimpanzees.

Chimpanzees and humans are capable of recognizing their own reflection in mirrors. Little is understood about the selective pressures that led to this evolved trait and about the mechanisms that underlie it. Here, we investigated the hypothesis that mirror self-recognition in chimpanzees is the byproduct of a developed form of self-awareness that was naturally selected for its adaptive use in social cognitive behaviors. We present here the first direct attempt to assess the social cognition hypothesis by analyzing the association between mirror self-recognition in chimpanzees, as measured by a mirror-mark test, and their performance on a variety of social cognition tests. Consistent with the social cognition hypothesis, chimpanzees who showed evidence of mirror self-recognition in the mark test tended to perform significantly better on the social cognition tasks than those who failed the mark test. Additionally, the data as a whole fit the social cognition hypothesis better than the main competing hypothesis of mirror self-recognition in great apes, the secondary representation hypothesis. Our findings strongly suggest that the evolutionary origins of great apes' and humans' capacity to understand ourselves, as revealed by our capacity to recognize ourselves in mirrors, are intimately linked to our ability to understand others.

Investigating individual differences in chimpanzee mirror self-recognition and cortical thickness: A vertex-based and region-of-interest analysis.

Mirror self-recognition (MSR), a recently evolved cognitive trait, is one of the most significant abilities that separate humans and great apes from more distantly related nonhuman primates. MSR may serve as the foundation for a number of related but more complex social cognitive abilities unique to humans and great apes including imitation, empathy, theory-of-mind, perspective taking and deception. However, our understanding of the neural basis of MSR in nonhuman primates remains largely unknown. The current study aimed to begin to fill this gap in the literature by investigating the neuroanatomical foundations of MSR in a sample of 67 captive chimpanzees. Vertex-based and region-of-interest analysis revealed significant differences in cortical thickness, particularly in males, in the cingulate cortex, inferior frontal gyrus and superior temporal and frontal cortex. The current study provides further evidence for the neuroanatomical foundations of mirror self-recognition abilities in chimpanzees.

Chimpanzees gesture to humans in mirrors: using reflection to dissociate seeing from line of gaze.

There is much experimental evidence suggesting that chimpanzees understand that others see. However, previous research has never experimentally ruled out the alternative explanation that chimpanzees are just responding to the geometric cue of 'direct line of gaze', the observable correlate of seeing in others. Here, we sought to resolve this ambiguity by dissociating seeing from direct line of gaze using a mirror. We investigated the frequency of chimpanzees' visual gestures towards a human experimenter who could see them (as a result of looking into a mirror) but who lacked a direct line of gaze to them (as a result of having his/her head turned away). Chimpanzees produced significantly more visual gestures when the experimenter could see them than when he/she could not, even when the experimenter did not have a direct line of gaze to them. Results suggest that chimpanzees, through a possible process of experience projection based on their own prior experience with mirrors, infer that an experimenter looking at the mirror can see them. We discuss our results in relation to the theory of mind hypothesis that chimpanzees understand seeing in others, and we evaluate possible alternative low-level explanations.

Genetic Factors and Orofacial Motor Learning Selectively Influence Variability in Central Sulcus Morphology in Chimpanzees (Pan troglodytes).

Captive chimpanzees (Pan troglodytes) have been shown to learn the use of novel attention-getting (AG) sounds to capture the attention of humans as a means of requesting or drawing their attention to a desired object or food. There are significant individual differences in the use of AG sounds by chimpanzees and, here, we examined whether changes in cortical organization of the central sulcus (CS) were associated with AG sound production. MRI scans were collected from 240 chimpanzees, including 122 that reliably produced AG sounds and 118 that did not. For each subject, the depth of CS was quantified along the superior-inferior plane with specific interest in the inferior portion corresponding to the region of the motor cortex where the mouth and orofacial movements are controlled. Results indicated that CS depth in the inferior, but not superior, portion was significantly greater in chimpanzees that reliably produced AG sounds compared with those who did not. Quantitative genetic analyses indicated that overall CS surface area and depth were significantly heritable, particularly in the superior regions, but less so in the inferior and central portions. Further, heritability in CS depth was altered as a function of acquisition of AG sounds. The collective results suggest that learning to produce AG sounds resulted in region-specific cortical reorganization within the inferior portion of the CS, a finding previously undocumented in chimpanzees or any nonhuman primate.SIGNIFICANCE STATEMENT Recent studies in chimpanzees (Pan troglodytes) have shown that some can learn to produce novel sounds by configuring different orofacial movement patterns and these sounds are used in communicatively relevant contexts. Here, we examined the neuromorphological correlates in the production of these sounds in chimpanzees. We show that chimpanzees that have learned to produce these sounds show significant differences in central sulcus (CS) morphology, particularly in the inferior region. We further show that overall CS morphology and regions within the superior portion are significantly heritable, whereas central and inferior portions of the CS are not. The collective findings suggest chimpanzees exhibit cortical plasticity in regions of the brain that were central to the emergence of speech functions in humans.

Handedness for Unimanual Grasping in 564 Great Apes: The Effect on Grip Morphology and a Comparison with Hand Use for a Bimanual Coordinated Task.

A number of factors have been proposed to influence within and between species variation in handedness in non-human primates. In the initial study, we assessed the influence of grip morphology on hand use for simple reaching in a sample of 564 great apes including 49 orangutans Pongo pygmaeus, 66 gorillas Gorilla gorilla, 354 chimpanzees Pan troglodytes and 95 bonobos Pan paniscus. Overall, we found a significant right hand bias for reaching. We also found a significant effect of the grip morphology of hand use. Grasping with the thumb and index finger was more prevalent in the right compared to left hand in all four species. There was no significant sex effect on the patterns of handedness. In a subsample of apes, we also compared consistency in hand use for simple reaching with previously published data on a task that measures handedness for bimanual actions. We found that the ratio of subjects with consistent right compared to left hand use was more prevalent in bonobos, chimpanzees and gorillas but not orangutans. However, for all species, the proportion of subjects with inconsistent hand preferences between the tasks was relatively high suggesting some measures may be more sensitive in assessing handedness than others.

Age-related motor dysfunction: Manual slowing in Gorilla gorilla gorilla.

Aging in humans and rhesus monkeys is commonly associated with motor function decrements including dexterity, speed, and strength. Despite their longevity and phylogenetic relatedness to humans, the effects of aging on motor function in non-human apes have been minimally studied. We conducted two experiments with western lowland gorillas (11-54 years of age) to determine whether aged gorillas exhibit motor deficits similar to those seen in other species. In experiment one, gorillas extracted up to 12 food rewards lodged in holes of a Lexan board. Extraction rates were calculated for eight test sessions. A repeated measures ANOVA revealed no main effects of session or sex on extraction rate, but a significant main effect of age. Comparisons between the first and last sessions showed that experience significantly improved extraction rates in young but not aged gorillas. In experiment two, gorillas retrieved a hex nut from three differently shaped rods with each hand for a reward. Latencies of retrieval were calculated for 16 test sessions. A repeated measures ANOVA revealed significant main effects of age class, sex, and session. There were significant interactions between session and sex, session and age, and session, sex, and age. These findings held when analyzing each rod shape separately. Post hoc comparisons revealed that young gorillas were significantly faster at the task than aged gorillas, and females were faster than males. This finding held only for the question mark shaped rod when analyzing each rod shape separately. Comparisons between the first and last sessions showed that experience did not significantly improve latencies in either age or sex class. The direction of these results are congruent with previous findings in humans and monkeys and suggest that aged gorillas experience deficits in bimanual coordination compared to younger gorillas and that age and sex influence fine motor ability in gorillas.