The impact of environment on the comprehension of declarative communication in apes

Empirical challenges from the comparative and developmental literature to the Shared Intentionality Theory - a review of alternative data on recursive mind reading, prosociality, imitation and cumulative culture

Humans have an irresistible inclination to coordinate actions with others, leading to species-unique forms of cooperation. According to the highly influential Shared Intentionality Theory (SITh), human cooperation is made possible by shared intentionality (SI), typically defined as a suite of socio-cognitive and motivational traits for sharing psychological states with others, thereby enabling individuals to engage in joint action in the mutually aware pursuit of shared goals. SITh theorises that SI evolved as late as 400,000 years ago, when our ancestors (in particular, Homo heidelbergensis) turned to a kind of food procurement that obligatorily required joint coordinated action. SI is, thus, hypothesized to be absent in other extant species, including our closest genetic relatives, the nonhuman great apes ("apes"). According to SITh, ape psychology is exclusively driven by individualistic motivations, as opposed to human psychology which is uniquely driven by altruistic motivations. The evolutionary scenario proposed by SITh builds on a series of findings from socio-cognitive research with apes and human children, and on the assumption that abilities expressed early in human development are human universals, unlikely to have been shaped by socio-cultural influences. Drawing on the primatological and developmental literature, we provide a systematic - albeit selective - review of SITh-inconsistent findings concerning psychological and behavioural traits theorised to be constitutive of SI. The findings we review pertain to all three thematic clusters typically addressed in SITh: (i) recursive mind reading; (ii) prosociality; (iii) imitation and cumulative culture. We conclude that such alternative data undermine two core SITh claims: the late evolutionary emergence of SI and the radical divide between ape and human psychology. We also discuss several conceptual and methodological limitations that currently hamper reliable comparative research on SI, in particular those engendered by Western-centric biases in the social sciences, where an overreliance on Western samples has promoted the formulation of Western-centric conceptualisations, operationalisations and methodologies.

Metarepresentation, trust, and "unleashed expression"

Heintz & Scott-Phillips's account of human expression leaves a number of central issues unclear - not least, whether the lack of expression in nonhuman species is attributable to their lack of the relevant metarepresentational abilities, an absence of trust, or a consequence of other factors. In place of their view, we propose a gradualistic account of the origins of human expression.

A Preliminary Assessment of Compassion Fatigue in Chimpanzee Caregivers

Compassion fatigue is defined as "traumatization of helpers through their efforts at helping others". It has negative effects on clinicians including reduced satisfaction with work, fatigue, irritability, dread of going to work, and lack of joy in life. It is correlated with patients' decreased satisfaction with care. Compassion fatigue occurs in a variety of helping professions including educators, social workers, mental health clinicians, and it also appears in nonhuman animal care workers. This study surveyed caregivers of chimpanzees using the ProQOL-V to assess the prevalence of compassion fatigue among this group. Compassion satisfaction is higher than many other types of animal care workers. Conversely, this group shows moderate levels of burnout and secondary traumatic stress; higher levels than other types of animal care workers and many medical professions. While compassion fatigue has an effect on the caregiver's experience, it has potential to affect animal welfare. Caregivers are an integral part of the chimpanzee social network. Compassion fatigue affects the caregiver's attitude, this could in turn affect the relationship and degrade the experience of care for captive chimpanzees. Compassion fatigue can be mitigated with professional development, mindfulness training, interrelationships among staff, and specialized training. This preliminary assessment indicates the work ahead is educating caregivers about compassion fatigue and implementing procedures in sanctuaries to mitigate burnout and secondary traumatic stress.

Neuroanatomical correlates of individual differences in the object choice task in chimpanzees (Pan troglodytes)

Declarative and imperative joint attention or joint engagement are important milestones in human infant development. These have been shown to be a significant predictor of later language development and are impaired in some individuals with, or at risk for, a diagnosis of autism spectrum disorder. Comparatively, while chimpanzees and other great apes have been reported to engage in imperative joint attention, evidence of declarative joint attention remains unclear based on existing studies. Some have suggested that differences in methods of assessing joint attention may have an influence on performance in nonhuman primates. Here, we report data on a measure of receptive joint attention (object choice task) in a sample of captive chimpanzees. Chimpanzees, as a group, performed significantly better than chance. By contrast, when considering individual performance, there was no significant difference in the number of those who passed and those who failed. Using quantitative genetic analyses, we found that performance on the object choice task was not significantly heritable nor were there any significant effects of sex, rearing history, or colony. Lastly, we found significant differences in gray matter covariation, between those who passed or failed the task. Those who passed contributed more to gray matter covariation in several brain regions within the social brain network, consistent with hypotheses regarding the importance of these regions in human and nonhuman primate social cognition.

Great ape communication as contextual social inference: a computational modelling perspective

Human communication has been described as a contextual social inference process. Research into great ape communication has been inspired by this view to look for the evolutionary roots of the social, cognitive and interactional processes involved in human communication. This approach has been highly productive, yet it is partly compromised by the widespread focus on how great apes use and understand individual signals. This paper introduces a computational model that formalizes great ape communication as a multi-faceted social inference process that integrates (a) information contained in the signals that make up an utterance, (b) the relationship between communicative partners and (c) the social context. This model makes accurate qualitative and quantitative predictions about real-world communicative interactions between semi-wild-living chimpanzees. When enriched with a pragmatic reasoning process, the model explains repeatedly reported differences between humans and great apes in the interpretation of ambiguous signals (e.g. pointing or iconic gestures). This approach has direct implications for observational and experimental studies of great ape communication and provides a new tool for theorizing about the evolution of uniquely human communication. This article is part of the theme issue 'Revisiting the human 'interaction engine': comparative approaches to social action coordination'.

The coordination of attention and action in great apes and humans

Great apes can discern what others are attending to and even direct others' attention to themselves in flexible ways. But they seemingly do not coordinate their attention with one another recursively—understanding that the other is monitoring their attention just as they are monitoring hers—in acts of joint attention, at least not in the same way as young human children. Similarly, great apes collaborate with partners in many flexible ways, but they seemingly do not coordinate with others to form mutually obligating joint goals and commitments, nor regulate the collaboration via acts of intentional communication, at least not in the same way as young human children. The hypothesis defended here is that it is precisely in their capacities to coordinate attention and action with others—that is, in their capacities for shared intentionality—that humans are most clearly distinguished from other great apes.

This article is part of the theme issue ‘Revisiting the human ‘interaction engine’: comparative approaches to social action coordination’.

When and how do non-human great apes communicate to support cooperation?

Several scholars have long suggested that human language and remarkable communicative abilities originate from the need and motivation to cooperate and coordinate actions with others. Yet, little work has focused on when and how great apes communicate during joint action tasks, partly because of the widely held assumption that animal communication is mostly manipulative, but also because non-human great apes' default motivation seems to be competitive rather than cooperative. Here, we review experimental cooperative tasks and show how situational challenges and the degree of asymmetry in terms of knowledge relevant for the joint action task affect the likelihood of communication. We highlight how physical proximity and strength of social bond between the participants affect the occurrence and type of communication. Lastly, we highlight how, from a production point of view, communicators appear capable of calibrating their signalling and controlling their delivery, showing clear evidence of first-order intentionality. On the other hand, recipients appear to struggle in terms of making use of referential information received. We discuss different hypotheses accounting for this asymmetry and provide suggestions concerning how future work could help us unveil to what degree the need for cooperation has shaped our closest living relatives' communicative behaviour.

This article is part of the theme issue ‘Revisiting the human ‘interaction engine’: comparative approaches to social action coordination’.

Interaction and ostension: the myth of 4th-order intentionality

Research in comparative cognition on allegedly uniquely human capacities considers the identification of these human capacities in other species as one of their main points of inquiry. Capacities are applied in their theoretical descriptions to promising empirical data. The conclusion then often is that even though, on a behavioural level, the human and nonhuman cases appear related, on a cognitive level there is no relation whatsoever because the underlying cognitive states diverge in quality. This result seems dissatisfying for two reasons: (1) there is ample empirical evidence that suggests the presence of the capacities in other species, and (2) the claim that the underlying states diverge often hinges on the reference to the theoretical definitions of these capacities only. This opinion piece focuses on the capacity of ostensive intentional communication to demonstrate that the original theoretical analyses often are not befitting a comparative endeavour and should therefore not be used as pivotal reference within comparative research. An outlook will be provided on more promising approaches to identifying ostensive communication, namely an interactive approach that will allow for ostension to not be perceived as a one-turn signalling behaviour, but as interactive, with the possibility of being established in a trial-and-error manner. This article is part of the theme issue 'Revisiting the human 'interaction engine': comparative approaches to social action coordination'.

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

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

Identifying animal complex cognition requires natural complexity

The search for human cognitive uniqueness often relied on low ecological tests with subjects experiencing unnatural ontogeny. Recently, neuroscience demonstrated the significance of a rich environment on the development of brain structures and cognitive abilities. This stresses the importance to consider the prior knowledge that subjects bring in any experiment. Second, recent developments in multivariate statistics control precisely for a number of factors and their interactions. Making controls in natural observations equivalent and sometimes superior to captive experimental studies without the drawbacks of the latter methods. Thus, we can now investigate complex cognition by accounting for many different factors, as required when solving tasks in nature. Combining both progresses allows us to move toward an “experience-specific cognition”, recognizing that cognition varies extensively in nature as individuals adapt to the precise challenges they experience in life. Such cognitive specialization makes cross-species comparisons more complex, while potentially identifying human cognitive uniqueness.

The performance of domestic dogs (Canis lupus familiaris) on two versions of the object choice task

Object choice task (OCT) studies are widely used to assess the phylogenetic and ontogenetic distribution of the understanding of communicative cues, with this understanding serving as a proxy for the discernment of communicative intentions. Recent reviews have found systematic procedural and methodological differences in studies which compare performances across species on the OCT. One such difference concerns the spatial configuration of the test set-up, specifically the distances between the two containers (inter-object distance) and the subject-experimenter distance. Here, we tested dogs on two versions of the task: a central version in which the containers were in the subjects' direct line of vision, and a peripheral version in which the position of the containers was distal to the subject. Half of the subjects were tested with a barrier in the testing environment (as nonhuman primates are tested) and the other half without. We found that dogs tested with a barrier performed significantly better in the central version and were more likely to fail to make a choice in the peripheral version. Dogs tested without a barrier showed comparable performance on the two versions. We thus failed to find support for the distraction hypothesis in dogs. We discuss potential explanations for this, highlighting how methodological differences in the presentation of the OCT can influence outcomes in studies using this paradigm.

Japanese Macaques' (Macaca fuscata) sensitivity to human gaze and visual perspective in contexts of threat, cooperation, and competition

Gaze sensitivity allows us to interpret the visual perspective of others, inferring their intentions and attentional states. In order to clarify the evolutionary history of this ability, we assessed the response of free-ranging Japanese macaques (Macaca fuscata) to human gaze in three contexts: threat (Experiment 1), cooperation (Experiment 2), and competition (Experiment 3). Subjects interpreted the direct gaze of an approaching human as a sign of threat, showing a greater flight initiation distance and more threats towards the human in this condition than when the human gazed in another direction. Subjects also adapted their behavior to the attentional cues of a human who gave them food, by for example moving into his visual field. However, the macaques did not seem to take the visual perspective of a human competing with them over food, as they failed to first retrieve the food that was not visible to the human (i.e., located behind an opaque barrier). Our results support the idea that Japanese macaques can respond to a human’s gaze flexibly depending on the context. Moreover, they highlight the importance of studying animal behavior across different species and contexts to better understand the selective pressures that might have led to its evolution.

The effects of distance on pointing comprehension in shelter dogs

The Object Choice Task is a methodology that has been increasingly popular for several decades and many strong claims have been made regarding the differential results between species. However, many studies use differing methodologies and individuals with systematically different backgrounds, which makes any comparisons suspect. One of the methodological differences that has been shown to result in differing responses is distance, both between the objects, and between the object and the gesture. Here, we systematically test these differences with a sample of shelter dogs and note the potential mechanisms underlying the results. Dogs were more successful if the objects were further apart (Distal Object) or the point was very close to the object (Proximal Cue). Success in both of these conditions can be most parsimoniously explained by mechanistic strategies, i.e. strategies that do not rely on mental representation or communicative mechanisms. We also note the results of some pilot data suggesting a non-communicative mechanism (body alignment through touch) by which shelter dogs and other animals may successfully respond when the objects and gestures are distant. We argue that the only point type that likely relies on communicative mechanisms is when the objects are close together, but the point is distant the condition in which dogs are least successful. Future research should take into consideration that individual dogs may use different strategies, or may switch between strategies, and note that all point-following is not necessarily indicative of communicative comprehension.

The role of early social rearing, neurological, and genetic factors on individual differences in mutual eye gaze among captive chimpanzees

Mutual eye gaze plays an important role in primate social development and communication. In the current study, we examined the underlying experiential, genetic, and neuroanatomical basis of mutual eye gaze variation in adult captive chimpanzees. A multivariate analysis of variance revealed a significant rearing effect on bout length, with human-reared chimpanzees engaging in longer bouts of mutual gaze compared to mother-reared and wild-born individuals. Next, we utilized source-based morphometry (SBM) to examine gray matter covariation in magnetic resonance imaging scans and determine the relationship between the resulting gray matter covariation components and mutual eye gaze. One SBM component was negatively correlated with gaze duration (nucleus accumbens and anterior insular cortex), while two components were positively correlated with bout length (posterior cingulate cortex, inferior occipital cortex, middle temporal cortex, hippocampus, and the precentral sulcus). Finally, heritability analyses revealed mutual eye gaze to be modestly heritable and significant genetic correlations between bout length and two gray matter covariation components. This study reveals that non-genetic factors, and to a lesser extent, genetic factors appear to influence mutual eye gaze in adult chimpanzees, and is the first to report neuroanatomical correlates of mutual eye gaze variation in chimpanzees.

Chimpanzees ( Pan troglodytes) coordinate by communicating in a collaborative problem-solving task

Successful collaboration often relies on individuals' capacity to communicate with each other. Despite extensive research on chimpanzee communication, there is little evidence that chimpanzees are capable, without extensive human training, of regulating collaborative activities via communication. This study investigated whether pairs of chimpanzees were capable of communicating to ensure coordination during collaborative problem-solving. The chimpanzee pairs needed two tools to extract fruits from an apparatus. The communicator in each pair could see the location of the tools (hidden in one of two boxes), whereas only the recipient could open the boxes. The subjects were first successfully tested for their capacity to understand the pointing gestures of a human who indicated the location of the tools. In a subsequent conspecifics test, the communicator increasingly communicated the tools' location, by approaching the baited box and giving the key needed to open it to the recipients. The recipient used these signals and obtained the tools, transferring one of the tools to the communicator so that the pair could collaborate in obtaining the fruits. The study suggests that chimpanzees have the necessary socio-cognitive skills to naturally develop a simple communicative strategy to ensure coordination in a collaborative task.

Testing dogs in ape-like conditions: the effect of a barrier on dogs' performance on the object-choice task

Recent reviews have found marked procedural and methodological differences in the testing of different taxonomic groups on the object-choice task. One such difference is the imposition of a barrier in the testing environment of nonhuman primates in the form of a cage, necessitated to ensure the experimenter's safety. Here, we conducted two studies with domestic dogs (Canis lupus familiaris) in which we compared the performance of dogs tested from within a child's playpen and dogs tested without this barrier present. In Study 1, in a within-subjects design, we found no effect of the barrier on dogs' ability to use a pointing cue, but there was an increase in instances in which dogs failed to choose a cup. In Study 2, in a between-subjects design, dogs tested with a barrier failed to perform above chance, and were also more likely to fail to make a choice. When dogs tested without a barrier made an incorrect response, these were more likely to be incorrect choices than no choice errors. We discuss the implications of these differences in behavioural responses in function of the presence of a barrier and the necessity of ensuring matched conditions when comparing across species.

Visually attending to a video together facilitates great ape social closeness

Humans create social closeness with one another through a variety of shared social activities in which they align their emotions or mental states towards an external stimulus such as dancing to music together, playing board games together or even engaging in minimal shared experiences such as watching a movie together. Although these specific behaviours would seem to be uniquely human, it is unclear whether the underlying psychology is unique to the species, or if other species might possess some form of this psychological mechanism as well. Here we show that great apes who have visually attended to a video together with a human (study 1) and a conspecific (study 2) subsequently approach that individual faster (study 1) or spend more time in their proximity (study 2) than when they had attended to something different. Our results suggest that one of the most basic mechanisms of human social bonding-feeling closer to those with whom we act or attend together-is present in both humans and great apes, and thus has deeper evolutionary roots than previously suspected.

Corpse-directed play parenting by a sterile adult female chimpanzee

The study of representational play in nonhuman primates, including chimpanzees (Pan troglodytes), provides interspecific perspectives on human cognitive development and evolution. A notable form of representational play in chimpanzees is play parenting, wherein parental behavior is directed at inanimate objects. Though observed in captivity, unambiguous examples of play parenting by wild chimpanzees are rare. Here, we report two cases from Ngogo in Kibale National Park, Uganda, in which a wild adult female chimpanzee (P. t. schweinfurthii) directed parental behaviors at corpses. Both cases involved the same adult female chimpanzee, aged 20-21 years. The first case was observed on 5 March 2016, and the play object was the corpse of a bushbaby (Galago thomasi); in the second case, observed on 6 May 2017, the play object was a recently deceased chimpanzee infant postmortally stolen from the mother. The chimpanzee possessed the first and second play objects for approximately 5.5 h and 1.8 h, respectively. In both cases, she performed a variety of maternal behaviors, including co-nesting, grooming, and dorsally carrying the play objects. In contrast to previous observations of play parenting in captivity, the play parent was a presumably sterile adult female, well beyond the average age of first birth. These observations contribute to the expanding literature on chimpanzee interactions with the corpses of both conspecifics and heterospecifics.

A gestural repertoire of 1- to 2-year-old human children: in search of the ape gestures

When we compare human gestures to those of other apes, it looks at first like there is nothing much to compare at all. In adult humans, gestures are thought to be a window into the thought processes accompanying language, and sign languages are equal to spoken language with all of its features. Some research firmly emphasises the differences between human gestures and those of other apes; however, the question about whether there are any commonalities is rarely investigated, and has mostly been confined to pointing gestures. The gestural repertoires of nonhuman ape species have been carefully studied and described with regard to their form and function-but similar approaches are much rarer in the study of human gestures. This paper applies the methodology commonly used in the study of nonhuman ape gestures to the gestural communication of human children in their second year of life. We recorded (n = 13) children's gestures in a natural setting with peers and caregivers in Germany and Uganda. Children employed 52 distinct gestures, 46 (89%) of which are present in the chimpanzee repertoire. Like chimpanzees, they used them both singly, and in sequences, and employed individual gestures flexibly towards different goals.

The mismeasure of ape social cognition

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

Multimodal communication and language origins: integrating gestures and vocalizations

The presence of divergent and independent research traditions in the gestural and vocal domains of primate communication has resulted in major discrepancies in the definition and operationalization of cognitive concepts. However, in recent years, accumulating evidence from behavioural and neurobiological research has shown that both human and non-human primate communication is inherently multimodal. It is therefore timely to integrate the study of gestural and vocal communication. Herein, we review evidence demonstrating that there is no clear difference between primate gestures and vocalizations in the extent to which they show evidence for the presence of key language properties: intentionality, reference, iconicity and turn-taking. We also find high overlap in the neurobiological mechanisms producing primate gestures and vocalizations, as well as in ontogenetic flexibility. These findings confirm that human language had multimodal origins. Nonetheless, we note that in great apes, gestures seem to fulfil a carrying (i.e. predominantly informative) role in close-range communication, whereas the opposite holds for face-to-face interactions of humans. This suggests an evolutionary shift in the carrying role from the gestural to the vocal stream, and we explore this transition in the carrying modality. Finally, we suggest that future studies should focus on the links between complex communication, sociality and cooperative tendency to strengthen the study of language origins.

Ontogeny vs. phylogeny in primate/canid comparisons: A meta-analysis of the object choice task

The Object Choice Task (OCT) is a widely used paradigm with which researchers measure the ability of a subject to comprehend deictic (directional) cues, such as pointing gestures and eye gaze. There is a widespread belief that nonhuman primates evince only a weak capacity to use deictic cues; in contrast, domestic dogs (Canis lupus familiaris) tend to demonstrate high success rates. This pattern of canid superiority has been taken to support the Domestication Hypothesis, which posits enhancing effects of artificial selection on the sociocognitive abilities of dogs and humans. Here we review nearly two decades of published findings, using variants of the OCT. We find systematic confounds with species classification in task-relevant preparation of the subjects, in the imposition of a barrier between reward and subject, and in the specific deictic cues used to indicate the location of hidden objects. Thus, the widespread belief that dogs outperform primates on OCTs is undermined by the systematic procedural differences in the assessments of these skills, differences that are confounded with taxonomic classification.

Separate brain areas for processing human and dog faces as revealed by awake fMRI in dogs (Canis familiaris)

Functional magnetic resonance imaging (fMRI) has emerged as a viable method to study the neural processing underlying cognition in awake dogs. Working dogs were presented with pictures of dog and human faces. The human faces varied in familiarity (familiar trainers and unfamiliar individuals) and emotional valence (negative, neutral, and positive). Dog faces were familiar (kennel mates) or unfamiliar. The findings revealed adjacent but separate brain areas in the left temporal cortex for processing human and dog faces in the dog brain. The human face area (HFA) and dog face area (DFA) were both parametrically modulated by valence indicating emotion was not the basis for the separation. The HFA and DFA were not influenced by familiarity. Using resting state fMRI data, functional connectivity networks (connectivity fingerprints) were compared and matched across dogs and humans. These network analyses found that the HFA mapped onto the human fusiform area and the DFA mapped onto the human superior temporal gyrus, both core areas in the human face processing system. The findings provide insight into the evolution of face processing.

Captive gibbons (Hylobatidae) use different referential cues in an object-choice task: insights into lesser ape cognition and manual laterality

BACKGROUND

Utilization of visual referential cues by non-human primates is a subject of constant scientific interest. However, only few primate species, mostly great apes, have been studied thoroughly in that regard, rendering the understanding of phylogenetic influences on the underlying cognitive patterns difficult.

METHODS

We tested six species of captive gibbons in an object-choice task (n = 11) for their ability to interpret two different pointing gestures, a combination of body orientation and gaze direction as well as glancing as referential cues. Hand preferences were tested in the object-choice task and in a bimanual tube task (n = 18).

RESULTS

We found positive responses to all signals except for the glancing cue at the individual as well as at the group level. The gibbons' success rates partially exceed results reported for great apes in comparable tests and appear to be similarly influenced by prior exposure to human communicative cues. Hand preferences exhibited by the gibbons in the object-choice task as well as in a bimanual tube task suggest that crested gibbons (Nomascus sp.) are strongly lateralized at individual but not at population level for tasks involving object manipulation.

DISCUSSION

Based on the available data, it can be assumed that the cognitive foundations to utilize different visual cues essential to human communication are conserved in extant hominoids and can be traced back at least to the common ancestor of great and lesser apes. However, future studies have to further investigate how the social environment of gibbons influences their ability to exploit referential signals. Gibbons' manual laterality patterns appear to differ in several aspects from the situation found in great apes. While not extensive enough to allow for general conclusions about the evolution of hand preferences in gibbons or apes in general, our results add to the expanding knowledge on manual lateralization in the Hylobatidae.

Human ostensive signals do not enhance gaze following in chimpanzees, but do enhance object-oriented attention

The previous studies have shown that human infants and domestic dogs follow the gaze of a human agent only when the agent has addressed them ostensively-e.g., by making eye contact, or calling their name. This evidence is interpreted as showing that they expect ostensive signals to precede referential information. The present study tested chimpanzees, one of the closest relatives to humans, in a series of eye-tracking experiments using an experimental design adapted from these previous studies. In the ostension conditions, a human actor made eye contact, called the participant's name, and then looked at one of two objects. In the control conditions, a salient cue, which differed in each experiment (a colorful object, the actor's nodding, or an eating action), attracted participants' attention to the actor's face, and then the actor looked at the object. Overall, chimpanzees followed the actor's gaze to the cued object in both ostension and control conditions, and the ostensive signals did not enhance gaze following more than the control attention-getters. However, the ostensive signals enhanced subsequent attention to both target and distractor objects (but not to the actor's face) more strongly than the control attention-getters-especially in the chimpanzees who had a close relationship with human caregivers. We interpret this as showing that chimpanzees have a simple form of communicative expectations on the basis of ostensive signals, but unlike human infants and dogs, they do not subsequently use the experimenter's gaze to infer the intended referent. These results may reflect a limitation of non-domesticated species for interpreting humans' ostensive signals in inter-species communication.

Primate social attention: Species differences and effects of individual experience in humans, great apes, and macaques

When viewing social scenes, humans and nonhuman primates focus on particular features, such as the models' eyes, mouth, and action targets. Previous studies reported that such viewing patterns vary significantly across individuals in humans, and also across closely-related primate species. However, the nature of these individual and species differences remains unclear, particularly among nonhuman primates. In large samples of human and nonhuman primates, we examined species differences and the effects of experience on patterns of gaze toward social movies. Experiment 1 examined the species differences across rhesus macaques, nonhuman apes (bonobos, chimpanzees, and orangutans), and humans while they viewed movies of various animals' species-typical behaviors. We found that each species had distinct viewing patterns of the models' faces, eyes, mouths, and action targets. Experiment 2 tested the effect of individuals' experience on chimpanzee and human viewing patterns. We presented movies depicting natural behaviors of chimpanzees to three groups of chimpanzees (individuals from a zoo, a sanctuary, and a research institute) differing in their early social and physical experiences. We also presented the same movies to human adults and children differing in their expertise with chimpanzees (experts vs. novices) or movie-viewing generally (adults vs. preschoolers). Individuals varied within each species in their patterns of gaze toward models' faces, eyes, mouths, and action targets depending on their unique individual experiences. We thus found that the viewing patterns for social stimuli are both individual- and species-specific in these closely-related primates. Such individual/species-specificities are likely related to both individual experience and species-typical temperament, suggesting that primate individuals acquire their unique attentional biases through both ontogeny and evolution. Such unique attentional biases may help them learn efficiently about their particular social environments.

Thirty years of great ape gestures

We and our colleagues have been doing studies of great ape gestural communication for more than 30 years. Here we attempt to spell out what we have learned. Some aspects of the process have been reliably established by multiple researchers, for example, its intentional structure and its sensitivity to the attentional state of the recipient. Other aspects are more controversial. We argue here that it is a mistake to assimilate great ape gestures to the species-typical displays of other mammals by claiming that they are fixed action patterns, as there are many differences, including the use of attention-getters. It is also a mistake, we argue, to assimilate great ape gestures to human gestures by claiming that they are used referentially and declaratively in a human-like manner, as apes’ “pointing” gesture has many limitations and they do not gesture iconically. Great ape gestures constitute a unique form of primate communication with their own unique qualities.

Convergent minds: ostension, inference and Grice's third clause

A prevailing view is that while human communication has an 'ostensive-inferential' or 'Gricean' intentional structure, animal communication does not. This would make the psychological states that support human and animal forms of communication fundamentally different. Against this view, I argue that there are grounds to expect ostensive communication in non-human clades. This is because it is sufficient for ostensive communication that one intentionally addresses one's utterance to one's intended interlocutor-something that is both a functional pre-requisite of successful communication and cognitively undemanding. Furthermore, while ostension is an important feature of intentional communication, the inferences required in Gricean communication may be minimal: ostension and inference may come apart. The grounds for holding that animal communication could not be Gricean are therefore weak. I finish by defending the idea that a 'minimally Gricean' model of communication is a valuable tool for characterizing the communicative interactions of many animal species.

Prelinguistic human infants and great apes show different communicative strategies in a triadic request situation

In the present research, we investigate the communicative strategies of 20 month old human infants and great apes when requesting rewards from a human experimenter. Infants and apes both adapted their signals to the attentional state of the experimenter as well as to the location of the reward. Yet, while infants frequently positioned themselves in front of the experimenter and pointed towards a distant reward, apes either remained in the experimenter’s line of sight and pointed towards him or moved out of sight and pointed towards the reward. Further, when pointing towards a reward that was placed at a distance from the experimenter, only the infants, and not the apes, took the experimenter’s attentional state into account. These results demonstrate that prelinguistic human infants and nonhuman apes use different means when guiding others’ attention to a location; indicating that differing cognitive mechanisms may underlie their pointing gestures.

Does size really matter? Investigating cognitive differences in spatial memory ability based on size in domestic dogs

The study of canine cognition can be useful in understanding the selective pressures affecting cognitive abilities. Dogs have undergone intensive artificial selection yielding distinctive breeds, which differ both phenotypically and behaviorally and no other species has a wider range in brain size. As brain size has long been hypothesized to relate to cognitive capacity, this species offers a useful model to further explore this relationship. The influence of physical size on canine cognition has not been thoroughly addressed, despite the fact that large dogs are often perceived to be 'smarter' than small dogs. To date, this preconception has only recently been addressed and supported in one study comparing large and small dogs in a social cognition task, where large dogs outperformed small dogs in a pointing choice task. We assessed large and small dogs using a series of spatial cognition tasks and detected no differences between the two groups. Further research is needed to clarify why our results failed to compliment previous findings. It is possible that differences found in social cognition tasks may not be due to differences in size, rather they may be based on other factors such as methodology, prior training experience, or past experience with humans in general.

Sea lions' (Zalophus californianus) use of human pointing gestures as referential cues

This experiment investigated the ability of four human-socialized sea lions to exploit human communicative gestures in three different object-choice tasks based on directional cues emitted by their caretakers. In Study 1, three of the tested subjects were able to generalize their choice of the pointed target to variations of the basic pointing gestures (i.e., cross-body point, elbow point, foot point, and gaze only), from the very first trials. Study 2 showed that the subjects could follow the pointing gestures geometrically and select the correct target among four possible targets, two on each side of the informant. In Study 3, we tested the robustness of their tendency to follow a pointing gesture by hiding targets behind barriers. One subject was able to follow pointing gestures towards targets not visible at the moment of their decision without any training, despite the presence of another visible and directly accessible one. Taken together, these results suggest that sea lions were able to use the referential property of the human pointing gesture, because they were able to rely on extrapolating precise linear vectors along different pointing body parts in order to identify a precise object rather than merely a general direction. These findings support previous arguments that some non-domesticated species might have as great an ability to respond appropriately to pointing gestures as domesticated dogs. The potential roles of human-socialization and specific features of wild sea lions ecology are discussed.

What's Special about Human Imitation? A Comparison with Enculturated Apes

What, if anything, is special about human imitation? An evaluation of enculturated apes’ imitation skills, a “best case scenario” of non-human apes’ imitation performance, reveals important similarities and differences between this special population of apes and human children. Candidates for shared imitation mechanisms include the ability to imitate various familiar transitive responses and object–object actions that involve familiar tools. Candidates for uniquely derived imitation mechanisms include: imitating novel transitive actions and novel tool-using responses as well as imitating opaque or intransitive gestures, regardless of familiarity. While the evidence demonstrates that enculturated apes outperform non-enculturated apes and perform more like human children, all apes, regardless of rearing history, generally excel at imitating familiar, over-rehearsed responses and are poor, relative to human children, at imitating novel, opaque or intransitive responses. Given the similarities between the sensory and motor systems of preschool age human children and non-human apes, it is unlikely that differences in sensory input and/or motor-output alone explain the observed discontinuities in imitation performance. The special rearing history of enculturated apes—including imitation-specific training—further diminishes arguments suggesting that differences are experience-dependent. Here, it is argued that such differences are best explained by distinct, specialized mechanisms that have evolved for copying rules and responses in particular content domains. Uniquely derived social and imitation learning mechanisms may represent adaptations for learning novel communicative gestures and complex tool-use. Given our species’ dependence on both language and tools, mechanisms that accelerated learning in these domains are likely to have faced intense selective pressures, starting with the earliest of human ancestors.

Early maternal loss affects social integration of chimpanzees throughout their lifetime

The long-term effects of early adverse experiences on later psychosocial functioning are well described in humans, but sparsely documented for chimpanzees. In our earlier studies, we investigated the effects of maternal and social deprivation on three groups of ex-laboratory chimpanzees who experienced either an early or later onset of long-term deprivation. Here we expand our research by adding data on subjects that came from two stable zoo groups. The groups comprised of early maternally deprived wild-caught chimpanzees and non-deprived zoo-born chimpanzees. We found that compared to zoo chimpanzees, ex-laboratory chimpanzees were more restricted regarding their association partners in the newly formed groups, but not during their second year of group-life, indicating that social stability has an important influence on the toleration of association partners close-by. Social grooming activity, however, was impaired in early long-term deprived ex-laboratory chimpanzees as well as in early maternally deprived zoo chimpanzees compared to non-deprived zoo chimpanzees. Thus, we conclude that early maternal loss has lifelong effects on the social integration of chimpanzees which becomes evident in their grooming networks. Although the retrospective nature of our study prevents a clear causal explanation, our results are of importance for understanding the development of social competence in chimpanzees.

Can vocal conditioning trigger a semiotic ratchet in marmosets?

The complexity of human communication has often been taken as evidence that our language reflects a true evolutionary leap, bearing little resemblance to any other animal communication system. The putative uniqueness of the human language poses serious evolutionary and ethological challenges to a rational explanation of human communication. Here we review ethological, anatomical, molecular, and computational results across several species to set boundaries for these challenges. Results from animal behavior, cognitive psychology, neurobiology, and semiotics indicate that human language shares multiple features with other primate communication systems, such as specialized brain circuits for sensorimotor processing, the capability for indexical (pointing) and symbolic (referential) signaling, the importance of shared intentionality for associative learning, affective conditioning and parental scaffolding of vocal production. The most substantial differences lie in the higher human capacity for symbolic compositionality, fast vertical transmission of new symbols across generations, and irreversible accumulation of novel adaptive behaviors (cultural ratchet). We hypothesize that increasingly-complex vocal conditioning of an appropriate animal model may be sufficient to trigger a semiotic ratchet, evidenced by progressive sign complexification, as spontaneous contact calls become indexes, then symbols and finally arguments (strings of symbols). To test this hypothesis, we outline a series of conditioning experiments in the common marmoset (Callithrix jacchus). The experiments are designed to probe the limits of vocal communication in a prosocial, highly vocal primate 35 million years far from the human lineage, so as to shed light on the mechanisms of semiotic complexification and cultural transmission, and serve as a naturalistic behavioral setting for the investigation of language disorders.

Distal Communication by Chimpanzees (Pan troglodytes): Evidence for Common Ground?

van der Goot et al. (2014) proposed that distal, deictic communication indexed the appreciation of the psychological state of a common ground between a signaler and a receiver. In their study, great apes did not signal distally, which they construed as evidence for the human uniqueness of a sense of common ground. This study exposed 166 chimpanzees to food and an experimenter, at an angular displacement, to ask, "Do chimpanzees display distal communication?" Apes were categorized as (a) proximal or (b) distal signalers on each of four trials. The number of chimpanzees who communicated proximally did not statistically differ from the number who signaled distally. Therefore, contrary to the claim by van der Goot et al., apes do communicate distally.

Meaning and ostension in great ape gestural communication

It is sometimes argued that while human gestures are produced ostensively and intentionally, great ape gestures are produced only intentionally. If true, this would make the psychological mechanisms underlying the different species' communication fundamentally different, and ascriptions of meaning to chimpanzee gestures would be inappropriate. While the existence of different underlying mechanisms cannot be ruled out, in fact claims about difference are driven less by empirical data than by contested assumptions about the nature of ostensive communication. On some accounts, there are no reasons to doubt that great ape gestural communication is ostensive. If these accounts are correct, attributions of meaning to chimpanzee gestures would be justified.

Production and Comprehension of Gestures between Orang-Utans (Pongo pygmaeus) in a Referential Communication Game

Orang-utans played a communication game in two studies testing their ability to produce and comprehend requestive pointing. While the ‘communicator’ could see but not obtain hidden food, the ‘donor’ could release the food to the communicator, but could not see its location for herself. They could coordinate successfully if the communicator pointed to the food, and if the donor comprehended his communicative goal and responded pro-socially. In Study 1, one orang-utan pointed regularly and accurately for peers. However, they responded only rarely. In Study 2, a human experimenter played the communicator’s role in three conditions, testing the apes’ comprehension of points of different heights and different degrees of ostension. There was no effect of condition. However, across conditions one donor performed well individually, and as a group orang-utans’ comprehension performance tended towards significance. We explain this on the grounds that comprehension required inferences that they found difficult – but not impossible. The finding has valuable implications for our thinking about the development of pointing in phylogeny.

Gestural and symbolic development among apes and humans: support for a multimodal theory of language evolution

What are the implications of similarities and differences in the gestural and symbolic development of apes and humans?This focused review uses as a starting point our recent study that provided evidence that gesture supported the symbolic development of a chimpanzee, a bonobo, and a human child reared in language-enriched environments at comparable stages of communicative development. These three species constitute a complete clade, species possessing a common immediate ancestor. Communicative behaviors observed among all species in a clade are likely to have been present in the common ancestor. Similarities in the form and function of many gestures produced by the chimpanzee, bonobo, and human child suggest that shared non-verbal skills may underlie shared symbolic capacities. Indeed, an ontogenetic sequence from gesture to symbol was present across the clade but more pronounced in child than ape. Multimodal expressions of communicative intent (e.g., vocalization plus persistence or eye-contact) were normative for the child, but less common for the apes. These findings suggest that increasing multimodal expression of communicative intent may have supported the emergence of language among the ancestors of humans. Therefore, this focused review includes new studies, since our 2013 article, that support a multimodal theory of language evolution.

The Importance of Development for Comparative Primatology

It is a truism to say that primates develop, but it is also important to acknowledge that development occurs across many domains, including motor behavior, socioemotional behavior, communication, and cognition. In this review, we focus on those aspects of development that impact social cognition outcomes in infancy. Triadic engagements, such as those of joint attention, cooperation, and intentional communication, develop in the first year of life in chimpanzees and humans. Joint attention, for example, occurs when infants coordinate their attention to a social partner while also attending to an object or event. Hominoids are strongly influenced by experiences during early development, especially experiences that are foundational for these coordinated triadic engagements. Purported species differences in triadic engagements are highlighted in current evolutionary theories of primate social cognition, but conclusions about species differences are unfounded when development is ignored. Developmental experiences must be matched, controlled, or systematically varied in experimental designs that make cross-species comparisons. Considerations of development, across species and across rearing experiences, would contribute to more accurate evolutionary theories of primate social cognition.

Emotional engagements predict and enhance social cognition in young chimpanzees

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

Training experience in gestures affects the display of social gaze in baboons' communication with a human

Gaze behaviour, notably the alternation of gaze between distal objects and social partners that accompanies primates' gestural communication is considered a standard indicator of intentionality. However, the developmental precursors of gaze behaviour in primates' communication are not well understood. Here, we capitalized on the training in gestures dispensed to olive baboons (Papio anubis) as a way of manipulating individual communicative experience with humans. We aimed to delineate the effects of such a training experience on gaze behaviour displayed by the monkeys in relation with gestural requests. Using a food-requesting paradigm, we compared subjects trained in requesting gestures (i.e. trained subjects) to naïve subjects (i.e. control subjects) for their occurrences of (1) gaze behaviour, (2) requesting gestures and (3) temporal combination of gaze alternation with gestures. We found that training did not affect the frequencies of looking at the human's face, looking at food or alternating gaze. Hence, social gaze behaviour occurs independently from the amount of communicative experience with humans. However, trained baboons-gesturing more than control subjects-exhibited most gaze alternation combined with gestures, whereas control baboons did not. By reinforcing the display of gaze alternation along with gestures, we suggest that training may have served to enhance the communicative function of hand gestures. Finally, this study brings the first quantitative report of monkeys producing requesting gestures without explicit training by humans (controls). These results may open a window on the developmental mechanisms (i.e. incidental learning vs. training) underpinning gestural intentional communication in primates.

Interpretation of human pointing by African elephants: generalisation and rationality

Factors influencing the abilities of different animals to use cooperative social cues from humans are still unclear, in spite of long-standing interest in the topic. One of the few species that have been found successful at using human pointing is the African elephant (Loxodonta africana); despite few opportunities for learning about pointing, elephants follow a pointing gesture in an object-choice task, even when the pointing signal and experimenter's body position are in conflict, and when the gesture itself is visually subtle. Here, we show that the success of captive African elephants at using human pointing is not restricted to situations where the pointing signal is sustained until the time of choice: elephants followed human pointing even when the pointing gesture was withdrawn before they had responded to it. Furthermore, elephants rapidly generalised their response to a type of social cue they were unlikely to have seen before: pointing with the foot. However, unlike young children, they showed no sign of evaluating the 'rationality' of this novel pointing gesture according to its visual context: that is, whether the experimenter's hands were occupied or not.

Different early rearing experiences have long-term effects on cortical organization in captive chimpanzees (Pan troglodytes)

Consequences of rearing history in chimpanzees (Pan troglodytes) have been explored in relation to behavioral abnormalities and cognition; however, little is known about the effects of rearing conditions on anatomical brain development. Human studies have revealed that experiences of maltreatment and neglect during infancy and childhood can have detrimental effects on brain development and cognition. In this study, we evaluated the effects of early rearing experience on brain morphology in 92 captive chimpanzees (ages 11-43) who were either reared by their mothers (n = 46) or in a nursery (n = 46) with age-group peers. Magnetic resonance brain images were analyzed with a processing program (BrainVISA) that extracts cortical sulci. We obtained various measurements from 11 sulci located throughout the brain, as well as whole brain gyrification and white and grey matter volumes. We found that mother-reared chimpanzees have greater global white-to-grey matter volume, more cortical folding and thinner grey matter within the cortical folds than nursery-reared animals. The findings reported here are the first to demonstrate that differences in early rearing conditions have significant consequences on brain morphology in chimpanzees and suggests potential differences in the development of white matter expansion and myelination.

Genetic influences on receptive joint attention in chimpanzees (Pan troglodytes)

Despite their genetic similarity to humans, our understanding of the role of genes on cognitive traits in chimpanzees remains virtually unexplored. Here, we examined the relationship between genetic variation in the arginine vasopressin V1a receptor gene (AVPR1A) and social cognition in chimpanzees. Studies have shown that chimpanzees are polymorphic for a deletion in a sequence in the 5′ flanking region of the AVPR1A, DupB, which contains the variable RS3 repetitive element, which has been associated with variation in social behavior in humans. Results revealed that performance on the social cognition task was significantly heritable. Furthermore, males with one DupB⁺ allele performed significantly better and were more responsive to socio-communicative cues than males homozygous for the DupB- deletion. Performance on a non-social cognition task was not associated with the AVPR1A genotype. The collective findings show that AVPR1A polymorphisms are associated with individual differences in performance on a receptive joint attention task in chimpanzees.