Anna Roberts - Academia.edu (original) (raw)
Papers by Anna Roberts
Biological Reviews, 2019
Mammals living in more complex social groups typically have large brains for their body size and ... more Mammals living in more complex social groups typically have large brains for their body size and many researchers have proposed that the primary driver of the increase in brain size through primate and hominin evolution was the selection pressures associated with sociality. Many mammals, and especially primates, use flexible signals that show a high degree of voluntary control and these signals may play an important role in forming and maintaining social relationships between group members. However, the specific role that cognitive skills play in this complex communication, and how in turn this relates to sociality, is still unclear. The hypothesis for the communicative roots of complex sociality and cognition posits that cognitive demands behind the communication needed to form and maintain bonded social relationships in complex social settings drives the link between brain size and sociality. We review the evidence in support of this hypothesis and why key features of cognitively complex communication such as intentionality and referentiality should be more effective in forming and maintaining bonded relationships as compared with less cognitively complex communication. Exploring the link between cognition, communication and sociality provides insights into how increasing flexibility in communication can facilitate the emergence of social systems characterised by bonded social relationships, such as those found in non-human primates and humans. To move the field forward and carry out both within-and among-species comparisons, we advocate the use of social network analysis, which provides a novel way to describe and compare social structure. Using this approach can lead to a new, systematic way of examining social and communicative complexity across species, something that is lacking in current comparative studies of social structure.
A key challenge for primates living in large, stable social groups is managing social relationshi... more A key challenge for primates living in large, stable social groups is managing social relationships. Chimpanzee gestures may act as a time-efficient social bonding mechanism, and the presence (homogeneity) and absence (heterogeneity) of overlap in repertoires in particular may play an important role in social bonding. However, how homogeneity and heterogeneity in the gestural repertoire of primates relate to social interaction is poorly understood. We used social network analysis and generalized linear mixed modelling to examine this question in wild chimpanzees. The repertoire size of both homogeneous and heterogeneous visual, tactile and auditory gestures was associated with the duration of time spent in social bonding behaviour, centrality in the social bonding network and demography. The audience size of partners who displayed similar or different characteristics to the signaller (e.g. same or opposite age or sex category) also influenced the use of homogeneous and heterogeneous gestures. Homogeneous and heterogeneous gestures were differentially associated with the presence of emotional reactions in response to the gesture and the presence of a change in the recipient's behaviour. Homogeneity and heterogeneity of gestural communication play a key role in maintaining a differentiated set of strong and weak social relationships in complex, multilevel societies.
Nature Communications, 2014
Humans routinely communicate to coordinate their activities, persisting and elaborating signals t... more Humans routinely communicate to coordinate their activities, persisting and elaborating signals to pursue goals that cannot be accomplished individually. Communicative persistence is associated with complex cognitive skills such as intentionality, because interactants modify their communication in response to another's understanding of their meaning. Here we show that two language-trained chimpanzees effectively use intentional gestures to coordinate with an experimentally naive human to retrieve hidden food, providing some of the most compelling evidence to date for the role of communicative flexibility in successful coordination in nonhumans. Both chimpanzees (named Panzee and Sherman) increase the rate of nonindicative gestures when the experimenter approaches the location of the hidden food. Panzee also elaborates her gestures in relation to the experimenter's pointing, which enables her to find food more effectively than Sherman. Communicative persistence facilitates effective communication during behavioural coordination and is likely to have been important in shaping language evolution.
Primates form strong and enduring social bonds with others and these bonds have important fitness... more Primates form strong and enduring social bonds with others and these bonds have important fitness consequences. However, how different types of communication are associated with different types of social bonds is poorly understood. Wild chimpanzees have a large repertoire of gestures, from visual gestures to tactile and auditory gestures. We used social network analysis to examine the association between proximity bonds (time spent in close proximity) and rates of gestural communication in pairs of chimpanzees when the intended recipient was within 10 m of the signaller. Pairs of chimpanzees with strong proximity bonds had higher rates of visual gestures, but lower rates of auditory long-range and tactile gestures. However, individual chimpanzees that had a larger number of proximity bonds had higher rates of auditory and tactile gestures and lower rates of visual gestures. These results suggest that visual gestures may be an efficient way to communicate with a small number of regular interaction partners, but that tactile and auditory gestures may be more effective at communicating with larger numbers of weaker bonds. Increasing flexibility of communication may have played an important role in managing differentiated social relationships in groups of increasing size and complexity in both primate and human evolution. Strong and enduring social bonds in primates are closely linked with fitness outcomes 1 , but the need to track and maintain these social relationships results in primate sociality being constrained by both temporal and cognitive factors 2. These constraints give rise to primate groups with a multilevel structure containing hierarchically nested social levels, with individuals having a differentiated set of stronger and weaker social relationships with conspe-cifics 3,4. Gestural communication–voluntary movements of the arms, head, body postures and locomotory gaits 5–7 –has been hypothesized to regulate social bonding in some multilevel societies 8 , because it provides an important channel through which individuals influence the behaviour and emotions of their social partners 5. Gestures vary in modality from less intense visual gestures to more intense auditory long-range or tactile gestures 5,6. Different types of social bonds may be associated with different patterns of gestural communication through the intensity of emotional arousal, whereby individuals express their own emotional arousal and also evaluate and process emotional arousal in others, in order to respond in an adaptive manner 9–17. Emotional arousal has been operationally defined as a state of physiological activation experienced as a change in heart rate 14,18 , cortisol secretion 19–23 or nasal temperature 24. Arousal change is associated with corresponding behavioural patterns, precipitated by exposure to different types of bonds with the social partners 25–28. For instance, when a primate interacts with a conspecific with whom they have a history of prior aggressive interactions , this is associated with an increase in heart rate, indicating an aroused response 14,18. In contrast, when a primate interacts with a conspecific with whom they have a positive social relationship, there is no increase in heart rate from baseline, indicating a lack of arousal 14,18. Various indicators of arousal relating to communication have also been examined. It has been proposed that indices of arousal and arousal change can be classified along the dimension of communication intensity or strength such as the potency of its presentation (loudness), frequency and duration 25,29. For instance, non-verbal loudness (rated on the scale from loud to soft) is correlated with self-reported measures of emotional arousal in humans 25. Louder human voices are associated with higher
The extent to which primates can flexibly adjust the production of gestural communication accordi... more The extent to which primates can flexibly adjust the production of gestural communication according to the presence and visual attention of the audience provides key insights into the social cognition underpinning gestural communication, such as an understanding of third party relationships. Gestures given in a mating context provide an ideal area for examining this flexibility, as frequently the interests of a male signaller, a female recipient and a rival male bystander conflict. Dominant chimpanzee males seek to monopolize matings, but subordinate males may use gestural communication flexibly to achieve matings despite their low rank. Here we show that the production of mating gestures in wild male East African chimpanzees (Pan troglodytes schweunfurthii) was influenced by a conflict of interest with females, which in turn was influenced by the presence and visual attention of rival males. When the conflict of interest was low (the rival male was present and looking away), chimpanzees used visual/ tactile gestures over auditory gestures. However, when the conflict of interest was high (the rival male was absent, or was present and looking at the signaller) chimpanzees used auditory gestures over visual/ tactile gestures. Further, the production of mating gestures was more common when the number of oestrous and non-oestrus females in the party increased, when the female was visually perceptive and when there was no wind. Females played an active role in mating behaviour, approaching for copulations more often when the number of oestrus females in the party increased and when the rival male was absent, or was present and looking away. Examining how social and ecological factors affect mating tactics in primates may thus contribute to understanding the previously unexplained reproductive success of subordinate male chimpanzees.
10.1038/ncomms4088, Jan 16, 2014
Humans routinely communicate to coordinate their activities, persisting and elaborating signals t... more Humans routinely communicate to coordinate their activities, persisting and elaborating signals to pursue goals that cannot be accomplished individually. Communicative persistence is associated with complex cognitive skills such as intentionality, because interactants modify their communication in response to another’s understanding of their meaning. Here we show that two language-trained chimpanzees effectively use intentional gestures to coordinate with an experimentally naive human to retrieve hidden food, providing some of the most compelling evidence to date for the role of communicative flexibility in successful coordination in nonhumans. Both chimpanzees (named Panzee and Sherman) increase the rate of non-indicative gestures when the experimenter approaches the location of the hidden food. Panzee also elaborates her gestures in relation to the experimenter’s pointing, which enables her to find food more effectively than Sherman. Communicative persistence facilitates effective communication during behavioural coordination and is likely to have been important in shaping language evolution.
A growing body of evidence suggests that human language may have emerged primarily in the gestura... more A growing body of evidence suggests that human language may have emerged primarily in the gestural rather than vocal domain, and that studying gestural communication in great apes is crucial to understanding language evolution. Although manual and bodily gestures are considered distinct at a neural level, there has been very limited consideration of potential differences at a behavioural level. In this study, we conducted naturalistic observations of adult wild East African chimpanzees (Pan troglodytes schweinfurthii) in order to establish a repertoire of gestures, and examine intentionality of gesture production, use and comprehension, comparing across manual and bodily gestures. At the population level, 120 distinct gesture types were identified, consisting of 65 manual gestures and 55 bodily gestures. Both bodily and manual gestures were used intentionally and effectively to attain specific goals, by signallers who were sensitive to recipient attention. However, manual gestures differed from bodily gestures in terms of communicative persistence, indicating a qualitatively different form of behavioural flexibility in achieving goals. Both repertoire size and frequency of manual gesturing were more affiliative than bodily gestures, while bodily gestures were more antagonistic. These results indicate that manual gestures may have played a significant role in the emergence of increased flexibility in great ape communication and social bonding.
We examine evidence for communicative intent during conspecific interactions in wild chimpanzees ... more We examine evidence for communicative intent during conspecific interactions in wild chimpanzees (Budongo Forest, Uganda), focusing on persistence in gestural communication. Previous research indicates that great apes have large gestural repertoires and produce gestural communication in a flexible and intentional manner, including the production of gesture sequences. Although there is a lack of consensus on the form and function of sequences, there is some evidence that sequences are produced when signallers fail to receive any response from a recipient. Here we provide first systematic evidence for communicative persistence in wild chimpanzees. Rather than examining only the presence or absence of a response, we used the most commonly observed response to assign meanings to gestures and examined sequence production in relation to response congruency. Chimpanzees ceased communication if successful, but persevered when unsuccessful. Chimpanzees repeated gestures when a response partially matched their goal but substituted the original gesture when a response was incongruent. Persistence was also mediated by recipient intent to respond, with more sequences produced within competitive than affiliative contexts. Gestures within sequences were homogenous in semantic meaning and signallers continued until the response matched the assigned meaning of the initial gesture. Gestural sequence production was not primarily affective; gesture intensity (in terms of modality) did not increase within sequences. Chimpanzee gestural sequences emerged to achieve specific outcomes; given variability in recipient behaviour following initial gestures, signallers were flexible in their persistence towards these goals.
Great ape gestural communication shares some important features with human language, namely flexi... more Great ape gestural communication shares some important features with human language, namely flexible and intentional signal use, and is important in understanding the evolutionary history of human communication. Research effort has primarily been devoted to compiling gestural repertoires in captive primates, while there are fewer systematic studies from the wild and in both contexts gesture reports are largely qualitative. We examined the gestural repertoire of a community of free-ranging East African chimpanzees, using focal animal sampling. Our goal was to quantify the morphological structure and variation of gestural signals. We used a morphological classification system based on 29 features, such as the trajectory and orientation during the preparatory and executive phases of a gesturing event. We then carried out hierarchical cluster and discriminant function analyses, which generated a repertoire of 30 morphologically distinct gesture types. Using a cross-validation technique we were able to correctly classify a majority of them, with incorrect classifications mainly for rare gesture types. We then compared the resulting clusters with previously reported repertoires; there was no systematic variation between captive and wild chimpanzee repertoires. Moreover, consensus was not greater for studies conducted within the same populations, highlighting the importance of systematic and well-documented inventories. Our morphologically based analyses indicate that manual gestures are best considered as graded rather than discrete communication signals, similar to some vocalisation systems. We discuss these findings in light of current theories of human language evolution.
Keywords: chimpanzee; gesture; great ape; communication; repertoire; structural inventory
Flexibility is considered a defining feature of great ape gestural communication. Previous resear... more Flexibility is considered a defining feature of great ape gestural communication. Previous research has suggested that there is a ‘means–ends’ dissociation between gesture type and context, whereby one signal may be used across contexts and several signals used within the same context. Such flexibility in signal production demands contextual comprehension, whereby recipients may perceive the context-free message of a given manual gesture, but also decide how to respond by inferring the signaller’s goals from the accompanying context. We conducted naturalistic observations of wild East African chimpanzee, Pan troglodytes schweinfurthii, manual gestures, focusing on recipient perspectives during communicative interactions. Our results indicate that chimpanzees recognize the context-free meanings of gestures and they are also able to respond flexibly by inferring the meaning from the combination of gesture and context, including relative rank. When analysed at the level of gesture type, some gestures were tightly associated with dominant responses and outcomes. Chimpanzee manual gestures are primarily used for directing a recipient’s movement or attention but the motivation underlying these gestural requests is inferred by the recipient from the context.
Keywords: chimpanzee; communication; contextual comprehension; gesture; gesture meaning; great ape; intentionality; manual gesture; Pan troglodytes schweinfurthii; semantic comprehension
Biological Reviews, 2019
Mammals living in more complex social groups typically have large brains for their body size and ... more Mammals living in more complex social groups typically have large brains for their body size and many researchers have proposed that the primary driver of the increase in brain size through primate and hominin evolution was the selection pressures associated with sociality. Many mammals, and especially primates, use flexible signals that show a high degree of voluntary control and these signals may play an important role in forming and maintaining social relationships between group members. However, the specific role that cognitive skills play in this complex communication, and how in turn this relates to sociality, is still unclear. The hypothesis for the communicative roots of complex sociality and cognition posits that cognitive demands behind the communication needed to form and maintain bonded social relationships in complex social settings drives the link between brain size and sociality. We review the evidence in support of this hypothesis and why key features of cognitively complex communication such as intentionality and referentiality should be more effective in forming and maintaining bonded relationships as compared with less cognitively complex communication. Exploring the link between cognition, communication and sociality provides insights into how increasing flexibility in communication can facilitate the emergence of social systems characterised by bonded social relationships, such as those found in non-human primates and humans. To move the field forward and carry out both within-and among-species comparisons, we advocate the use of social network analysis, which provides a novel way to describe and compare social structure. Using this approach can lead to a new, systematic way of examining social and communicative complexity across species, something that is lacking in current comparative studies of social structure.
A key challenge for primates living in large, stable social groups is managing social relationshi... more A key challenge for primates living in large, stable social groups is managing social relationships. Chimpanzee gestures may act as a time-efficient social bonding mechanism, and the presence (homogeneity) and absence (heterogeneity) of overlap in repertoires in particular may play an important role in social bonding. However, how homogeneity and heterogeneity in the gestural repertoire of primates relate to social interaction is poorly understood. We used social network analysis and generalized linear mixed modelling to examine this question in wild chimpanzees. The repertoire size of both homogeneous and heterogeneous visual, tactile and auditory gestures was associated with the duration of time spent in social bonding behaviour, centrality in the social bonding network and demography. The audience size of partners who displayed similar or different characteristics to the signaller (e.g. same or opposite age or sex category) also influenced the use of homogeneous and heterogeneous gestures. Homogeneous and heterogeneous gestures were differentially associated with the presence of emotional reactions in response to the gesture and the presence of a change in the recipient's behaviour. Homogeneity and heterogeneity of gestural communication play a key role in maintaining a differentiated set of strong and weak social relationships in complex, multilevel societies.
Nature Communications, 2014
Humans routinely communicate to coordinate their activities, persisting and elaborating signals t... more Humans routinely communicate to coordinate their activities, persisting and elaborating signals to pursue goals that cannot be accomplished individually. Communicative persistence is associated with complex cognitive skills such as intentionality, because interactants modify their communication in response to another's understanding of their meaning. Here we show that two language-trained chimpanzees effectively use intentional gestures to coordinate with an experimentally naive human to retrieve hidden food, providing some of the most compelling evidence to date for the role of communicative flexibility in successful coordination in nonhumans. Both chimpanzees (named Panzee and Sherman) increase the rate of nonindicative gestures when the experimenter approaches the location of the hidden food. Panzee also elaborates her gestures in relation to the experimenter's pointing, which enables her to find food more effectively than Sherman. Communicative persistence facilitates effective communication during behavioural coordination and is likely to have been important in shaping language evolution.
Primates form strong and enduring social bonds with others and these bonds have important fitness... more Primates form strong and enduring social bonds with others and these bonds have important fitness consequences. However, how different types of communication are associated with different types of social bonds is poorly understood. Wild chimpanzees have a large repertoire of gestures, from visual gestures to tactile and auditory gestures. We used social network analysis to examine the association between proximity bonds (time spent in close proximity) and rates of gestural communication in pairs of chimpanzees when the intended recipient was within 10 m of the signaller. Pairs of chimpanzees with strong proximity bonds had higher rates of visual gestures, but lower rates of auditory long-range and tactile gestures. However, individual chimpanzees that had a larger number of proximity bonds had higher rates of auditory and tactile gestures and lower rates of visual gestures. These results suggest that visual gestures may be an efficient way to communicate with a small number of regular interaction partners, but that tactile and auditory gestures may be more effective at communicating with larger numbers of weaker bonds. Increasing flexibility of communication may have played an important role in managing differentiated social relationships in groups of increasing size and complexity in both primate and human evolution. Strong and enduring social bonds in primates are closely linked with fitness outcomes 1 , but the need to track and maintain these social relationships results in primate sociality being constrained by both temporal and cognitive factors 2. These constraints give rise to primate groups with a multilevel structure containing hierarchically nested social levels, with individuals having a differentiated set of stronger and weaker social relationships with conspe-cifics 3,4. Gestural communication–voluntary movements of the arms, head, body postures and locomotory gaits 5–7 –has been hypothesized to regulate social bonding in some multilevel societies 8 , because it provides an important channel through which individuals influence the behaviour and emotions of their social partners 5. Gestures vary in modality from less intense visual gestures to more intense auditory long-range or tactile gestures 5,6. Different types of social bonds may be associated with different patterns of gestural communication through the intensity of emotional arousal, whereby individuals express their own emotional arousal and also evaluate and process emotional arousal in others, in order to respond in an adaptive manner 9–17. Emotional arousal has been operationally defined as a state of physiological activation experienced as a change in heart rate 14,18 , cortisol secretion 19–23 or nasal temperature 24. Arousal change is associated with corresponding behavioural patterns, precipitated by exposure to different types of bonds with the social partners 25–28. For instance, when a primate interacts with a conspecific with whom they have a history of prior aggressive interactions , this is associated with an increase in heart rate, indicating an aroused response 14,18. In contrast, when a primate interacts with a conspecific with whom they have a positive social relationship, there is no increase in heart rate from baseline, indicating a lack of arousal 14,18. Various indicators of arousal relating to communication have also been examined. It has been proposed that indices of arousal and arousal change can be classified along the dimension of communication intensity or strength such as the potency of its presentation (loudness), frequency and duration 25,29. For instance, non-verbal loudness (rated on the scale from loud to soft) is correlated with self-reported measures of emotional arousal in humans 25. Louder human voices are associated with higher
The extent to which primates can flexibly adjust the production of gestural communication accordi... more The extent to which primates can flexibly adjust the production of gestural communication according to the presence and visual attention of the audience provides key insights into the social cognition underpinning gestural communication, such as an understanding of third party relationships. Gestures given in a mating context provide an ideal area for examining this flexibility, as frequently the interests of a male signaller, a female recipient and a rival male bystander conflict. Dominant chimpanzee males seek to monopolize matings, but subordinate males may use gestural communication flexibly to achieve matings despite their low rank. Here we show that the production of mating gestures in wild male East African chimpanzees (Pan troglodytes schweunfurthii) was influenced by a conflict of interest with females, which in turn was influenced by the presence and visual attention of rival males. When the conflict of interest was low (the rival male was present and looking away), chimpanzees used visual/ tactile gestures over auditory gestures. However, when the conflict of interest was high (the rival male was absent, or was present and looking at the signaller) chimpanzees used auditory gestures over visual/ tactile gestures. Further, the production of mating gestures was more common when the number of oestrous and non-oestrus females in the party increased, when the female was visually perceptive and when there was no wind. Females played an active role in mating behaviour, approaching for copulations more often when the number of oestrus females in the party increased and when the rival male was absent, or was present and looking away. Examining how social and ecological factors affect mating tactics in primates may thus contribute to understanding the previously unexplained reproductive success of subordinate male chimpanzees.
10.1038/ncomms4088, Jan 16, 2014
Humans routinely communicate to coordinate their activities, persisting and elaborating signals t... more Humans routinely communicate to coordinate their activities, persisting and elaborating signals to pursue goals that cannot be accomplished individually. Communicative persistence is associated with complex cognitive skills such as intentionality, because interactants modify their communication in response to another’s understanding of their meaning. Here we show that two language-trained chimpanzees effectively use intentional gestures to coordinate with an experimentally naive human to retrieve hidden food, providing some of the most compelling evidence to date for the role of communicative flexibility in successful coordination in nonhumans. Both chimpanzees (named Panzee and Sherman) increase the rate of non-indicative gestures when the experimenter approaches the location of the hidden food. Panzee also elaborates her gestures in relation to the experimenter’s pointing, which enables her to find food more effectively than Sherman. Communicative persistence facilitates effective communication during behavioural coordination and is likely to have been important in shaping language evolution.
A growing body of evidence suggests that human language may have emerged primarily in the gestura... more A growing body of evidence suggests that human language may have emerged primarily in the gestural rather than vocal domain, and that studying gestural communication in great apes is crucial to understanding language evolution. Although manual and bodily gestures are considered distinct at a neural level, there has been very limited consideration of potential differences at a behavioural level. In this study, we conducted naturalistic observations of adult wild East African chimpanzees (Pan troglodytes schweinfurthii) in order to establish a repertoire of gestures, and examine intentionality of gesture production, use and comprehension, comparing across manual and bodily gestures. At the population level, 120 distinct gesture types were identified, consisting of 65 manual gestures and 55 bodily gestures. Both bodily and manual gestures were used intentionally and effectively to attain specific goals, by signallers who were sensitive to recipient attention. However, manual gestures differed from bodily gestures in terms of communicative persistence, indicating a qualitatively different form of behavioural flexibility in achieving goals. Both repertoire size and frequency of manual gesturing were more affiliative than bodily gestures, while bodily gestures were more antagonistic. These results indicate that manual gestures may have played a significant role in the emergence of increased flexibility in great ape communication and social bonding.
We examine evidence for communicative intent during conspecific interactions in wild chimpanzees ... more We examine evidence for communicative intent during conspecific interactions in wild chimpanzees (Budongo Forest, Uganda), focusing on persistence in gestural communication. Previous research indicates that great apes have large gestural repertoires and produce gestural communication in a flexible and intentional manner, including the production of gesture sequences. Although there is a lack of consensus on the form and function of sequences, there is some evidence that sequences are produced when signallers fail to receive any response from a recipient. Here we provide first systematic evidence for communicative persistence in wild chimpanzees. Rather than examining only the presence or absence of a response, we used the most commonly observed response to assign meanings to gestures and examined sequence production in relation to response congruency. Chimpanzees ceased communication if successful, but persevered when unsuccessful. Chimpanzees repeated gestures when a response partially matched their goal but substituted the original gesture when a response was incongruent. Persistence was also mediated by recipient intent to respond, with more sequences produced within competitive than affiliative contexts. Gestures within sequences were homogenous in semantic meaning and signallers continued until the response matched the assigned meaning of the initial gesture. Gestural sequence production was not primarily affective; gesture intensity (in terms of modality) did not increase within sequences. Chimpanzee gestural sequences emerged to achieve specific outcomes; given variability in recipient behaviour following initial gestures, signallers were flexible in their persistence towards these goals.
Great ape gestural communication shares some important features with human language, namely flexi... more Great ape gestural communication shares some important features with human language, namely flexible and intentional signal use, and is important in understanding the evolutionary history of human communication. Research effort has primarily been devoted to compiling gestural repertoires in captive primates, while there are fewer systematic studies from the wild and in both contexts gesture reports are largely qualitative. We examined the gestural repertoire of a community of free-ranging East African chimpanzees, using focal animal sampling. Our goal was to quantify the morphological structure and variation of gestural signals. We used a morphological classification system based on 29 features, such as the trajectory and orientation during the preparatory and executive phases of a gesturing event. We then carried out hierarchical cluster and discriminant function analyses, which generated a repertoire of 30 morphologically distinct gesture types. Using a cross-validation technique we were able to correctly classify a majority of them, with incorrect classifications mainly for rare gesture types. We then compared the resulting clusters with previously reported repertoires; there was no systematic variation between captive and wild chimpanzee repertoires. Moreover, consensus was not greater for studies conducted within the same populations, highlighting the importance of systematic and well-documented inventories. Our morphologically based analyses indicate that manual gestures are best considered as graded rather than discrete communication signals, similar to some vocalisation systems. We discuss these findings in light of current theories of human language evolution.
Keywords: chimpanzee; gesture; great ape; communication; repertoire; structural inventory
Flexibility is considered a defining feature of great ape gestural communication. Previous resear... more Flexibility is considered a defining feature of great ape gestural communication. Previous research has suggested that there is a ‘means–ends’ dissociation between gesture type and context, whereby one signal may be used across contexts and several signals used within the same context. Such flexibility in signal production demands contextual comprehension, whereby recipients may perceive the context-free message of a given manual gesture, but also decide how to respond by inferring the signaller’s goals from the accompanying context. We conducted naturalistic observations of wild East African chimpanzee, Pan troglodytes schweinfurthii, manual gestures, focusing on recipient perspectives during communicative interactions. Our results indicate that chimpanzees recognize the context-free meanings of gestures and they are also able to respond flexibly by inferring the meaning from the combination of gesture and context, including relative rank. When analysed at the level of gesture type, some gestures were tightly associated with dominant responses and outcomes. Chimpanzee manual gestures are primarily used for directing a recipient’s movement or attention but the motivation underlying these gestural requests is inferred by the recipient from the context.
Keywords: chimpanzee; communication; contextual comprehension; gesture; gesture meaning; great ape; intentionality; manual gesture; Pan troglodytes schweinfurthii; semantic comprehension