Early communicative gestures in human and chimpanzee 1-year-olds observed across diverse socioecological settings.

We investigated the communicative gestures used by chimpanzee and human infants. In contrast to previous studies, we compared the species at the same age (12-14 months) and used multiple groups living in diverse socioecological settings for both species. We recorded gestures produced by infants and those produce by others and directed toward infants. We classified the gestures into the following types: human-usual, chimpanzee-usual, and species-common; and searched for within species and between species differences. We found no significant differences between groups or species in overall rates of infant-produced or infant-received gestures, suggesting that all of these infants produced and received gestures at similar levels. We did find significant differences, however, when we considered the three types of gesture. Chimpanzee infants produced significantly higher rates of chimpanzee-usual gestures, and human infants produced significantly higher rates of human-usual gestures, but there was no significant species difference in the species-common gestures. Reports of species differences in gesturing in young infants, therefore, could be influenced by investigators' choice of gesture type. Interestingly, we found that 1-year-old infants produced the gesture of "hold mutual gaze" and that the chimpanzee infants had a significantly higher rate than the human infants. We did not find strong evidence that the specific types of gestural environment experienced by young infants influenced the types of gestures that infants produce. We suggest that at this point in development (before human infants use lots of speech), nonverbal communicative gestures may be equally important for human and chimpanzee infants.

Testing for personality consistency across naturally occurring behavioral contexts in sanctuary chimpanzees (Pan troglodytes).

Personality is both a reflection of the bio-behavioral profile of individuals and a summary of how they typically interact with their physical and social world. Personality is usually defined as having distinct behavioral characteristics, which are assumed to be consistent over time and across contexts. Like other mammals, primates have individual differences in personality. Although temporal consistency is sometimes measured in primates, and contextual consistency is sometimes measured across experimental contexts, it is rare to measure both in the same individuals and outside of experimental settings. Here, we aim to measure both temporal and contextual consistency in chimpanzees, assessing their personality with behavioral observations from naturally occurring contexts (i.e., real-life settings). We measured personality-based behaviors in 22 sanctuary chimpanzees, in the contexts of feeding, affiliation, resting, and solitude, across two time periods, spanning 4 years. Of the 22 behaviors recorded, about 64% were consistent across two to four contexts and 50% were consistent over time. Ten behaviors loaded significantly onto three trait components: explorativeness, boldness-sociability, and anxiety-sociability, as revealed by factor analysis. Like others, we documented individual differences in the personality of chimpanzees based on reliably measured observations in real-life contexts. Furthermore, we demonstrated relatively strong, but not absolute, temporal, and contextual consistency in personality-based behaviors. We also found another aspect of individual differences in personality, specifically, the extent to which individual chimpanzees show consistency. Some individuals showed contextual and temporal consistency, whereas others show significant variation across behaviors, contexts, and/or time. We speculate that the relative degree of consistency in personality may vary within chimpanzees. It may be that different primate species vary in the extent to which individuals show consistency of personality traits. Our behavioral-based assessment can be used with wild populations, increasing the validity of personality studies, facilitating comparative studies and potentially being applicable to conservation efforts.

Joint Attention in Human and Chimpanzee Infants in Varied Socio-Ecological Contexts.

Joint attention (JA) is an early manifestation of social cognition, commonly described as interactions in which an infant looks or gestures to an adult female to share attention about an object, within a positive emotional atmosphere. We label this description the JA phenotype. We argue that characterizing JA in this way reflects unexamined assumptions which are, in part, due to past developmental researchers' primary focus on western, middle-class infants and families. We describe a range of cultural variations in caregiving practices, socialization goals, and parenting ethnotheories as an essential initial step in viewing joint attention within inclusive and contextualized perspectives. We begin the process of conducting a decolonized study of JA by considering the core construct of joint attention (i.e., triadic connectedness) and adopting culturally inclusive definitions (labeled joint engagement [JE]). Our JE definitions allow for attention and engagement to be expressed in visual and tactile modalities (e.g., for infants experiencing distal or proximal caregiving), with various social partners (e.g., peers, older siblings, mothers), with a range of shared topics (e.g., representing diverse socialization goals, and socio-ecologies with and without toys), and with a range of emotional tone (e.g., for infants living in cultures valuing calmness and low arousal, and those valuing exuberance). Our definition of JE includes initiations from either partner (to include priorities for adult-led or child-led interactions). Our next foundational step is making an ecological commitment to naturalistic observations (Dahl, 2017, Child Dev Perspect, 11(2), 79-84): We measure JE while infants interact within their own physical and social ecologies. This commitment allows us to describe JE as it occurs in everyday contexts, without constraints imposed by researchers. Next, we sample multiple groups of infants drawn from diverse socio-ecological settings. Moreover, we include diverse samples of chimpanzee infants to compare with diverse samples of human infants, to investigate the extent to which JE is unique to humans, and to document diversity both within and between species. We sampled human infants living in three diverse settings. U.K. infants (n = 8) were from western, middle-class families living near universities in the south of England. Nso infants (n = 12) were from communities of subsistence farmers in Cameroon, Africa. Aka infants (n = 10) were from foraging communities in the tropical rain forests of Central African Republic, Africa. We coded behavioral details of JE from videotaped observations (taken between 2004 and 2010). JE occurred in the majority of coded intervals (Mdn = 68%), supporting a conclusion that JE is normative for human infants. The JA phenotype, in contrast, was infrequent, and significantly more common in the U.K. (Mdn = 10%) than the other groups (Mdn < 3%). We found significant within-species diversity in JE phenotypes (i.e., configurations of predominant forms of JE characteristics). We conclude that triadic connectedness is very common in human infants, but there is significant contextualization of behavioral forms of JE. We also studied chimpanzee infants living in diverse socio-ecologies. The PRI/Zoo chimpanzee infants (n = 7) were from captive, stable groups of mixed ages and sexes, and included 4 infants from the Chester Zoo, U.K. and 3 from the Primate Research Institute, Kyoto University, Japan. The Gombe chimpanzee infants (n = 12) were living in a dynamically changing, wild community in the Gombe National Park, Tanzania, Africa. Additionally, we include two Home chimpanzee infants who were reared from birth by a female scientist, in the combined U.S., middle-class contexts of home and university cognition laboratory. JE was coded from videotaped observations (taken between 1993 and 2006). JE occurred during the majority of coded intervals (Mdn = 64%), consistent with the position that JE is normative for chimpanzee infants. The JA phenotype, in contrast, was rare, but more commonly observed in the two Home chimpanzee infants (in 8% and 2% of intervals) than in other chimpanzee groups (Mdns = 0%). We found within-species diversity in the configurations comprising the JE phenotypes. We conclude that triadic connectedness is very common in chimpanzee infants, but behavioral forms of joint engagement are contextualized. We compared JE across species, and found no species-uniqueness in behavioral forms, JE characteristics, or JE phenotypes. Both human and chimpanzee infants develop contextualized social cognition. Within-species diversity is embraced when triadic connectedness is described with culturally inclusive definitions. In contrast, restricting definitions to the JA phenotype privileges a behavioral form most valued in western, middle-class socio-ecologies, irrespective of whether the interactions involve human or chimpanzee infants. Our study presents a model for how to decolonize an important topic in developmental psychology. Decolonization is accomplished by defining the phenomenon inclusively, embracing diversity in sampling, challenging claims of human-uniqueness, and having an ecological commitment to observe infant social cognition as it occurs within everyday socio-ecological contexts. It is essential that evolutionary and developmental theories of social cognition are re-built on more inclusive and decolonized empirical foundations.

The mismeasure of ape social cognition.

In his classic analysis, Gould (The mismeasure of man, WW Norton, New York, 1981) demolished the idea that intelligence was an inherent, genetic trait of different human groups by emphasizing, among other things, (a) its sensitivity to environmental input, (b) the incommensurate pre-test preparation of different human groups, and (c) the inadequacy of the testing contexts, in many cases. According to Gould, the root cause of these oversights was confirmation bias by psychometricians, an unwarranted commitment to the idea that intelligence was a fixed, immutable quality of people. By virtue of a similar, systemic interpretive bias, in the last two decades, numerous contemporary researchers in comparative psychology have claimed human superiority over apes in social intelligence, based on two-group comparisons between postindustrial, Western Europeans and captive apes, where the apes have been isolated from European styles of social interaction, and tested with radically different procedures. Moreover, direct comparisons of humans with apes suffer from pervasive lapses in argumentation: Research designs in wide contemporary use are inherently mute about the underlying psychological causes of overt behavior. Here we analyze these problems and offer a more fruitful approach to the comparative study of social intelligence, which focuses on specific individual learning histories in specific ecological circumstances.

The gesture 'Touch': Does meaning-making develop in chimpanzees' use of a very flexible gesture?

In this bottom-up study of gesture, we focused on the details of a single gesture, Touch. We compared characteristics of use by three young chimpanzees with those of 11 adults, their interactive partners, housed in a semi-natural social group at the Kyoto University Primate Research Institute (KUPRI) in Japan. Five hundred eighty-one observations of the gesture Touch were collected across a four-year time span. This single gesture had 36 different forms, was directed to 70 different target locations on the body of social partners, and occurred in 26 different contexts. Significant differences were found between infant and adult initiators in the form, target locations, and contexts of the gesture Touch. There was a wide diversity in form-location patterns within each context, and there were no form-location patterns specific to particular contexts. Thus, we demonstrate that this gesture exhibits flexibility in form and flexibility in use. The results from this study illustrate the importance of contextualized meaning in understanding flexibility in the gesture use of great apes.

Early Socioemotional Intervention Mediates Long-Term Effects of Atypical Rearing on Structural Covariation in Gray Matter in Adult Chimpanzees.

Atypical rearing has deleterious effects on chimpanzee behavior during development, some of which can be ameliorated with a responsive care intervention (RCI). Here, we obtained in vivo magnetic resonance images of adult brains of 27 chimpanzees given institutional care, with and without RCI, and compared them with those of 16 chimpanzees mother-reared from birth. We found significant long-term rearing effects on structural covariation and gray matter volume, specifically in the basal forebrain (i.e., caudate, putamen, nucleus accumbens, rectus gyrus, and orbital prefrontal cortex), indicating that RCI prevented brain changes due to atypical rearing. A significant correlation between covariation in these brain areas and caregiver nurturing, experienced in the first month of life, suggests a possible developmental mechanism for the effect of early experience on brain networks. We identified an early intervention that prevents changes in the basal forebrain that otherwise emerge as a consequence of institutionalized rearing without species-typical socioemotional experiences.

Apes communicate about absent and displaced objects: methodology matters.

Displaced reference is the ability to refer to an item that has been moved (displaced) in space and/or time, and has been called one of the true hallmarks of referential communication. Several studies suggest that nonhuman primates have this capability, but a recent experiment concluded that in a specific situation (absent entities), human infants display displaced reference but chimpanzees do not. Here, we show that chimpanzees and bonobos of diverse rearing histories are capable of displaced reference to absent and displaced objects. It is likely that some of the conflicting findings from animal cognition studies are due to relatively minor methodological differences, but are compounded by interpretation errors. Comparative studies are of great importance in elucidating the evolution of human cognition; however, greater care must be taken with methodology and interpretation for these studies to accurately reflect species differences.

Triggering social interactions: chimpanzees respond to imitation by a humanoid robot and request responses from it.

Even the most rudimentary social cues may evoke affiliative responses in humans and promote social communication and cohesion. The present work tested whether such cues of an agent may also promote communicative interactions in a nonhuman primate species, by examining interaction-promoting behaviours in chimpanzees. Here, chimpanzees were tested during interactions with an interactive humanoid robot, which showed simple bodily movements and sent out calls. The results revealed that chimpanzees exhibited two types of interaction-promoting behaviours during relaxed or playful contexts. First, the chimpanzees showed prolonged active interest when they were imitated by the robot. Second, the subjects requested 'social' responses from the robot, i.e. by showing play invitations and offering toys or other objects. This study thus provides evidence that even rudimentary cues of a robotic agent may promote social interactions in chimpanzees, like in humans. Such simple and frequent social interactions most likely provided a foundation for sophisticated forms of affiliative communication to emerge.

Emotional engagements predict and enhance social cognition in young chimpanzees.

Social cognition in infancy is evident in coordinated triadic engagements, that is, infants attending jointly with social partners and objects. Current evolutionary theories of primate social cognition tend to highlight species differences in cognition based on human-unique cooperative motives. We consider a developmental model in which engagement experiences produce differential outcomes. We conducted a 10-year-long study in which two groups of laboratory-raised chimpanzee infants were given quantifiably different engagement experiences. Joint attention, cooperativeness, affect, and different levels of cognition were measured in 5- to 12-month-old chimpanzees, and compared to outcomes derived from a normative human database. We found that joint attention skills significantly improved across development for all infants, but by 12 months, the humans significantly surpassed the chimpanzees. We found that cooperativeness was stable in the humans, but by 12 months, the chimpanzee group given enriched engagement experiences significantly surpassed the humans. Past engagement experiences and concurrent affect were significant unique predictors of both joint attention and cooperativeness in 5- to 12-month-old chimpanzees. When engagement experiences and concurrent affect were statistically controlled, joint attention and cooperation were not associated. We explain differential social cognition outcomes in terms of the significant influences of previous engagement experiences and affect, in addition to cognition. Our study highlights developmental processes that underpin the emergence of social cognition in support of evolutionary continuity.

Gestures and social-emotional communicative development in chimpanzee infants.

Communicative skills of chimpanzees are of significant interest across many domains, such as developmental psychology (how does communication emerge in prelinguistic beings?), evolution (e.g., did human language evolve from primate gestures?), and in comparative psychology (how does the nonverbal communication of chimpanzees and humans compare?). Here we ask about how gestures develop in chimpanzee infants (n = 16) that were raised in an interactive program designed to study skill development. Data on socio-communicative development were collected following 4 hr of daily interaction with each infant, longitudinally from birth through the first year of life. A consistent and significant developmental pattern was found across the contexts of tickle play, grooming, and chase play: Infant chimpanzees first engaged in interactions initiated by others, then they initiated interactions, and finally, they requested others to join them in the interaction. Gestures were documented for initiating and requesting tickle play, for initiating and requesting grooming, and for initiating and requesting chase play. Gestural requests emerged significantly later than gestural initiations, but the age at which gestures emerged was significantly different across contexts. Those gestures related to hierarchical rank relations, that is, gestures used by subordinates in interaction with more dominant individuals, such as wrist presenting and rump presenting, did not emerge in the same manner as the other gestures. This study offers a new view on the development of gestures, specifically that many develop through interaction and communicate socio-emotional desires, but that not all gestures emerge in the same manner.

Effects of cage mesh on pointing: hand shapes in chimpanzees (Pan troglodytes).

It has been speculated that cage mesh exerts a shaping influence on reaching behavior by captive apes, which is then misconstrued as pointing by human observers. Although this notion is clearly falsified by the pointing of captive language-trained apes-who point in the absence of intervening cage mesh-nevertheless, the degree to which cage mesh might influence pointing hand shapes by captive great apes in other housing environments remains relatively unexplored. We examined 259 pointing gestures displayed in archival footage from over 18 h of observation by three nonlanguage-trained chimpanzees housed at a biomedical research center. We coded points in relation to how close to the boundaries of the diamond-shaped cage mesh their points were displayed. We found that points with the whole hand were significantly more likely to be displayed away from the mesh boundaries, relative to points with the index finger or other single-digit points. However, points of each hand shape were displayed at each location, demonstrating that these physical parameters do not fully account for the number of fingers extended while pointing by chimpanzees.

Neurobehavioral Integrity of Chimpanzee Newborns: Comparisons across groups and across species reveal gene-environment interaction effects.

The aims of this article are to describe the neurobehavioral integrity of chimpanzee newborns, to investigate how early experiences affect the neurobehavioral organization of chimpanzees, and to explore species differences by comparing chimpanzee newborns to a group of typically developing human newborns. Neurobehavioral integrity related to orientation, motor performance, arousal, and state regulation of 55 chimpanzee (raised in four different settings) and 42 human newborns was measured with the Neonatal Behavioral Assessment Scale (NBAS) a semi-structured 25-minute interactive assessment. Thirty-eight chimpanzees were tested every other day from birth, and analyses revealed significant developmental changes in 19 of 27 NBAS scores. The cross-group and cross-species comparisons were conducted at 2 and 30 days of age. Among the 4 chimpanzee groups, significant differences were found in 23 of 24 NBAS scores. Surprisingly, the cross-species comparisons revealed that the human group was distinct in only 1 of 25 NBAS scores (the human group had significantly less muscle tone than all the chimpanzee groups). The human group was indistinguishable from at least one of the chimpanzee groups in the remaining 24 of 25 NBAS scores. The results of this study support the conclusion that the interplay between genes and environment, rather than genes alone or environment alone, accounts for phenotypic expressions of newborn neurobehavioral integrity in hominids.

BIZARRE chimpanzees do not represent "the chimpanzee".

Henrich et al. convincingly caution against the overgeneralization of findings from particular human populations, but fail to apply their own compelling reasoning to our nearest living relatives, the great apes. Here we argue that rearing history is every bit as important for understanding cognition in other species as it is in humans.

Enhancement of attachment and cognitive development of young nursery-reared chimpanzees in responsive versus standard care.

Forty-six nursery-reared chimpanzee infants (22 females and 24 males) receiving either standard care (n = 29) or responsive care (n = 17) at the Great Ape Nursery at Yerkes participated in this study. Standard care (ST) consisted primarily of peer-rearing, with humans providing essential health-related care. Responsive care (RC) consisted of an additional 4 hr of interaction 5 days a week with human caregivers who were specially trained to enhance species-typical chimpanzee socio-emotional and communicative development. At 9 months, ST and RC chimpanzees were examined with the Bayley Scales for Infant Development to assess their Mental Development Index (MDI). At 12 months, the chimpanzees were assessed with their human caregivers in the Ainsworth Strange Situation Procedure (SSP). In this first study to use the SSP in chimpanzees, nursery-reared chimpanzees exhibited the definite patterns of distress, proximity seeking, and exploration that underpin the SSP for human infants. In ST chimpanzees the attachment classification distribution was similar to that of human infants raised in Greek or Romanian orphanages. RC chimpanzees showed less disorganized attachment to their human caregivers, had a more advanced cognitive development, and displayed less object attachment compared to ST chimpanzees. Responsive care stimulates chimpanzees' cognitive and emotional development, and is an important factor in ameliorating some of the adverse effects of institutional care.

Increases in theta CSD power and coherence during a calibrated stop-signal task: implications for goal-conflict processing and the Behavioural Inhibition System.

Psychologists have identified multiple different forms of conflict, such as information processing conflict and goal conflict. As such, there is a need to examine the similarities and differences in neurology between each form of conflict. To address this, we conducted a comprehensive electroencephalogram (EEG) analysis of Shadli, Glue, McIntosh, and McNaughton's calibrated stop-signal task (SST) goal-conflict task. Specifically, we examined changes in scalp-wide current source density (CSD) power and coherence across a wide range of frequency bands during the calibrated SST (n = 34). We assessed differences in EEG between the high and low goal-conflict conditions using hierarchical analyses of variance (ANOVAs). We also related goal-conflict EEG to trait anxiety, neuroticism, Behavioural Inhibition System (BIS)-anxiety and revised BIS (rBIS) using regression analyses. We found that changes in CSD power during goal conflict were limited to increased midfrontocentral theta. Conversely, coherence increased across 23 scalp-wide theta region pairs and one frontal delta region pair. Finally, scalp-wide theta significantly predicted trait neuroticism but not trait anxiety, BIS-anxiety or rBIS. We conclude that goal conflict involves increased midfrontocentral CSD theta power and scalp-wide theta-dominated coherence. Therefore, compared with information processing conflict, goal conflict displays a similar EEG power profile of midfrontocentral theta but a much wider coherence profile. Furthermore, the increases in theta during goal conflict are the characteristic of BIS-driven activity. Therefore, future research should confirm whether these goal-conflict effects are driven by the BIS by examining whether the effects are attenuated by anxiolytic drugs. Overall, we have identified a unique network of goal-conflict EEG during the calibrated SST.

Effects of sex and early rearing condition on adult behavior, health, and well-being in captive chimpanzees (Pan troglodytes).

Scientific evaluation of management strategies for captive species is part of the establishment of best practices for animal welfare. Here we report the effects of sex, rearing, and a sex-by-rearing interaction on adult, captive chimpanzees' (Pan troglodytes) behavior, health, well-being, personality, and orientation towards humans based on multiple methods (observation, animal records, and surveys). Chimpanzees raised in three conditions, mother-reared (MR), standard nursery (ST) and an experimental nursery (RC), were assessed approximately 20 years after their differential rearing experiences concluded. Sex had a significant effect on behavior towards conspecifics (aggression [M > F]; affiliation [F > M]), on abnormal behavior (rocking [M > F]), and on likelihood of incurring at least one injury (between ages 6 and 10 [M > F]). Rearing condition had a significant impact on behavior towards humans (negative solicitation [RC = ST > MR = ST]; neutral behavior [RC > ST > MR], yawning (RC = ST > MR = ST), subjective well-being (MR = ST > RC = ST), and on GI illness frequency (RC > ST = MR). Sex interacted with rearing on aggression towards humans (for males, RC > MR = ST), frequency of upper respiratory infection (URI: for males RC > MR = ST)) and likelihood of at least one URI between the ages of 11 and 15 (RC males > ST males). Our findings support the conclusion that there are long-term effects of both early rearing and sex on captive adult chimpanzee welfare.

Perceived differences between chimpanzee (Pan troglodytes) and human (Homo sapiens) facial expressions are related to emotional interpretation.

Human face perception is a finely tuned, specialized process. When comparing faces between species, therefore, it is essential to consider how people make these observational judgments. Comparing facial expressions may be particularly problematic, given that people tend to consider them categorically as emotional signals, which may affect how accurately specific details are processed. The bared-teeth display (BT), observed in most primates, has been proposed as a homologue of the human smile (J. A. R. A. M. van Hooff, 1972). In this study, judgments of similarity between BT displays of chimpanzees (Pan troglodytes) and human smiles varied in relation to perceived emotional valence. When a chimpanzee BT was interpreted as fearful, observers tended to underestimate the magnitude of the relationship between certain features (the extent of lip corner raise) and human smiles. These judgments may reflect the combined effects of categorical emotional perception, configural face processing, and perceptual organization in mental imagery and may demonstrate the advantages of using standardized observational methods in comparative facial expression research.

Classifying chimpanzee facial expressions using muscle action.

The Chimpanzee Facial Action Coding System (ChimpFACS) is an objective, standardized observational tool for measuring facial movement in chimpanzees based on the well-known human Facial Action Coding System (FACS; P. Ekman & W. V. Friesen, 1978). This tool enables direct structural comparisons of facial expressions between humans and chimpanzees in terms of their common underlying musculature. Here the authors provide data on the first application of the ChimpFACS to validate existing categories of chimpanzee facial expressions using discriminant functions analyses. The ChimpFACS validated most existing expression categories (6 of 9) and, where the predicted group memberships were poor, the authors discuss potential problems with ChimpFACS and/or existing categorizations. The authors also report the prototypical movement configurations associated with these 6 expression categories. For all expressions, unique combinations of muscle movements were identified, and these are illustrated as peak intensity prototypical expression configurations. Finally, the authors suggest a potential homology between these prototypical chimpanzee expressions and human expressions based on structural similarities. These results contribute to our understanding of the evolution of emotional communication by suggesting several structural homologies between the facial expressions of chimpanzees and humans and facilitating future research.

Dyadic interactions, attachment and the presence of triadic interactions in chimpanzees and humans.

From a developmental perspective, dyadic interactions with social partners and dyadic interactions with objects underpin early social cognition in humans and chimpanzees. In humans, dyadic social relationships form in the first three months of life, dyadic relations with objects form in the first 6 months of life, and triadic relations begin around 8-12 months. In chimpanzees, a similar developmental pattern is evident with dyadic social relationships forming in the first three months of life, dyadic relations with objects forming in the first 5 months of life, and triadic relations in the latter half of the first year of life. During ontogeny humans and chimpanzees experience emotional engagements, both with social partners and with objects, and these impact outcomes in social cognition. Rather than being considered too complex, diversity of socio-emotional experiences during development can be embraced, with the goal to specify how they influence social cognition outcomes in humans and in chimpanzees. This process may provide the evolutionary and biological foundations for plasticity.

Intramuscular electrical stimulation of facial muscles in humans and chimpanzees: Duchenne revisited and extended.

The pioneering work of Duchenne (1862/1990) was replicated in humans using intramuscular electrical stimulation and extended to another species (Pan troglodytes: chimpanzees) to facilitate comparative facial expression research. Intramuscular electrical stimulation, in contrast to the original surface stimulation, offers the opportunity to activate individual muscles as opposed to groups of muscles. In humans, stimulation resulted in appearance changes in line with Facial Action Coding System (FACS) action units (AUs), and chimpanzee facial musculature displayed functional similarity to human facial musculature. The present results provide objective identification of the muscle substrate of human and chimpanzee facial expressions- data that will be useful in providing a common language to compare the units of human and chimpanzee facial expression.