The human faculty for object-mediated action, including tool use and imitation, exceeds that of even our closest primate relatives and is a key foundation of human cognitive and cultural uniqueness. In humans and macaques, observing object-directed grasping actions activates a network of frontal, parietal, and occipitotemporal brain regions, but differences in human and macaque activation suggest that this system has been a focus of selection in the primate lineage. To study the evolution of this system, we performed functional neuroimaging in humans' closest living relatives, chimpanzees. We compare activations during performance of an object-directed manual grasping action, observation of the same action, and observation of a mimed version of the action that consisted of only movements without results. Performance and observation of the same action activated a distributed frontoparietal network similar to that reported in macaques and humans. Like humans and unlike macaques, these regions were also activated by observing movements without results. However, in a direct chimpanzee/human comparison, we also identified unique aspects of human neural responses to observed grasping. Chimpanzee activation showed a prefrontal bias, including significantly more activity in ventrolateral prefrontal cortex, whereas human activation was more evenly distributed across more posterior regions, including significantly more activation in ventral premotor cortex, inferior parietal cortex, and inferotemporal cortex. This indicates a more “bottom-up” representation of observed action in the human brain and suggests that the evolution of tool use, social learning, and cumulative culture may have involved modifications of frontoparietal interactions.
Monitoring adrenocortical activity in wild primate populations is critical, given the well-documented relationship between stress, health and reproduction. Although many primate studies have quantified fecal glucocorticoid metabolite (FGM) concentrations, it is imperative that researchers validate their method for each species. Here, we describe and validate a technique for field extraction and storage of FGMs in wild chimpanzees (Pan troglodytes). Our method circumvents many of the logistical challenges associated with field studies while yielding similar results to a commonly-used laboratory method. We further validate that our method accurately reflects stress physiology using an ACTH challenge in a captive chimpanzee and a FGM peak at parturition in a wild subject. Finally, we quantify circadian patterns for FGMs for the first time in this species. Understanding these patterns may allow researchers to directly link specific events with the stress response.
chimpanzee; fecal glucocorticoid metabolites; field methods; ACTH; parturition; circadian rhythm
Humans and chimpanzees demonstrate numerous cognitive specializations for processing faces, but comparative studies with monkeys suggest that these may be the result of recent evolutionary adaptations. The present study utilized the novel approach of face space, a powerful theoretical framework used to understand the representation of face identity in humans, to further explore species differences in face processing. According to the theory, faces are represented by vectors in a multidimensional space, the centre of which is defined by an average face. Each dimension codes features important for describing a face’s identity, and vector length codes the feature’s distinctiveness. Chimpanzees and rhesus monkeys discriminated male and female conspecifics’ faces, rated by humans for their distinctiveness, using a computerized task. Multidimensional scaling analyses showed that the organization of face space was similar between humans and chimpanzees. Distinctive faces had the longest vectors and were the easiest for chimpanzees to discriminate. In contrast, distinctiveness did not correlate with the performance of rhesus monkeys. The feature dimensions for each species’ face space were visualized and described using morphing techniques. These results confirm species differences in the perceptual representation of conspecific faces, which are discussed within an evolutionary framework.
Face space; Multidimensional scaling; Face identity; Rhesus monkey; Chimpanzee; Species differences
Early life stress (ELS) is a risk factor for anxiety, mood disorders and alterations in stress responses. Less is known about the long-term neurobiological impact of ELS. We used [18F]-fluorodeoxyglucose Positron Emission Tomography (FDG-PET) to assess neural responses to a moderate stress test in adult monkeys that experienced ELS as infants. Both groups of monkeys showed hypothalamic-pituitary-adrenal (HPA) axis stress-induced activations and cardiac arousal in response to the stressor. A whole brain analysis detected significantly greater regional cerebral glucose metabolism (rCGM) in superior temporal sulcus, putamen, thalamus, and inferotemporal cortex of ELS animals compared to controls. Region of interest (ROI) analyses performed in areas identified as vulnerable to ELS showed greater activity in the orbitofrontal cortex of ELS compared to control monkeys, but greater hippocampal activity in the control compared to ELS monkeys. Together, these results suggest hyperactivity in emotional and sensory processing regions of adult monkeys with ELS, and greater activity in stress-regulatory areas in the controls. Despite these neural responses, no group differences were detected in neuroendocrine, autonomic or behavioral responses, except for a trend towards increased stillness in the ELS monkeys. Together, these data suggest hypervigilance in the ELS monkeys in the absence of immediate danger.
Early life stress; rearing; HPA axis; monkey; PET; social brain
Neurological experiments have revealed a complex network of areas in the human brain that respond more to faces than to other categories of objects and thus have been implemented in face categorization. The aim of this study was to investigate whether chimpanzees (N = 5), our closest living relatives, detect and categorize faces on the basis of first-order information. Further, whether this sensitivity is specific to faces or generalizes to other objects. In service to this aim, we created multiple categories of two-tone ‘Mooney’ objects (chimpanzee faces, shoes, human hands) because, by maximizing contrast, the Mooney transformation selectively degrades second-order information (the basis for individual discrimination in humans), leaving only first-order information intact. Two experiments were carried out using a 2AFC MTS procedure. The first experiment providing strong evidence that, like humans, chimpanzees categorize Mooney faces as faces. However, without second-order information, the chimpanzees could not match Mooney faces at the individual-level. In Experiment 2 four of the five chimpanzees found it easier to categorize Mooney faces than Mooney shoes. Neurological evidence strongly implicates a dedicated neural mechanism for face categorization in the human brain and our data suggest that chimpanzees share this level of specialization.
Face recognition is a complex skill that requires the integration of facial features across the whole face, e.g., holistic processing. It is unclear whether, and to what extent, other species process faces in a manner that is similar to humans. Previous studies on the inversion effect, a marker of holistic processing, in nonhuman primates have revealed mixed results in part because many studies have failed to include alternative image categories necessary to understand whether the effects are truly face-specific. The present study re-examined the inversion effect in rhesus monkeys and chimpanzees using comparable testing methods and a variety of high quality stimuli including faces and nonfaces. The data support an inversion effect in chimpanzees only for conspecifics’ faces (expert category), suggesting face-specific holistic processing similar to humans. Rhesus monkeys showed inversion effects for conspecifics, but also for heterospecifics’ faces (chimpanzees), and nonfaces images (houses), supporting important species differences in this simple test of holistic face processing.
face recognition; inversion effect; holistic processing; matching-to-sample; comparative
Understanding how individual identity is processed from faces remains a complex problem. Contrast reversal, showing faces in photographic negative, impairs face recognition in humans and demonstrates the importance of surface-based information (shading and pigmentation) in face recognition. We tested the importance of contrast information for face encoding in chimpanzees and rhesus monkeys using a computerized face-matching task. Results showed that contrast reversal (positive to negative) selectively impaired face processing in these two species, although the impairment was greater for chimpanzees. Unlike chimpanzees, however, monkeys performed just as well matching negative to positive faces, suggesting that they retained some ability to extract identity information from negative faces. A control task showed that chimpanzees, but not rhesus monkeys, performed significantly better matching face parts compared with whole faces after a contrast reversal, suggesting that contrast reversal acts selectively on face processing, rather than general visual-processing mechanisms. These results confirm the importance of surface-based cues for face processing in chimpanzees and humans, while the results were less salient for rhesus monkeys. These findings make a significant contribution to understanding the evolution of cognitive specializations for face processing among primates, and suggest potential differences between monkeys and apes.
face recognition; contrast reversal; configuration; chimpanzee; rhesus monkey; evolution
Numerous studies have shown that familiarity strongly influences how well humans recognize faces. This is particularly true when faces are encountered across a change in viewpoint. In this situation, recognition may be accomplished by matching partial or incomplete information about a face to a stored representation of the known individual, whereas such representations are not available for unknown faces. Chimpanzees, our closest living relatives, share many of the same behavioral specializations for face processing as humans, but the influence of familiarity and viewpoint have never been compared in the same study. Here, we examined the ability of chimpanzees to match the faces of familiar and unfamiliar conspecifics in their frontal and 3/4 views using a computerized task. Results showed that, while chimpanzees were able to accurately match both familiar and unfamiliar faces in their frontal orientations, performance was significantly impaired only when unfamiliar faces were presented across a change in viewpoint. Therefore, like in humans, face processing in chimpanzees appears to be sensitive to individual familiarity. We propose that familiarization is a robust mechanism for strengthening the representation of faces and has been conserved in primates to achieve efficient individual recognition over a range of natural viewing conditions.
All primates can recognize faces and do so by analyzing the subtle variation that exists between faces. Through a series of three experiments, we attempted to clarify the nature of second-order information processing in nonhuman primates. Experiment one showed that both chimpanzees (Pan troglodytes) and rhesus monkeys (Macaca mulatta) tolerate geometric distortions along the vertical axis, suggesting that information about absolute position of features does not contribute to accurate face recognition. Chimpanzees differed from monkeys, however, in that they were more sensitive to distortions along the horizontal axis, suggesting that when building a global representation of facial identity, horizontal relations between features are more diagnostic of identity than vertical relations. Two further experiments were performed to determine whether the monkeys were simply less sensitive to horizontal relations compared to chimpanzees or were instead relying on local features. The results of these experiments confirm that monkeys can utilize a holistic strategy when discriminating between faces regardless of familiarity. In contrast, our data show that chimpanzees, like humans, use a combination of holistic and local features when the faces are unfamiliar, but primarily holistic information when the faces become familiar. We argue that our comparative approach to the study of face recognition reveals the impact that individual experience and social organization has on visual cognition.
Visual cognition; Comparative psychology; Face perception; Face recognition
The ability to recognize faces is an important socio-cognitive skill that is associated with a number of cognitive specializations in humans. While numerous studies have examined the presence of these specializations in non-human primates, species where face recognition would confer distinct advantages in social situations, results have been mixed. The majority of studies in chimpanzees support homologous face-processing mechanisms with humans, but results from monkey studies appear largely dependent on the type of testing methods used. Studies that employ passive viewing paradigms, like the visual paired comparison task, report evidence of similarities between monkeys and humans, but tasks that use more stringent, operant response tasks, like the matching-to-sample task, often report species differences. Moreover, the data suggest that monkeys may be less sensitive than chimpanzees and humans to the precise spacing of facial features, in addition to the surface-based cues reflected in those features, information that is critical for the representation of individual identity. The aim of this paper is to provide a comprehensive review of the available data from face-processing tasks in non-human primates with the goal of understanding the evolution of this complex cognitive skill.
face recognition; primates; holistic processing
Faces provide important information about identity, age, and even kinship. A previous study in chimpanzees reported greater similarity between the faces of mothers and sons compared with mothers and daughters, or unrelated individuals. This was interpreted as an inbreeding avoidance mechanism where females, the dispersing gender, should avoid mating with any male that resembles their mother. Alternatively, male faces may be more distinctive than female faces, biasing attention toward males. To test these hypotheses, chimpanzees and rhesus monkeys matched conspecifics’ faces of unfamiliar mothers and fathers with their sons and daughters. Results showed no evidence of male distinctiveness, rather a cross-gender effect was found: chimpanzees were better matching moms with sons and fathers with daughters. Rhesus monkeys, however, showed an overwhelming bias toward male-distinctiveness. They were faster to learn male faces, performed better on father– offspring and parent–son trials, and were best matching fathers with sons. This suggests that for the rhesus monkey, inbreeding avoidance involves something other than facial phenotypic matching but that among chimpanzees, the visual recognition of facial similarities may play an important role.
kin recognition; face; phenotypic matching; male distinctiveness; inbreeding
A comparative perspective has remained central to the study of human facial expressions since Darwin’s [(1872/1998). The expression of the emotions in man and animals (3rd ed.). New York: Oxford University Press] insightful observations on the presence and significance of cross-species continuities and species-unique phenomena. However, cross-species comparisons are often difficult to draw due to methodological limitations. We report the application of a common methodology, the Facial Action Coding System (FACS) to examine facial movement across two species of hominoids, namely humans and chimpanzees. FACS [Ekman & Friesen (1978). Facial action coding system. CA: Consulting Psychology Press] has been employed to identify the repertoire of human facial movements. We demonstrate that FACS can be applied to other species, but highlight that any modifications must be based on both underlying anatomy and detailed observational analysis of movements. Here we describe the ChimpFACS and use it to compare the repertoire of facial movement in chimpanzees and humans. While the underlying mimetic musculature shows minimal differences, important differences in facial morphology impact upon the identification and detection of related surface appearance changes across these two species.
Comparative anatomy; Chimpanzees; Facial action coding system (FACS); Facial morphology
Humans are subject to the composite illusion: two identical top halves of a face are perceived as “different” when they are presented with different bottom halves. This observation suggests that when building a mental representation of a face, the underlying system perceives the whole face, and has difficulty decomposing facial features. We adapted a behavioural task that measures the composite illusion to examine the perception of faces in two nonhuman species. Specifically we had spider (Ateles geoffroyi) and rhesus monkeys (Macaca mulatta) perform a two-forced choice, match-to-sample task where only the top half of sample was relevant to the task. The results of Experiment 1 show that spider monkeys (N = 2) process the faces of familiar species (conspecifics and humans, but not chimpanzees, sheep, or sticks), holistically. The second experiment tested rhesus monkeys (N = 7) with the faces of humans, chimpanzees, gorillas, sheep and sticks. Contrary to prediction, there was no evidence of a composite effect in the human (or familiar primate) condition. Instead, we present evidence of a composite illusion in the chimpanzee condition (an unfamiliar primate). Together, these experiments show that visual expertise does not predict the composite effect across the primate order.
comparative psychology; face perception; holistic processing
Facial expression is a common mode of visual communication in mammals but especially so in primates. Rhesus macaques (Macaca mulatta) have a well-documented facial expression repertoire that is controlled by the facial/mimetic musculature as in all mammals. However, little is known about the musculature itself and how it compares to those of other primates. Here we present a detailed description of the facial musculature in rhesus macaques in behavioral, evolutionary, and comparative contexts. Foramlin-fixed faces from six adult male specimens were dissected using a novel technique. The morphology, attachments, three-dimensional relationships, and variability of muscles were noted and compared with chimpanzees (Pan troglodytes) and with humans. Results showed that there was a greater number of facial muscles in rhesus macaques than previously described (24) including variably present (and previously unmentioned) zygomaticus minor, levator labii superioris alaeque nasi, depressor septi, anterior auricularis, inferior auricularis, and depressor supercilii muscles. Facial muscles of the rhesus macaque were very similar to those in chimpanzees and in humans but M. mulatta did not possess a risorius muscle. These results support previous studies that describe a highly graded and intricate facial expression repertoire in rhesus macaques. Furthermore, these results indicate that phylogenetic position is not the primary factor governing structure of primate facial musculature and that other factors such as social behavior are probably more important. Results from the present study may provide valuable input to both biomedical studies that use rhesus macaques as a model for human disease and disorder that includes assessment of facial movement and studies into the evolution of primate societies and communication.
macaque; facial muscle; mimetic; chimpanzee; evolution; facial expression; primate
Faces are salient stimuli for primates that rely predominantly on visual cues for recognizing conspecifics and maintaining social relationships. While previous studies have shown similar face discrimination processes in chimpanzees and humans, data from monkeys are unclear. Therefore, three studies examined face processing in rhesus monkeys using the face inversion effect, a fractured face task, and an individual recognition task. Unlike chimpanzees and humans, the monkeys showed a general face inversion effect reflected by significantly better performance on upright compared to inverted faces (conspecifics, human and chimpanzees faces) regardless of the subjects’ expertise with those categories. Fracturing faces alters first- and second-order configural manipulations whereas previous studies in chimpanzees showed selective deficits for second-order configural manipulations. Finally, when required to individuate conspecific’s faces, i.e., matching two different photographs of the same conspecific, monkeys showed poor discrimination and repeated training. These results support evolutionary differences between rhesus monkeys and Hominoids in the importance of configural cues and their ability to individuate conspecifics’ faces, suggesting a lack of face expertise in rhesus monkeys.
face processing; configural cues; inversion effect; rhesus monkey
The ability to recognize and accurately interpret facial expressions is critically important for nonhuman primates that rely on these nonverbal signals for social communication. Despite this, little is known about how nonhuman primates, particularly monkeys, discriminate between facial expressions. In the present study, seven rhesus monkeys were required to discriminate four categories of conspecific facial expressions using a matching-to-sample task. In experiment 1, the matching pair showed identical photographs of facial expressions, paired with every other expression type as the nonmatch. The identity of the nonmatching stimulus monkey differed from the one in the sample. Subjects performed above chance on session 1, with no difference in performance across the four expression types. In experiment 2, the identity of all three monkeys differed in each trial, and a neutral portrait was also included as the nonmatching stimulus. Monkeys discriminated expressions across individual identity when the non-match was a neutral stimulus, but they had difficulty when the nonmatch was another expression type. We analysed the degree to which specific feature redundancy could account for these error patterns using a multidimensional scaling analysis which plotted the perceived dissimilarity between expression dyads along a two-dimensional axis. One axis appeared to represent mouth shape, stretched open versus funnelled, while the other appeared to represent a combination of lip retraction and mouth opening. These features alone, however, could not account for overall performance and suggest that monkeys do not rely solely on distinctive features to discriminate among different expressions.
facial expression; facial features; FACS; Macaca mulatta; matching-to-sample; rhesus monkey
The face inversion effect, or impaired recognition of upside down compared to upright faces, is used as a marker for the configural processing of faces in primates. The inversion effect in humans and chimpanzees is strongest for categories of stimuli for which subjects have considerable expertise, primarily conspecifics' faces. Moreover, discrimination performance decreases linearly as faces are incrementally rotated from upright to inverted. This suggests that rotated faces must be transformed, or normalized back into their most typical viewpoint before configural processing can ensue, and the greater the required normalization, the greater the likelihood of errors resulting. Previous studies in our lab have demonstrated a general face inversion effect in rhesus monkeys that was not influenced by expertise. Therefore, the present study examined the influence of rotation angle on the visual perception of face and nonface stimuli that varied in their level of expertise to further delineate the processes underlying the inversion effect in rhesus monkeys. Five subjects discriminated images in five orientation angles. Results showed significant linear impairments for all stimulus categories, including houses. However, compared to the upright images, only rhesus faces resulted in worse performance at rotation angles greater than 45°, suggesting stronger configural processing for stimuli for which subjects had the greatest expertise.
Face processing; Expertise; Configural cues; Rotation
There has been little research over the past few decades focusing on similarities and differences in the form and function of emotional signals in nonhuman primates, or whether these communication systems are homologous with those of humans. This is, in part, due to the fact that detailed and objective measurement tools to answer such questions have not been systematically developed for nonhuman primate research. Despite this, emotion research in humans has benefited for over 30 years from an objective, anatomically based facial-measurement tool: the Facial Action Coding System. In collaboration with other researchers, we have now developed a similar system for chimpanzees (ChimpFACS) and, in the process, have made exciting new discoveries regarding chimpanzees’ perception and categorization of emotional facial expressions, similarities in the facial anatomy of chimpanzees and humans, and we have identified homologous facial movements in the two species. Investigating similarities and differences in primate emotional communication systems is essential if we are to understand unique evolutionary specializations among different species.
emotion; facial expression; chimpanzee Facial Action Coding System; evolution; communication
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.
facial muscles; intramuscular electrical stimulation; primates; Duchenne
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.
ChimpFACS; facial expressions; emotion; homology
Previous studies have demonstrated the sensitivity of chimpanzees to facial configurations. Three studies further these findings by showing this sensitivity to be specific to second-order relational properties. In humans, this type of configural processing requires prolonged experience and enables subordinate-level discriminations of many individuals. Chimpanzees showed evidence of a composite-like effect for conspecific but not human faces despite extensive experience with humans. Chimpanzee face recognition was impaired only when manipulations targeted second-order properties. Finally, face processing was impaired when individual features were blurred through pixelation. Results confirm that chimpanzee face discrimination, like humans, depends on the integrity of second-order relational properties.
Face processing; Configuration; Expertise; Chimpanzee; Composite effect
The ability to recognize and accurately interpret facial expressions are critical social cognition skills in primates, yet very few studies have examined how primates discriminate these social signals and which features are the most salient. Four experiments examined chimpanzee facial expression processing using a set of standardized, prototypical stimuli created using the new ChimpFACS coding system. First, chimpanzees were found to accurately discriminate between these expressions using a computerized matching-to-sample task, and recognition was impaired for all but one expression category when they were inverted. Third, a multidimensional scaling analysis examined the perceived dissimilarity among these facial expressions revealing 2 main dimensions, the degree of mouth closure and extent of lip-puckering and retraction. Finally, subjects were asked to match each facial expression category using only individual component features. For each expression category, at least 1 component movement was more salient or representative of that expression than the others. However, these were not necessarily the only movements implicated in subject's overall pattern of errors. Therefore, similar to humans, both configuration and component movements are important during chimpanzee facial expression processing.
facial expression; FACS; social cognition; visual processing; inversion
The social brain hypothesis proposes that large neocortex size in Homonoids evolved to cope with the increasing demands of complex group living and greater numbers of interindividual relationships. Group living requires that individuals communicate effectively about environmental and internal events. Recent data have highlighted the complexity of chimpanzee communication, including graded facial expressions and referential vocalizations. Among Hominoids, elaborate facial communication is accompanied by specializations in brain areas controlling facial movement. Finally, the evolution of empathy, or emotional awareness, might have a neural basis in specialized cells in the neocortex, that is, spindle cells that have been associated with self-conscious emotions, and mirror neurons that have recently been shown to activate in response to communicative facial gestures.
Among humans, face recognition involves highly specialized cognitive and neural processes that enable the recognition of specific individuals [1–5]. While comparative studies suggest similar cognitive processes underlie face recognition in chimpanzees and humans [6–8, SOM#1], it remains unknown whether chimpanzees also show face-selective activity in ventral temporal cortex. This study is the first to examine regional cerebral glucose metabolism using 18F-Flurodeoxyglucose Positron Emission Tomography in chimpanzees after they performed computerized tasks matching conspecifics’ faces and nonface objects (SOM#2). A whole brain analysis comparing these two tasks directly in five chimpanzees revealed significant face-selective activity in brain regions known to comprise the distributed cortical face processing network in humans, including superior temporal sulcus and orbitofrontal cortex [9–11]. In order to identify regions that were exclusively active during one task, but not the other, a resting-state condition was subtracted from each task and the activity exclusive to each task was identified. This revealed numerous distinct patches of face-selective activity in the fusiform gyrus that were interspersed within a large expanse of object-selective cortex. This pattern suggests similar object form topography in the ventral temporal cortex of chimpanzees and humans, in which faces may represent a special class of visual stimulus.
The orbicularis oris muscle (OOM) plays a role in the production of primate facial expressions and vocalizations, nutrient intake, and in some non-human primates it is used as a prehensile, manipulative tool. Since the chimpanzee (Pan troglodytes) is the closest living relative of humans, a comparison of the OOM between these species may increase our understanding of the morphological specializations related to differing functional demands of their lips and the factors responsible for their divergent evolution. To these ends, this study compares the microanatomy of the mid-line upper fibers of the OOM between chimpanzees and humans. A mid-line portion of the OOM was harvested from upper lips of three chimpanzee and five human cadavers. The sampled blocks included the area between the lateral borders of the nasal alar cartilages in both species. Each sample was processed for paraffin histology, sectioned, and stained with a variety of protocols. Sections were examined for fiber direction and relative thickness of muscle layers. Ratios of cross-sectional connective tissue area vs. cross-sectional muscle tissue area, muscle fiber diameter, and relative dermal thickness were calculated for each species. In both species, a clear pars marginalis layer was recognized, contrary to previous reports that only humans possess this layer. In chimpanzees, relative fiber diameter and the relative amount of muscle tissue (i.e., based on connective tissue area: muscle tissue area ratio) were significantly (p<0.05) greater than in humans. In contrast, measurements of relative dermal thickness showed that humans have a greater average dermal thickness of the upper lip than chimpanzees. Taken together, these results suggest that both human and chimpanzee OOM upper fibers meet the specific functional demands associated with their divergent vocal and facial display repertoires, the development of human speech, and the use of the upper lip as a prehensile tool in chimpanzees.
chimpanzee; lips; orbicularis oris; histomorphometrics; wadging; speech; evolution