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"Liebal, Katja"
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Social negotiation and “accents” in Western lowland gorillas’ gestural communication
Recent findings on chimpanzee infants’ gestural development show that they use some gesture types flexibly and adjust them depending on their interaction partner and social context, suggesting that gestural communication is partly learnt and partly genetically determined. However, how gesture types are shaped by social and demographic factors remains unclear. We addressed this question by focusing on gesture type morphology and conducted a fined-grained analysis of gestural form during intraspecific social-play interactions in two captive groups of Western lowland gorillas (
Gorilla gorilla gorilla
). We focused on the most frequent gesture types (
beat chest
,
slap body
,
slap ground
and
touch body
) produced by subadults (infants, juveniles and adolescents). We considered twelve morphological gesture characteristics (e.g., horizontal and vertical hand trajectories, fingers flexion and spread). Our multifactorial investigation shows that morphological characteristics of distinct gesture types can be shaped by social factors, namely signaller’s sociodemographic characteristics (group and kinship), signaller’s behavioural characteristics (body posture) and context-related characteristics (recipient’s sex, attentional state and position in the signaller’s visual field). We nurtured the lively debate concerning gesture origins by revealing the existence of “accents” in non-verbal communication and the highly variable adjustment of gestural form to different conspecifics and interactional characteristics, which supports the revised social negotiation hypothesis.
Journal Article
The origins of non-human primates' manual gestures
The increasing body of research into human and non-human primates' gestural communication reflects the interest in a comparative approach to human communication, particularly possible scenarios of language evolution. One of the central challenges of this field of research is to identify appropriate criteria to differentiate a gesture from other non-communicative actions. After an introduction to the criteria currently used to define non-human primates' gestures and an overview of ongoing research, we discuss different pathways of how manual actions are transformed into manual gestures in both phylogeny and ontogeny. Currently, the relationship between actions and gestures is not only investigated on a behavioural, but also on a neural level. Here, we focus on recent evidence concerning the differential laterality of manual actions and gestures in apes in the framework of a functional asymmetry of the brain for both hand use and language.
Journal Article
A longitudinal comparison of maternal behaviour in German urban humans (Homo sapiens) and captive chimpanzees (Pan troglodytes)
Comparative perspectives are crucial in the study of human development, yet longitudinal comparisons of humans and other primates are still relatively uncommon. Here, we combined theoretical frameworks from cross-cultural and comparative psychology, to study maternal style in 10 mother–infant pairs of German urban humans (
Homo sapiens
) and 10 mother–infant pairs of captive chimpanzees (
Pan troglodytes
), during the first year of infants’ development. We conducted focal observations of different behaviours (i.e. nursing, carrying, body contact, touching, grooming, restraining, approaching, leaving, rejection, aggression, mutual gaze, object stimulation), during natural interactions. Analyses revealed a more distal maternal style in WEIRD humans than in captive chimpanzees, with different behaviours being generally more common in one of the two species throughout development. For other behaviours (i.e. nursing), developmental trajectories differed between WEIRD humans and captive chimpanzees, although differences generally decreased through infants’ development. Overall, our study confirms functional approaches as a valid tool for comparative longitudinal studies.
Journal Article
The goal of ape pointing
Captive great apes regularly use pointing gestures in their interactions with humans. However, the precise function of this gesture is unknown. One possibility is that apes use pointing primarily to direct attention (as in \"please look at that\"); another is that they point mainly as an action request (such as \"can you give that to me?\"). We investigated these two possibilities here by examining how the looking behavior of recipients affects pointing in chimpanzees (Pan troglodytes) and bonobos (Pan paniscus). Upon pointing to food, subjects were faced with a recipient who either looked at the indicated object (successful-look) or failed to look at the indicated object (failed-look). We predicted that, if apes point primarily to direct attention, subjects would spend more time pointing in the failed-look condition because the goal of their gesture had not been met. Alternatively, we expected that, if apes point primarily to request an object, subjects would not differ in their pointing behavior between the successful-look and failed-look conditions because these conditions differed only in the looking behavior of the recipient. We found that subjects did differ in their pointing behavior across the successful-look and failed-look conditions, but contrary to our prediction subjects spent more time pointing in the successful-look condition. These results suggest that apes are sensitive to the attentional states of gestural recipients, but their adjustments are aimed at multiple goals. We also found a greater number of individuals with a strong right-hand than left-hand preference for pointing.
Journal Article
Understanding others’ preferences: A comparison across primate species and human societies
We investigated children’s and non-human great apes’ ability to anticipate others’ choices from their evident food preferences—regardless of whether these preferences deviate or align with one’s own. We assessed children from three culturally-diverse societies (Namibia, Germany, and Samoa; N = 71; age range = 5–11) and four non-human great ape species (chimpanzees ( Pan troglodytes ), bonobos ( Pan paniscus ), gorillas ( Gorilla gorilla ), and orangutans ( Pongo abelii ); N = 25; age range = 7–29) regarding their choices in a dyadic food-retrieval task. Across conditions, participants’ preferences were either aligned ( same preference condition) or opposed ( opposite preference condition) to those of their competitors. Children across societies altered their choices based on their competitor’s preferences, indicating a cross-culturally recurrent capacity to anticipate others’ choices relying on preferences-based inferences. In contrast to human children, all non-human great apes chose according to their own preferences but independent of those of their competitors. In sum, these results suggest that the tendency to anticipate others’ choices based on their food preferences is cross-culturally robust and, among the great apes, most likely specific to humans.
Journal Article
Gestural communication in wild spider monkeys (Ateles geoffroyi)
Gestures play a central role in the communication systems of several animal families, including primates. In this study, we provide a first assessment of the gestural systems of a Platyrrhine species, Geoffroy’s spider monkeys (
Ateles geoffroyi
). We observed a wild group of 52 spider monkeys and assessed the distribution of visual and tactile gestures in the group, the size of individual repertoires and the intentionality and effectiveness of individuals’ gestural production. Our results showed that younger spider monkeys were more likely than older ones to use tactile gestures. In contrast, we found no inter-individual differences in the probability of producing visual gestures. Repertoire size did not vary with age, but the probability of accounting for recipients’ attentional state was higher for older monkeys than for younger ones, especially for gestures in the visual modality. Using vocalizations right before the gesture increased the probability of gesturing towards attentive recipients and of receiving a response, although age had no effect on the probability of gestures being responded. Overall, our study provides first evidence of gestural production in a Platyrrhine species, and confirms this taxon as a valid candidate for research on animal communication.
Journal Article
Perception of facial expressions reveals selective affect-biased attention in humans and orangutans
Rapid detection and recognition of another individual’s emotional state plays a pivotal role for humans and, most likely, other social species. Proper reactions contribute to an individual’s survival in potentially dangerous situations; this is ensured by a preferential attention towards salient cues. The predisposition to attend to certain categories of affectively salient stimuli– also referred to as affect-biased attention - is likely shared with other species, since fast detection of and appropriate reaction to threats is crucial to survival. We compared human children and one of our close relatives, Sumatran orangutans (Pongo abelii), and predicted that both look more attentively and longer at emotionally salient facial expressions of their own and corresponding other species, compared to neutral faces. However, in contrast to a bias towards emotions providing relevant information by indicating a threat, both species preferentially looked at the fear-related, but not the angry faces of humans and consistently preferred the silent-bared teeth espressions in orangutans. The differential attention towards certain expressions might derive from their social function and the need to detect a potential threat in the environment. Our findings are consistent with claims rooting this affect-biased attention characteristic of human perception in our evolutionary history.
Journal Article
GorillaFACS: The Facial Action Coding System for the Gorilla spp
The Facial Action Coding System (FACS) is an objective observation tool for measuring human facial behaviour. It avoids subjective attributions of meaning by objectively measuring independent movements linked to facial muscles, called Action Units (AUs). FACS has been adapted to 11 other taxa, including most apes, macaques and domestic animals, but not yet gorillas. To carry out cross species studies of facial expressions within and beyond apes, gorillas need to be included in such studies. Hence, we developed the GorillaFACS for the Gorilla spp . We followed similar methodology as previous FACS: First, we examined the facial muscular plan of the gorilla. Second, we analysed gorilla videos in a wide variety of contexts to identify their spontaneous facial movements. Third, we classified the individual facial movements according to appearance changes produced by the corresponding underlying musculature. A diverse repertoire of 42 facial movements was identified in the gorilla, including 28 AUs and 14 Action Descriptors, with several new movements not identified in the HumanFACS. Although some of the movements in gorillas differ from humans, the total number of AUs is comparable to the HumanFACS (32 AUs). Importantly, the gorilla’s range of facial movements was larger than expected, suggesting a more relevant role in social interactions than what was previously assumed. GorillaFACS is a scientific tool to measure facial movements, and thus, will allow us to better understand the gorilla’s expressions and communication. Furthermore, GorillaFACS has the potential be used as an important tool to evaluate this species welfare, particularly in settings of close proximity to humans.
Journal Article
The proximate regulation of prosocial behaviour: towards a conceptual framework for comparative research
Humans and many other animal species act in ways that benefit others. Such prosocial behaviour has been studied extensively across a range of disciplines over the last decades, but findings to date have led to conflicting conclusions about prosociality across and even within species. Here, we present a conceptual framework to study the proximate regulation of prosocial behaviour in humans, non-human primates and potentially other animals. We build on psychological definitions of prosociality and spell out three key features that need to be in place for behaviour to count as prosocial: benefitting others, intentionality, and voluntariness. We then apply this framework to review observational and experimental studies on sharing behaviour and targeted helping in human children and non-human primates. We show that behaviours that are usually subsumed under the same terminology (e.g. helping) can differ substantially across and within species and that some of them do not fulfil our criteria for prosociality. Our framework allows for precise mapping of prosocial behaviours when retrospectively evaluating studies and offers guidelines for future comparative work.
Journal Article