In What Way Is Natural Selection Related to Primate Communication?
Abstract
To understand the evolution of emotional communication, comparative enquiry on facial expression similarities between humans and related species is essential. Chimpanzees display a complex, flexible facial expression repertoire with many concrete and functional similarities to humans. This paper reviews what is known about these facial expression repertoires, discusses the importance of social arrangement in agreement the meaning of different expressions, and introduces a new coding system, the ChimpFACS, and describes how it can exist used to decide homologies betwixt human and chimpanzee facial expressions. Finally, information technology reviews previous studies on the categorization of facial expressions past chimpanzees using computerized tasks, and discusses the importance of configural processing for this skill in both humans and chimpanzees. Futurity directions for understanding the evolution of emotional communication will include studies on the social role of facial expressions in ongoing social interactions, the development of facial expression communication and more studies that examine the perception of these important social signals.
NATURAL ETHOLOGY OF FACIAL EXPRESSION: FORM AND FUNCTION
Many animal species communicate using a variety of highly conspicuous signals, including acoustic, tactile, olfactory and visual displays that have been tuned by natural selection to impact the listener in a reliable way (Smith, 1977 ; Dawkins and Krebs, 1978 ). Amid primates, the visual and auditory domains have become the two nearly prominently involved in social communications. At that place is, for example, a general supposition that facial expressions convey a variety of data near an private's motivation, intentions and emotions (van Hooff, 1967 ; Ekman, 1997 ; Parr, et al ., 2002 ). As such, facial expressions are critically important for coordinating social interaction, facilitating group cohesion and maintaining individual social relationships (Parr et al ., 2002 ). Despite the importance of facial expressions in the evolution of complex societies, at that place is fiddling piece of work comparing either the course or role of facial expression across singled-out phylogenetic groups. Moreover, apart from a handful of fantabulous ethograms that depict the advice repertoires for a diversity of species, including chimpanzees, bonobos, rhesus monkeys, capuchin monkeys and canids (Hinde and Rowell, 1962 ; van Hooff, 1962 , 1967 , 1973 ; Andrew, 1963 ; Goodall, 1968 ; Pull a fast one on, 1969 ; Bolwig, 1978 ; Weigel, 1979 ; de Waal, 1988 ; Preuschoft and van Hooff, 1997 ; Redican, 1982 ; Parr, et al ., 2005 ), at that place has been little attempt to standardize the clarification of their facial and vocal displays in a manner that facilitates comparative and evolutionary studies.
Fridlund ( 1994 ) has proposed that facial expressions are all-time understood equally chatty signals and as such, researchers should focus on their functional consequences during social interaction, or how they impact the listener. Moreover, the just way to fully sympathise why facial expressions have evolved to convey a specific meaning is to compare similar facial expressions between evolutionarily related species, examining any factors that may take influenced their social function, including ecological pressures and social factors like authority style and social organization (Preuschoft and van Hooff, 1997 ). Some facial expressions appear to be well represented beyond diverse taxonomic groups, making them good models for understanding social and emotional function, while others announced to be species-specific. The bared-teeth display, too referred to equally the fear grin, or grimace, is one of the most conspicuous and well-studied facial expressions in ethology and has been reported in a diverseness of mammalian species from canids to primates. Enquiry has shown, however, that the communicative function of this expression can differ quite broadly depending on the species, their type of social organization and social context. In wolves, for example, retraction of the lips horizontally over the teeth results in a 'submissive grinning' which is used past cubs and subordinates when actively greeting adult conspecifics, or humans (Fox, 1969 ). Antithetical to this expression is a vertical lip retraction which is given by dominant animals during aggressive interactions, very similar facial movements just with vastly different social functions (Darwin, 1872 ; Fox, 1969 ).
Among primates, the function of the bared-teeth also has unlike meanings depending on the species and their blazon of social organization. Amidst macaques species that take despotic social systems characterized by strict, linear say-so hierarchies, i.e. rhesus monkeys, the bared-teeth brandish appears to exist a point of submission, or rank recognition in that it is only given by subordinates to higher ranking individuals (van Hooff, 1976 ; de Waal and Luttrell, 1985 ). This expression has been referred to as a formal point of authorisation in the rhesus monkey because it is highly ritualized in appearance and has long-term predictability in determining dominance relationships despite brusque-term variation in social contexts (de Waal and Luttrell, 1985 ). In this report, bared-teeth displays performed by subordinate individuals occurred most often in response to the arroyo of a dominant monkey, and the most frequent response was for the subordinate to withdraw from whatsoever social interaction (de Waal and Luttrell, 1985 ). However, the pregnant of the bared-teeth display is quite different when used by species with more egalitarian social systems, including some macaques, mandrills, Gelada baboons and chimpanzees (van Hooff, 1967 ; Preuschoft and van Hooff, 1997 ). In these species, the bared-teeth brandish is more appeasing and functions to increase social attraction and affiliation. It communicates beneficial intent in that the signaler wishes no harm, and that in that location is no risk of aggression (van Hooff, 1967 ; van Hooff, 1976 ; Waller and Dunbar, 2005 ). It can also occur during affiliative contexts, such every bit grooming, sexual solicitation and reconciliations, and thus functions to increase affiliative tendencies and reduce proximity between individuals (van Hooff, 1973 ; Preuschoft and van Hooff, 1997 ; Parr et al ., 2005 ; Waller and Dunbar, 2005 ).
Numerous authors have gone on to propose that the bared-teeth display is homologous with the human being smile, meaning that they are the result of mutual evolutionary descent (van Hooff, 1972 ; Preuschoft and van Hooff, 1997 ; Waller and Dunbar, 2005 ). This conclusion is based in part on the physical similarity in the advent of the bared-teeth brandish and the homo smile, which are both characterized by retraction of the lip corners exposing, in most cases, the upper and lower teeth. Nonetheless, the homology is also based on similarity in the social function of these expressions, indicating appeasement, reassurance, increasing social bonding, and thus its important role in facilitating social cohesion amongst primates (Preuschoft and van Hooff, 1997 ). These examples suggest that while similar appearing expressions can oft have different meanings, or serve different social functions, depending on the species and their type of social organisation, they share a common evolutionary root. Therefore, in order to empathise the evolution of human emotional expressions, similar the smile, it is extremely informative to accept a comparative arroyo and evaluate whether the form and part of these expressions are uniquely human, present in many related species, or perhaps just shared by very closely related species, like Hominoids (Fridlund, 1994 ). The picture is less clear for other expressions, and only a few other direct comparisons have been made, i.e. homology between human laughter and the nonhuman primate 'play face' (Preuschoft and van Hooff, 1967; van Hooff, 1972 ). This is undoubtedly due to the fact that researchers, to date, have lacked a mutual linguistic communication, or standardized system that could exist used to identify potential homologues. Ideally, any cess of homology should be based on something other than physical advent, so a consideration of the underlying musculature is of import (Waller et al ., 2006 ). Among primates, it is highly likely that many expressions may share a common beginnings, and thus a comparative treatment is warranted, fifty-fifty if the expressions look different in terms of their characteristic features.
Evolution OF COMPARABLE CODING SYSTEMS FOR PRIMATE FACIAL EXPRESSIONS
Recently, researchers have developed an objective, standardized method for measuring facial motion in the chimpanzee that is directly comparable to humans, thus facilitating inquiry on facial expression homologues (Vick et al ., in press; world wide web.chimpfacs.com ). This system, referred to as ChimpFACS, is based on the well-known homo Facial Action Coding System, or FACS, developed by Ekman and colleagues to provide an objective tool for studying the biological basis for human expressions and emotion (Ekman and Friesen, 1978 ). Both of these systems are unique in that they identify the well-nigh minimal units of facial motion according to the part of the underlying musculature. Therefore, both systems enable the objective description of facial appearance changes in the human face and the chimpanzee face that are associated with movements of the underlying facial musculature. Each movement is described using a numeric code, referred to as an Action Unit of measurement (AU), and researchers are trained to utilize these codes reliably through a standardized testing process, making all users of the system statistically reliable with 1 another ( http://face-and-emotion.com ). There are several advantages to using such a standardized arrangement for comparative studies. Start, because the facial musculature of chimpanzees and humans is highly comparable (Burrows et al ., 2006 ; Waller et al ., 2006 ), the ii systems provide a basis for understanding homologous facial expression construction in the 2 species. Second, by orienting attention towards individual facial movements, researchers are encouraged to view the face up objectively and non exist biased by the humans' natural tendency to focus on overall expression configuration (Wallbott and Ricci-Bitti, 1993 ; Calder et al ., 2000 ). Thus, the face can be described, or coded, objectively using minimal units of measurement without interference from its holistic emotional quality. Finally, considering the systems are both standardized using a graded testing process, researchers from different groups are now able to compare their results directly using a common nomenclature. The value of the system is obvious in the examples given above; when attempting to decide whether expressions are homologous across chimpanzees and humans, similar the bared-teeth display and the smiling, researchers can brainstorm by using subjective ratings of their similarity, but and then utilize the more objective, standardized assessment of their anatomical similarity by comparing private action units in both species.
After the creation of the ChimpFACS, researchers were interested in testing its accuracy to determine whether this bottom-up arroyo (action unit of measurement to expression configuration) could actually validate the existing categories of chimpanzee facial expressions made previously by experts in chimpanzee communicative behavior. This was done using a discriminate functions assay (DFA) which enabled a statistical comparison of whether the a priori expression categories could be predicted by mutual patterns of underlying facial movements, or patterns of AUs, coded from each expression using ChimpFACS. Over 250 facial expressions were categorized by Parr using published ethograms of chimpanzee behavior (Parr et al ., 2005 ), and so coded using ChimpFACS past Waller, a certified ChimpFACS expert (Vick et al ., in printing). Ix expression categories, and 15 activeness units, accounted for the 250+ examples and this included an ambiguous category that was used for expressions that could not easily exist classified. The results of the DFA showed a high percentage of agreement for well-nigh all of the expression categories (Parr et al ., in press). The only notable exception was the pout, which was near oft identified as a pant-hoot due to a similarity in 1 AU movement (AU22-lip funneler) which was present in both of these expressions. Therefore, objective coding of naturally occurring chimpanzee facial expressions according to minimal facial movements using ChimpFACS produced functionally meaningful expression categories. Moreover, the analysis enabled the identification of unique combinations of muscle movements (AUs) for all expression categories. Thus, the DFA provided a clear clarification of prototypical facial expression configurations based on standardized muscle action (Parr et al ., in printing). These configurations were so predictive that naïve researchers were able to identify expression categories with over 80% accurateness solely by reading the AU configuration (Parr et al ., in press). An analogy of each prototypical expression configuration and the per centum of category agreement betwixt AU codes and a priori classifications can be seen in Figure 1 .
Fig. 1
An illustration of prototypical chimpanzee facial expressions. These are listed in pairs. The example on the left side of the pair shows the Poser animated expression, while the example on the correct shows a naturalistic chimpanzee expression. Under the Poser expression is the prototypical AU configuration as identified by the Discriminant Functions Analysis, and nether the naturalistic expression is the percentage agreement between AU configuration and a priori classification for that category.
Fig. one
An illustration of prototypical chimpanzee facial expressions. These are listed in pairs. The example on the left side of the pair shows the Poser animated expression, while the example on the right shows a naturalistic chimpanzee expression. Nether the Poser expression is the prototypical AU configuration as identified by the Discriminant Functions Analysis, and under the naturalistic expression is the percentage agreement between AU configuration and a priori classification for that category.
THE STANDARDIZATION OF CHIMPANZEE FACIAL EXPRESSIONS USING CHIMPFACS
The identification of prototypical expression configurations is extremely important for ongoing research on chimpanzee social cognition as it provides a blueprint for standardizing the images used in computerized tasks, such every bit expression categorization studies (Parr et al ., 1998 ). Using the results from the DFA, researchers at present have a listing of prototypical facial configurations that account for each major expression category, and an objective tool in ChimpFACS for identifying these. Even with these tools, even so, acquiring photographs of chimpanzee expressions at their peak intensity is extremely difficult because expressions are dynamic, oftentimes occur during highly charged social contexts where there is fast movement, and subjects are more often than not faced away from the photographer, making it hard to capture frontal pictures, or to standardize these in terms of head orientation and posture.
To overcome the difficulties in obtaining high quality naturalistic photographs of chimpanzee expressions, researchers take now turned to custom 3-dimentional (3D) blitheness software to create these configurations manually. There are two types of expressions shown in Figure 1 , a naturalistic photograph and a drawing-like chimpanzee face up. The cartoon-like face was created using the 3D animation software Poser 6.0 ( world wide web.efrontiers.com ), which enables detailed, custom animation of facial movements by allowing the user to impose an artificial muscle structure onto the confront and then animating this 'muscle' to touch on facial advent in a highly controlled and naturalistic manner. To illustrate, the Poser expressions in the figure were made using anatomically accurate muscle structure and realistic AU movements and are posed in the prototypical facial configurations identified by the DFA (Parr et al ., in press). Because chimpanzee researchers cannot position chimpanzee faces using verbal instruction, as is the principal method for creating homo expression stimuli for use in emotion perception studies, the Poser chimpanzee allows researchers control over head orientation, gaze direction, expression configuration and intensity. The application of these techniques volition considerably advance comparative cognition enquiry as it eliminates the aforementioned difficulties in creating controlled stimulus libraries where each instance expression is required to be photographed at peak intensity, in full-frontal posture from freely behaving chimpanzees. Moreover, in combination with the ChimpFACS, researchers can utilize these models to pose individual action units, realistic combinations of action units, such as those seen in Figure 1 , and even artificial combinations of facial movements to specifically examine which elements of expressions are virtually meaningful for the chimpanzee's emotional perception.
Comparison HOMOLOGOUS FACIAL MOVEMENTS IN CHIMPANZEES AND HUMANS
With the development of a standardized system for measuring facial movement in chimpanzees (ChimpFACS) that is anatomically and organizationally comparable to humans (FACS), the stage is gear up for a more comprehensive investigation into the evolution of facial expressions and facial emotion (Parr et al ., in press; Vick et al ., in press; Waller et al ., 2006 ). There are several ways to endeavor such an investigation and information technology will probable be quite some fourth dimension earlier there are enough information to make whatever definitive conclusions. At this very preliminary state, one tin can reasonably describe similarities in the facial movements between prototypical chimpanzee expressions and human emotional expressions (Parr et al ., in press). In their study, these authors near closely matched the AUs' configurations of the chimpanzee bared-teeth display and man smile, chimpanzee screams to human screams, the chimpanzee jutting lip face to human anger, chimpanzee laughter to man laughter, only could not find a good human equivalent for the chimpanzee pant-hoot. When humans accept been asked to judge the emotional quality of chimpanzee expressions using basic emotions labels, the post-obit comparisons have emerged: chimpanzee bared-teeth display = happiness, chimpanzee play face = happiness, chimpanzee pant-hoot = happy, chimpanzee scream = acrimony (Fernandez-Carriba et al ., unpublished data). Thus, in these very preliminary comparisons, similarities emerged.
An alternative, and arguably a more than intuitive, arroyo is to kickoff match human and chimpanzee expressions based on the muscular components, then brand inferences almost the emotional quality of human faces that are structurally homologous to chimpanzee expressions. Figure 2 shows such a comparison where the human images are taken direct from the FACS transmission (Ekman et al ., 2002 ) and show facial movements that are structurally homologous to the prototypical chimpanzee expressions. The identical AUs shared by the 2 expression examples are highlighted in bold italics. In some cases there is an extra movement in either the human or chimpanzee example. Non surprisingly, one of the most hit comparisons hither is between the chimpanzee bared-teeth display and its human being equivalent. Every bit discussed earlier, previous researchers have suggested a homology between the chimpanzee bared-teeth display and the man smile (van Hooff, 1972 ; Preuschoft and van Hooff, 1995 ; Waller and Dunbar, 2005 ; Parr et al ., in press). Nevertheless, the human bared-teeth expression in Figure two strongly resembles a grimace, or a forced smile, but information technology does non give the countenance of happiness. Contrast this to the human play face up, which gives a strong impression of happiness and supports others who take suggested that this expression is homologous to laughter in humans (van Hooff, 1972 ; Preuschoft and van Hooff, 1995 ; Waller and Dunbar, 2005 ; Parr et al ., in press). One caption may exist that genuine human smiles, those associated with emotional happiness or enjoyment, include an AU6-cheek raiser, which is missing in the human being bared-teeth expression (Ekman and Friesen, 1982 ; Ekman et al ., 1990 ; Frank et al ., 1993 ). Interestingly, when people accept been asked to rate man faces that contain either individual movements (single AUs) or combinations of movements (multiple AUs), researchers report that the configuration AU10 + 12 + 16 + 25 (the same movements as in the prototypical chimpanzee bared-teeth face) is near ofttimes described as showing fear (Wallbott and Ricci-Bitti, 1993 ). While it might seem unlikely that the add-on of a unmarried AU (the AU6) could change the entire emotional estimation of an expression, the information strongly back up this conclusion. Because humans process facial expressions configurally, and exercise not selectively attend to individual features, the configuration AU10 + 12 + 16 + 25 is interpreted differently than AU6 + 10 + 12 + 25 (Wallbott and Ricci-Bitti, 1993 ). If the product of AU6 does indeed correlate with genuine happiness, so information technology is arguably very important to distinguish between these two expressions, and thus configural processing has a clear and crucial function. Interestingly, the man scream face in Effigy ii contains an AU6, and information technology does not appear to have a strong negative emotional countenance. These findings provide a strong argument for the need to more than fully understand how chimpanzees procedure their own facial expressions, what the role of configural processing is in expression categorization and whether chimpanzees are as sensitive to individual movements as humans. Therefore, in order to more fully sympathise the evolution of advice and the social function of facial expressions, a comparative perspective on facial expression categorization is needed.
Fig. 2
Prototypical chimpanzee facial expressions and homologous facial movements in a man (Ekman et al ., 2002 ).
Fig. 2
Prototypical chimpanzee facial expressions and homologous facial movements in a homo (Ekman et al ., 2002 ).
PREVIOUS STUDIES ON FACIAL EXPRESSION CATEGORIZATION By CHIMPANZEES
Studies of emotional advice have typically taken one of two approaches, either they focus on the perception of the brandish, how it is processed and an examination of salient features or configuration necessary for accurate interpretation, or they focus on the social function of the expression, how information technology affects the behavior of a receiver within a social surroundings. Ideally, it is the combination of these 2 approaches that will provide the most complete understanding of emotional communication in ongoing social interactions. Therefore, two disquisitional questions for understanding the development of emotional advice are; beginning, whether chimpanzees and humans employ similar perceptual cues to discriminate amidst facial expression categories, and second, what is the functional outcome of these expressions for ongoing social interactions. In addressing the first approach, human being studies take shown strong configural preferences for both face identity recognition and facial expression categorization (Calder et al ., 2000 ; Etcoff and Magee, 1992 ; Wallbott and Ricci-Bitti, 1993 ). Configural cues refer to the relative size, shape and spatial arrangement of features within the face (Maurer et al ., 2002 ). While numerous studies have demonstrated a strong configural bias for face up identity processing in the chimpanzee (Parr et al ., 2000 ; Parr et al ., 2006 ; Parr and Heintz, in press), the perceptual cues important for facial expression categorization remain unclear (Parr et al ., 1998 ).
Parr and colleagues have performed several studies to accost this first approach: how practise chimpanzees discriminate facial expressions, and what are the relevant features for this perceptual procedure (Parr et al ., 1998 ; Parr and Maestripieri, 2003 )? The goal of these studies was first to make up one's mind whether chimpanzees could visually match different examples of expression types and 2d, to try and understand how these categorizations were achieved. Were subjects, for instance, extracting specific salient features, similar the presence of teeth, oral fissure position, etc., or were they relying on the overall expression configuration, like humans? These studies accept been performed using a computerized joystick-testing paradigm and matching-to-sample (MTS) format. Subjects have had years of expertise with this testing situation and eagerly participate in daily testing sessions (Parr et al ., 2000 , Parr and de Waal, 1999 ). The basic format for MTS is that subjects are first presented with a sample stimulus, a conspecific facial expression, for example, and a cursor on the computer screen over a black groundwork. They take been trained that when they can command the movements of the cursor on the monitor by manipulating the joystick. The sample stimulus is the image to lucifer and they starting time must orient towards information technology past contacting information technology with the joystick-controlled cursor. After this, the sample stimulus clears the screen and they are presented with two alternative stimuli, i matches the sample on predetermined stimulus dimension, i.eastward. expression type (target epitome), while the other does not match (foil).
In a previous expression matching task, Parr and colleagues ( 1998 ) presented chimpanzees with a sample photograph showing an unfamiliar conspecific making one of the vi facial expressions, bared-teeth, pant-hoot, play face, relaxed-lip face, scream and neutral. The target image was another individual making the aforementioned facial expression, so the matching pair of photographs was unique, and the foil showed a tertiary individual making a neutral expression. Successful matching, therefore, required discrimination based on expression type, not the identity of the individual making the expression. On the beginning testing session, all six chimpanzees performed significantly above chance (50%) on the majority of the expression categories (bared-teeth = 67%, play face = 68%, scream = 64%), demonstrating that not only are expressions highly salient, only that expression type tin be used spontaneously as a means for bigotry (Parr et al ., 1998 ).
In a follow-upward to this report, Parr and colleagues ( 1998 ) investigated how expressions were discriminated, using overall configuration or the extraction of specific features. To exercise this, each expression was characterized according to the presence or absence of specific salient features using subjective criteria, such equally mouth open, eyes open up, teeth visible. This was washed for five expression types, bared-teeth, pant-hoot, play face, relaxed-lip and scream. In the experimental job, each dyadic combination of expressions was so paired together, totaling 20 dissimilar combinations for the v expression categories. For half of the expression pairs ( N = ten), the target (friction match) and foil (not-match) expressions shared three or more than features in mutual, while the other 10 pairs were very distinctive, sharing ii or fewer features in common. Effigy 3 shows examples of a trial where the expression pair shares features in common ( Effigy 3 A) or has little feature overlap ( Effigy 3 B). The performance on expression dyads that were similar versus distinctive was then compared. The hypothesis was that if the chimpanzees were extracting specific salient features, such as teeth visible, when performing expression discriminations, their performance should be improve on distinct dyads compared with like dyads. Moreover, a negative correlation would exist expected overall betwixt the number of shared features in each of the 20 expression dyads and performance matching the expressions in those trials, i.e. the more feature overlap, the worse performance would be. The information supported the outset prediction: overall, subjects' functioning was significantly better discriminating expression dyads that were singled-out compared to similar, t (4) = 2.94, P < 0.05. There was, yet, little back up for the second prediction. The overall correlation between shared features and performance was only weakly negative, r = −0.11, NS (Parr et al ., 1998 ). In fact, the role of distinctive, non-overlapping facial features in providing an advantage for expression discrimination was merely detected for some expression categories and not others. Thus, no conclusive evidence was found for or confronting configural facial expression processing in this species.
Fig. 3
An illustration of an expression matching trial where the expression pairs share similar features (A), or in which the expression pair contains distinctive features (B).
Fig. 3
An illustration of an expression matching trial where the expression pairs share similar features (A), or in which the expression pair contains distinctive features (B).
One serious problem with these before studies was the inability to objectively categorize each expression in terms of its relevant features. With the creation of ChimpFACS and the development of the standardized, prototypical Poser expressions described earlier, the question of how chimpanzees process facial expressions, configurally or using feature-based cues, can exist re-addressed in a more controlled way by manipulating activity units individually and in combinations and presenting these to subjects using similar discrimination studies every bit described earlier. These studies are currently underway at the Yerkes Primate Heart and volition make an important contribution towards understanding the evolution of emotional communication by addressing similarities and differences in the perceptual discrimination of facial expressions past chimpanzees compared to humans.
In conclusion, a slap-up deal still remains to exist learned about the evolution of social signals, like facial expressions. This article has reviewed and expanded on information suggesting homologies between the primate bared-teeth display and the human smile, and the primate play face and human laughter. But to what extent are other facial expressions evolutionarily similar beyond related species? Accent has likewise been placed on the general lack of knowledge concerning the social function of facial expressions in not-human being primates. Only a handful of studies take thus far been attempted, withal the question of how these expressions function in a social context is critical for a more complete understanding of their evolutionary significance and continuity. Finally, this cursory review did non address bug of development or social learning, i.east. how do primates learn the social office and/or meaning of these expressions? Is there an innate component, or is learning totally dependent on experience? These developmental questions pertain to more than just understanding the meaning of expressions, merely also in knowing how to produce expressions, every bit in appearance, so as to convey appropriate information to conspecifics, and then knowing when to use them in the advisable social contexts.
Tools such as the ChimpFACS provide an exciting new methodology for agreement similarities and differences in chimpanzee and human expressions by comparison homologous facial movements. Thus, this new tool volition help facilitate comparisons of facial expressions co-ordinate to their structural similarities and enable researchers to expand on whether facial expressions other than the bared-teeth display and the play face may accept structural homologs in humans. Moreover, combining studies on the structural similarity of these expressions with tasks that investigate their perceptual classification will provide insights into which features are important for expression categorization, and how the cognitive processes that underlie the categorization of these signals are indeed like to what is known near man expression categorization. This latter question may tie in with recent piece of work on human being neuroimaging that suggests that neural systems important for face identity recognition and processing emotional content from facial expressions are, in fact, dissociable (Haxby et al ., 2000 ; Kesler-Westward et al ., 2001 ; Phan et al ., 2004 ). Finally, sorely defective are studies that examine the social function of facial expressions both in chimpanzees and in humans. Agreement their function is critical for making whatsoever inferences about evolutionary continuity, their emotional significant and the co-development of signaler and receiver to maximize efficient advice inside social groups.
Conflict of Involvement
None declared.
Back up for this commodity was provided by RR-00165 from the NIH/NCRR to the Yerkes National Primate Research Center, R01-MH068791 to 50.A.P. Special thanks to Matt Heintz for assistance with Poser 6.0, Paul Ekman for the use of photographic stimuli, and Ralph Adolphs, David Skuse and the Cold Springs Harbor Institute for organizing the special briefing on the Biology of Social Cognition where these ideas were disseminated.
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