Use of experimenter-given cues during object-choice tasks by chimpanzees (Pan troglodytes), an orangutan (Pongo pygmaeus), and human infants (Homo sapiens)

TANGO: A reliable, open-source, browser-based task to assess individual differences in gaze understanding in 3 to 5-year-old children and adults

Traditional measures of social cognition used in developmental research often lack satisfactory psychometric properties and are not designed to capture variation between individuals. Here, we present the TANGO (Task for Assessing iNdividual differences in Gaze understanding-Open); a brief (approx. 5-10min), reliable, open-source task to quantify individual differences in the understanding of gaze cues. Localizing the attentional focus of an agent is crucial in inferring their mental states, building common ground, and thus, supporting cooperation. Our interactive browser-based task works across devices and enables in-person and remote testing. The implemented spatial layout allows for discrete and continuous measures of participants' click imprecision and is easily adaptable to different study requirements. Our task measures inter-individual differences in a child (N = 387) and an adult (N = 236) sample. Our two study versions and data collection modes yield comparable results that show substantial developmental gains: the older children are, the more accurately they locate the target. High internal consistency and test-retest reliability estimates underline that the captured variation is systematic. Associations with social-environmental factors and language skills speak to the validity of the task. This work shows a promising way forward in studying individual differences in social cognition and will help us explore the structure and development of our core social-cognitive processes in greater detail.

Following the human point: Research with nonhuman animals since Povinelli, Nelson, and Boysen (1990)

For this special issue in honor of Dr. Sarah (Sally) Boysen's career, we review studies on point following in nonhuman animals. Of the 126 papers that we documented on this topic published since the publication of Povinelli, Nelson, and Boysen (1990, Journal of Comparative Psychology, 104, 203-210), 94 (75%) were published in the past 15 years, including 22 in the past 5 years, indicating that this topic is still an active area of interest in the field of animal behavior and cognition. We present results of a survey of publication trends, discussing the species tested and the sample sizes, and we note methodological considerations and current multilaboratory approaches. We then categorize and synthesize the research questions addressed in these studies, which have been at both the ultimate level (e.g., questions related to evolutionary adaptiveness and phylogenetic differences) and proximate level (e.g., questions related to experiential and temperamental processes). Throughout, we consider future directions for this area of research.

The evolutionary drivers of primate scleral coloration

The drivers of divergent scleral morphologies in primates are currently unclear, though white sclerae are often assumed to underlie human hyper-cooperative behaviours. Humans are unusual in possessing depigmented sclerae whereas many other extant primates, including the closely-related chimpanzee, possess dark scleral pigment. Here, we use phylogenetic generalized least squares (PGLS) analyses with previously generated species-level scores of proactive prosociality, social tolerance (both n = 15 primate species), and conspecific lethal aggression (n = 108 primate species) to provide the first quantitative, comparative test of three existing hypotheses. The 'self-domestication' and 'cooperative eye' explanations predict white sclerae to be associated with cooperative, rather than competitive, environments. The 'gaze camouflage' hypothesis predicts that dark scleral pigment functions as gaze direction camouflage in competitive social environments. Notably, the experimental evidence that non-human primates draw social information from conspecific eye movements is unclear, with the latter two hypotheses having recently been challenged. Here, we show that white sclerae in primates are associated with increased cooperative behaviours whereas dark sclerae are associated with reduced cooperative behaviours and increased conspecific lethal violence. These results are consistent with all three hypotheses of scleral evolution, suggesting that primate scleral morphologies evolve in relation to variation in social environment.

Chimpanzee (Pan troglodytes) gaze is conspicuous at ecologically-relevant distances

Chimpanzee (Pan troglodytes) sclera appear much darker than the white sclera of human eyes, to such a degree that the direction of chimpanzee gaze may be concealed from conspecifics. Recent debate surrounding this topic has produced mixed results, with some evidence suggesting that (1) primate gaze is indeed concealed from their conspecifics, and (2) gaze colouration is among the suite of traits that distinguish uniquely social and cooperative humans from other primates (the cooperative eye hypothesis). Using a visual modelling approach that properly accounts for specific-specific vision, we reexamined this topic to estimate the extent to which chimpanzee eye coloration is discriminable. We photographed the faces of captive chimpanzees and quantified the discriminability of their pupil, iris, sclera, and surrounding skin. We considered biases of cameras, lighting conditions, and commercial photography software along with primate visual acuity, colour sensitivity, and discrimination ability. Our visual modeling of chimpanzee eye coloration suggests that chimpanzee gaze is visible to conspecifics at a range of distances (within approximately 10 m) appropriate for many species-typical behaviours. We also found that chimpanzee gaze is discriminable to the visual system of primates that chimpanzees prey upon, Colobus monkeys. Chimpanzee sclera colour does not effectively conceal gaze, and we discuss this result with regard to the cooperative eye hypothesis, the evolution of primate eye colouration, and methodological best practices for future primate visual ecology research.

Young children's developing ability to integrate gestural and emotional cues

In three studies, children aged 22 to 46 months (N = 180) needed to integrate pointing gestures or gaze cues with positive and negative facial expressions to succeed in an object-choice task. Finding a toy required children to either choose (positive expression) or avoid (negative expression) the indicated target. Study 1 showed that 22-month-olds are better at integrating a positive facial expression with a pointing gesture compared with a negative facial expression with a pointing gesture. Study 2 tracked the integration of negative expressions and pointing across development, finding an unexpected, U-shaped trajectory with group-level success only at 46 months. Study 3 showed that already 34-month-olds succeeded when pointing was replaced with communicative gaze. These findings suggest that at the end of the second year of life, children are generally able to integrate emotional displays and communicative cues such as gestures and ostensive gaze to reevaluate and contextualize utterances. In addition, pointing gestures appear to be understood by young children as a call to act on a referenced object. Findings illustrate that communicative cues should be studied in conjunction with emotional displays to draw an ecologically valid picture of communicative development.

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.

Flexible gaze-following in rhesus monkeys

Humans are characterized by complex social cognitive abilities that emerge early in development. Comparative studies of nonhuman primates can illuminate the evolutionary history of these social capacities. We examined the cognitive skills that rhesus monkeys (Macaca mulatta) use to follow gaze, a foundational skill in human social development. While rhesus monkeys can make inferences about others' gaze when competing, it is unclear how they think about gaze information in other contexts. In study 1, monkeys (n = 64) observed a demonstrator look upwards either in a barrier condition where a box was overhead, so that monkeys could not see the target of her gaze, or a no barrier condition where nothing blocked her view. In study 2, monkeys (n = 59) could approach to observe the target of the demonstrator's gaze when the demonstrator looked behind a barrier on the ground or, in the no barrier condition, behind a window frame in the same location. Monkeys were more likely to directly look up in study 1 if they could initially see the location where the demonstrator was looking, but they did not preferentially reorient their bodies to observe the out-of-view location when they could not see that location. In study 2, monkeys did preferentially reorient, but at low rates. This indicates that rhesus monkeys can use social cognitive processes outside of competitive contexts to model what others can or cannot see, but may not be especially motivated to see what others look at in non-competitive contexts, as they reorient infrequently or in an inconsistent fashion. These similarities and differences between gaze-following in monkeys and children can help to illuminate the evolution of human social cognition.

The use of a human's location and social cues by Asian elephants in an object-choice task

Asian elephants have previously demonstrated an ability to follow olfactory cues, but not human-provided social cues like pointing and gazing or orienting to find hidden food (Plotnik et al. in PLoS One 8:e61174, 2013; Anim Behav 88:91-98, 2014). In a study conducted with African elephants, however, elephants were able to follow a combination of these social cues to find food, even when the experimenter's position was counter to the location of the food. The authors of the latter study argued that the differences in the two species' performances might have been due to methodological differences in the study designs (Smet and Byrne in Curr Biol 23(20):2033-2037, 2013). To further investigate the reasons for these potential differences, we partially adapted Smet and Byrne (2013)'s design for a group of Asian elephants in Thailand. In a two-object-choice task in which only one of two buckets was baited with food, we found that, as a group, the elephants did not follow cues provided by an experimenter when she was positioned either equidistant between the buckets or closer to the incorrect bucket when providing the cues. The elephants did, however, follow cues when the experimenter was closer to the correct bucket. In addition, there was individual variability in the elephants' performance within and across experimental conditions. This indicates that in general, for Asian elephants, the pointing and/or gazing cues alone may not be salient enough; local enhancement in the form of the experimenter's position in relation to the food reward may represent a crucial, complementary cue. These results suggest that the variability within and between the species in their performance on these tasks could be due to a number of factors, including methodology, the elephants' experiences with their handlers, ecological differences in how Asian and African elephants use non-visual sensory information to find food in the wild, or some combination of the three.

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.

Chimpanzee uses manipulative gaze cues to conceal and reveal information to foraging competitor

Tactical deception has been widely reported in primates on a functional basis, but details of behavioral mechanisms are usually unspecified. We tested a pair of chimpanzees (Pan troglodytes) in the informed forager paradigm, in which the subordinate saw the location of hidden food and the dominant did not. We employed cross-correlations to examine temporal contingencies between chimpanzees' behavior: specifically how the direction of the subordinate's gaze and movement functioned to manipulate the dominant's searching behavior through two tactics, withholding, and misleading information. In Experiment 1, not only did the informed subordinate tend to stop walking toward a single high value food, but she also refrained from gazing toward it, thus, withholding potentially revealing cues from her searching competitor. In a second experiment, in which a moderate value food was hidden in addition to the high value food, whenever the subordinate alternated her gaze between the dominant and the moderate value food, she often paused walking for 5 s; this frequently recruited the dominant to the inferior food, functioning as a "decoy." The subordinate flexibly concealed and revealed gaze toward a goal, which suggests that not only can chimpanzees use visual cues to make predictions about behavior, but also that chimpanzees may understand that other individuals can exploit their gaze direction. These results substantiate descriptive reports of how chimpanzees use gaze to manipulate others, and to our knowledge are the first quantitative data to identify behavioral mechanisms of tactical deception.

RESEARCH HIGHLIGHTS

Cross correlations show a subordinate chimpanzee tactically deceived a dominant by not gazing toward a valuable food (withholding), and recruiting to a "decoy" food (misleading). Chimpanzees understand that others can exploit their gaze direction.

Do dogs follow behavioral cues from an unreliable human?

Dogs are known to consistently follow human pointing gestures. In this study, we asked whether dogs "automatically" do this or whether they flexibly adjust their behavior depending upon the reliability of the pointer, demonstrated in an immediately preceding event. We tested pet dogs in a version of the object choice task in which a piece of food was hidden in one of the two containers. In Experiment 1, Phase 1, an experimenter pointed at the baited container; the second container was empty. In Phase 2, after showing the contents of both containers to the dogs, the experimenter pointed at the empty container. In Phase 3, the procedure was exactly as in Phase 1. We compared the dogs' responses to the experimenter's pointing gestures in Phases 1 and 3. Most dogs followed pointing in Phase 1, but many fewer did so in Phase 3. In Experiment 2, dogs followed a new experimenter's pointing in Phase 3 following replication of procedures of Phases 1 and 2 in Experiment 1. This ruled out the possibility that dogs simply lost motivation to participate in the task in later phases. These results suggest that not only dogs are highly skilled at understanding human pointing gestures, but also they make inferences about the reliability of a human who presents cues and consequently modify their behavior flexibly depending on the inference.

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.

Using cross correlations to investigate how chimpanzees (Pan troglodytes) use conspecific gaze cues to extract and exploit information in a foraging competition

In a dyadic informed forager task, chimpanzees (Pan troglodytes) are known to exploit the knowledge of informed subordinates; however, the behavioral mechanisms they employ are unknown. It is tempting to interpret outcome measures, such as which individual obtained the food, in a cognitively richer way than the outcomes may justify. We employed a different approach from prior research, asking how chimpanzees compete by maneuvering around each other, whether they use gaze cues to acquire information from others, and what information they use in moment-to-moment decision-making. We used cross correlations, which plot the correlation between two variables as a function of time, systematically to examine chimpanzee interactions in a series of dyadic informed forager contests. We used cross correlations as a "proof of concept" so as to determine whether the target actions were contingent on, or occurred in a time-locked pattern relative to, the referent actions. A subordinate individual was given privileged knowledge of food location. As expected, an ignorant dominant followed the informed subordinate's movement in the enclosure. The dominant also followed the subordinate's gaze direction: after she looked at the subordinate, she was more likely to gaze toward this same direction within one second. In contrast, the subordinate only occasionally followed the dominant's movement and gaze. The dominant also changed her own direction of movement to converge on the location to which the subordinate directed her gaze and movement. Cross correlation proves an effective technique for charting contingencies in social interactions, an important step in understanding the use of cognition in natural situations.

Seeing the experimenter influences the response to pointing cues in long-tailed macaques

Methodological variations in experimental conditions can strongly influence animals' performances in cognitive tests. Specifically, the procedure of the so-called object-choice task has been controversially discussed; here, a human experimenter indicates the location of hidden food by pointing or gazing at one of two or more containers. Whereas dogs usually succeed, results for nonhuman primates are ambiguous. In the standard version of the task the majority of subjects do not respond appropriately to human pointing. However, modifying the task setup, such as placing the containers further apart, seems to improve subjects' performances, suggesting that cue salience may be an important variable. Here we investigated whether the visibility of the experimenter inhibits long-tailed macaques' (Macaca fascicularis) usage of the pointing cue. In our baseline condition, with the experimenter fully visible, the monkeys chose the correct container in 61% of the trials. The performance increased significantly, however, when the experimenter was hidden behind a curtain and only the arm of the experimenter, a doll's arm, or a stick was visible. Furthermore, the monkeys performed significantly better when the tip of the pointing finger or device was close to the target compared to the more distant condition. Intriguingly, after these experiments the monkeys' performance was also significantly improved in the baseline condition (70%). Apparently, the monkeys were first distracted by the presence of the experimenter, but then learned to use the cue. These findings highlight the importance of the test conditions, and question some of the assumptions about species-specific differences in the object-choice task.

Use of experimenter-given cues in dogs

Since the observations of O. Pfungst the use of human-provided cues by animals has been well-known in the behavioural sciences ("Clever Hans effect"). It has recently been shown that rhesus monkeys (Macaca mulatta) are unable to use the direction of gazing by the experimenter as a cue for finding food, although after some training they learned to respond to pointing by hand. Direction of gaze is used by chimpanzees, however. Dogs (Canis familiaris) are believed to be sensitive to human gestural communication but their ability has never been formally tested. In three experiments we examined whether dogs can respond to cues given by humans. We found that dogs are able to utilize pointing, bowing, nodding, head-turning and glancing gestures of humans as cues for finding hidden food. Dogs were also able to generalize from one person (owner) to another familiar person (experimenter) in using the same gestures as cues. Baseline trials were run to test the possibility that odour cues alone could be responsible for the dogs' performance. During training individual performance showed limited variability, probably because some dogs already "knew" some of the cues from their earlier experiences with humans. We suggest that the phenomenon of dogs responding to cues given by humans is better analysed as a case of interspecific communication than in terms of discrimination learning.

Are chimpanzees really so poor at understanding imperative pointing? Some new data and an alternative view of canine and ape social cognition

There is considerable interest in comparative research on different species’ abilities to respond to human communicative cues such as gaze and pointing. It has been reported that some canines perform significantly better than monkeys and apes on tasks requiring the comprehension of either declarative or imperative pointing and these differences have been attributed to domestication in dogs. Here we tested a sample of chimpanzees on a task requiring comprehension of an imperative request and show that, though there are considerable individual differences, the performance by the apes rival those reported in pet dogs. We suggest that small differences in methodology can have a pronounced influence on performance on these types of tasks. We further suggest that basic differences in subject sampling, subject recruitment and rearing experiences have resulted in a skewed representation of canine abilities compared to those of monkeys and apes.

Intracranial arachnoid cysts in a chimpanzee (Pan troglodytes)

An intracranial arachnoid cyst was detected in a 32-year-old, 44.6-kg, female chimpanzee at the Primate Research Institute, Kyoto University. Magnetic resonance imaging (MRI) and computed tomography (CT) were performed and the cognitive studies in which she participated were reviewed. MRI revealed that the cyst was present in the chimpanzee's right occipital convexity, and was located in close proximity to the posterior horn of the right lateral ventricle without ventriculomegaly. CT confirmed the presence of the cyst and no apparent signs indicating previous skull fractures were found. The thickness of the mandible was asymmetrical, whereas the temporomandibular joints and dentition were symmetrical. She showed no abnormalities in various cognitive studies since she was 3 years old, except a different behavioural pattern during a recent study, indicating a possible visual field defect. Detailed cognitive studies, long-term observation of her physical condition and follow-up MRI will be continued.

Are Clark's Nutcrackers (Nucifraga Columbiana) Able to Discriminate Knowledge States of Human Experimenters during an Object-Choice Task?

Corvids and primates have been shown to possess similar cognitive adaptations, yet these animals are seldom tested using similar procedures. Object-choice tasks, which have commonly been used to test whether an animal is able to infer the mental state of a human experimenter based on a gestural cue, provide one potential means of testing these animals using a similar paradigm. The current study used an object-choice task to examine whether the corvid, Clark's nutcracker ( Nucifraga columbiana), is able to use a cognitive strategy to discriminate between the knowledge states of two human experimenters. One experimenter was informed, and the other uninformed, as to the location of a food reward hidden inside one of two opaque containers. During the Uninformed Gesture condition, the nutcrackers were given probe tests during which only the person performing as the uninformed experimenter provided a gesture. Thus, the nutcrackers could not use the experimenter's gesture to reliably find the food. During the Gesture Conflict condition, the nutcrackers were presented with a cue conflict. During probe tests, both the informed and the uninformed experimenter gestured to separate containers. Thus, to find the food the nutcrackers had to use the gesture from the informed experimenter and refrain from using the gesture of the uninformed experimenter. Our results showed that when the uninformed experimenter's gesture was presented alone, the birds continued to follow the gesture even though it was not consistently predictive of the food's location. However, when provided with two conflicting gestures, as a group the nutcrackers responded to the gesture of the informed experimenter at above chance levels. These results suggest that the birds had learned that the gesture was informative, perhaps by associative learning, yet when this mechanism was not reliable the nutcrackers were able to use either the human experimenters' presence/absence during the baiting process, or possibly their knowledge states, to determine which gesture to rely upon.

Differences in the early cognitive development of children and great apes

There is very little research comparing great ape and human cognition developmentally. In the current studies we compared a cross-sectional sample of 2- to 4-year-old human children (n=48) with a large sample of chimpanzees and bonobos in the same age range (n=42, hereafter: apes) on a broad array of cognitive tasks. We then followed a group of juvenile apes (n=44) longitudinally over 3 years to track their cognitive development in greater detail. In skills of physical cognition (space, causality, quantities), children and apes performed comparably at 2 years of age, but by 4 years of age children were more advanced (whereas apes stayed at their 2-year-old performance levels). In skills of social cognition (communication, social learning, theory of mind), children out-performed apes already at 2 years, and increased this difference even more by 4 years. Patterns of development differed more between children and apes in the social domain than the physical domain, with support for these patterns present in both the cross-sectional and longitudinal ape data sets. These results indicate key differences in the pattern and pace of cognitive development between humans and other apes, particularly in the early emergence of specific social cognitive capacities in humans.

Visual cues given by humans are not sufficient for Asian elephants (Elephas maximus) to find hidden food

Recent research suggests that domesticated species – due to artificial selection by humans for specific, preferred behavioral traits – are better than wild animals at responding to visual cues given by humans about the location of hidden food. \Although this seems to be supported by studies on a range of domesticated (including dogs, goats and horses) and wild (including wolves and chimpanzees) animals, there is also evidence that exposure to humans positively influences the ability of both wild and domesticated animals to follow these same cues. Here, we test the performance of Asian elephants (Elephas maximus) on an object choice task that provides them with visual-only cues given by humans about the location of hidden food. Captive elephants are interesting candidates for investigating how both domestication and human exposure may impact cue-following as they represent a non-domesticated species with almost constant human interaction. As a group, the elephants (n = 7) in our study were unable to follow pointing, body orientation or a combination of both as honest signals of food location. They were, however, able to follow vocal commands with which they were already familiar in a novel context, suggesting the elephants are able to follow cues if they are sufficiently salient. Although the elephants’ inability to follow the visual cues provides partial support for the domestication hypothesis, an alternative explanation is that elephants may rely more heavily on other sensory modalities, specifically olfaction and audition. Further research will be needed to rule out this alternative explanation.

Head-mounted eye tracking of a chimpanzee under naturalistic conditions

This study offers a new method for examining the bodily, manual, and eye movements of a chimpanzee at the micro-level. A female chimpanzee wore a lightweight head-mounted eye tracker (60 Hz) on her head while engaging in daily interactions with the human experimenter. The eye tracker recorded her eye movements accurately while the chimpanzee freely moved her head, hands, and body. Three video cameras recorded the bodily and manual movements of the chimpanzee from multiple angles. We examined how the chimpanzee viewed the experimenter in this interactive setting and how the eye movements were related to the ongoing interactive contexts and actions. We prepared two experimentally defined contexts in each session: a face-to-face greeting phase upon the appearance of the experimenter in the experimental room, and a subsequent face-to-face task phase that included manual gestures and fruit rewards. Overall, the general viewing pattern of the chimpanzee, measured in terms of duration of individual fixations, length of individual saccades, and total viewing duration of the experimenter’s face/body, was very similar to that observed in previous eye-tracking studies that used non-interactive situations, despite the differences in the experimental settings. However, the chimpanzee viewed the experimenter and the scene objects differently depending on the ongoing context and actions. The chimpanzee viewed the experimenter’s face and body during the greeting phase, but viewed the experimenter’s face and hands as well as the fruit reward during the task phase. These differences can be explained by the differential bodily/manual actions produced by the chimpanzee and the experimenter during each experimental phase (i.e., greeting gestures, task cueing). Additionally, the chimpanzee’s viewing pattern varied depending on the identity of the experimenter (i.e., the chimpanzee’s prior experience with the experimenter). These methods and results offer new possibilities for examining the natural gaze behavior of chimpanzees.

Olive baboons communicate intentionally by pointing

A pointing gesture creates a referential triangle that incorporates distant objects into the relationship between the signaller and the gesture's recipient. Pointing was long assumed to be specific to our species. However, recent reports have shown that pointing emerges spontaneously in captive chimpanzees and can be learned by monkeys. Studies have demonstrated that both human children and great apes use manual gestures (e.g. pointing), and visual and vocal signals, to communicate intentionally about out-of-reach objects. Our study looked at how monkeys understand and use their learned pointing behaviour, asking whether it is a conditioned, reinforcement-dependent response or whether monkeys understand it to be a mechanism for manipulating the attention of a partner (e.g. a human). We tested nine baboons that had been trained to exhibit pointing, using operant conditioning. More specifically, we investigated their ability to communicate intentionally about the location of an unreachable food reward in three contexts that differed according to the human partner's attentional state. In each context, we quantified the frequency of communicative behaviour (auditory and visual signals), including gestures and gaze alternations between the distal food and the human partner. We found that the baboons were able to modulate their manual and visual communicative signals as a function of the experimenter's attentional state. These findings indicate that monkeys can intentionally produce pointing gestures and understand that a human recipient must be looking at the pointing gesture for them to perform their attention-directing actions. The referential and intentional nature of baboons' communicative signalling is discussed.

Gibbons (Hylobates pileatus, H. moloch, H. lar, Symphalangus syndactylus) follow human gaze, but do not take the visual perspective of others

We investigated four gibbon species of two different genera (Hylobates pileatus, H. moloch, H. lar, Symphalangus syndactylus) in terms of their looking behavior in response to a human who either looked up or looked at the gibbon. Comparing those two conditions, gibbons as a group looked up more when the human was looking up, but they also performed more looks in other directions and thus generally looked more in this condition. Unlike great apes, gibbons did not respond differently between conditions when only the first look on every trial was considered. Furthermore, they did not perform double looks up to check where the human was looking and also did not habituate to the human's looks up. This suggests that gibbons co-orient with human gaze, but unlike great apes, they do not take the visual perspective of others.

Gaze direction - a cue for hidden food in rooks (Corvus frugilegus)?

Other individual's head- and eye-directions can be used as social cues indicating the presence of important events. Among birds, ravens and rooks have been shown to co-orient with conspecifics and with humans by following their gaze direction into distant space and behind visual screens. Both species use screens to cache food in private; also, it had been suggested that they may rely on gaze cues to detect hidden food. However, in an object-choice task, ravens failed to do so, and their competitive lifestyle may have prevented them from relying on these cues. Here we tested closely related and cooperative rooks. Food was hidden in one of two cups and the experimenter gazed at the baited cup. In a second experiment, we aimed to increase the birds' motivation to choose correctly by increasing the investment needed to obtain the reward. To do so, the birds had to pull on a string to obtain the cup. Here, the birds as a group tended to rely on gaze cues. In addition, individual birds quickly learned to use the cue in both experiments. Although rooks may not use gaze cues to find hidden food spontaneously, they may quickly learn to do so.

Environmental Influences on Joint Attention in Great Apes: Implications for Human Cognition

In humans, cultural differences in advanced social cognition have been well demonstrated. In tasks pertaining to theory of mind, for instance, individuals pass standard assessments of false-belief understanding at substantially different ages in different cultural environments. Less well-studied are more basic sociocognitive capacities, such as joint attention, which are held by many to constitute the foundational skill set for advanced sociocognitive reasoning. Here, we review the striking group differences in joint attention displayed by great apes as a consequence of being raised in different cultural environments, including wild habitats, institutional settings such as zoos and biomedical research centers, and home-rearing or language-training settings. Like humans, apes develop tactics for joint attention that are adaptive to the particular environments of their early rearing experiences. Great apes serve as animal models for environmental influences on sociocognitive capacities in our own species.

Clark's nutcrackers (Nucifraga columbiana) use gestures to identify the location of hidden food

Heterospecific cues, such as gaze direction and body position, may be an important source of information that an animal can use to infer the location of resources like food. The use of heterospecific cues has been largely investigated using primates, dogs, and other mammals; less is known about whether birds can also use heterospecific gestures. We tested six Clark's nutcrackers in a two-way object-choice task using touch, point, and gaze cues to investigate whether these birds can use human gestures to find food. Most of the birds were able to use a touch gesture during the first trial of testing and were able to learn to use point and gaze (eyes and head alternation) cues after a limited number of trials. This study is the first to test a non-social corvid on the object-choice task. The performance of non-social nutcrackers is similar to that of more social and related corvids, suggesting that species with different evolutionary histories can utilize gestural information.

BIZARRE chimpanzees do not represent "the chimpanzee"

Henrich et al. convincingly caution against the overgeneralization of findings from particular human populations, but fail to apply their own compelling reasoning to our nearest living relatives, the great apes. Here we argue that rearing history is every bit as important for understanding cognition in other species as it is in humans.

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

A series of recent reports have questioned the ability of great apes to comprehend declarative communication and have suggested that this ability is biologically based and may have driven the evolution of human language. We tested three groups of differently reared chimpanzees and bonobos for their ability to understand declarative signals in an object-choice task. The scores of the two groups of apes that were reared in a sociolinguistically complex environment were significantly higher than the scores of the standard-reared group. The results further showed that bonobos did not outperform chimpanzees. Our results demonstrate that environmental factors, particularly access to a sociolinguistically rich environment, directly influence great apes' ability to comprehend declarative signals and suggest that, contrary to recent claims, apes have the biological capacity to utilize purely informative communication.

Visual search for human gaze direction by a Chimpanzee (Pan troglodytes)

Background

Humans detect faces with direct gazes among those with averted gazes more efficiently than they detect faces with averted gazes among those with direct gazes. We examined whether this “stare-in-the-crowd” effect occurs in chimpanzees (Pan troglodytes), whose eye morphology differs from that of humans (i.e., low-contrast eyes, dark sclera).

Methodology/Principal Findings

An adult female chimpanzee was trained to search for an odd-item target (front view of a human face) among distractors that differed from the target only with respect to the direction of the eye gaze. During visual-search testing, she performed more efficiently when the target was a direct-gaze face than when it was an averted-gaze face. This direct-gaze superiority was maintained when the faces were inverted and when parts of the face were scrambled. Subsequent tests revealed that gaze perception in the chimpanzee was controlled by the contrast between iris and sclera, as in humans, but that the chimpanzee attended only to the position of the iris in the eye, irrespective of head direction.

Conclusion/Significance

These results suggest that the chimpanzee can discriminate among human gaze directions and are more sensitive to direct gazes. However, limitations in the perception of human gaze by the chimpanzee are suggested by her inability to completely transfer her performance to faces showing a three-quarter view.

A comparative approach to dogs' (Canis familiaris) and human infants' comprehension of various forms of pointing gestures

We investigated whether dogs and 2-, and 3-year-old human infants living, in some respects, in very similar social environments are able to comprehend various forms of the human pointing gesture. In the first study, we looked at their ability to comprehend different arm pointing gestures (long cross-pointing, forward cross-pointing and elbow cross-pointing) to locate a hidden object. Three-year-olds successfully used all gestures as directional cues, while younger children and dogs could not understand the elbow cross-pointing. Dogs were also unsuccessful with the forward cross-pointing. In the second study, we used unfamiliar pointing gestures i.e. using a leg as indicator (pointing with leg, leg cross-pointing, pointing with knee). All subjects were successful with leg pointing gestures, but only older children were able to comprehend the pointing with knee. We suggest that 3-year-old children are able to rely on the direction of the index finger, and show the strongest ability to generalize to unfamiliar gestures. Although some capacity to generalize is also evident in younger children and dogs, especially the latter appear biased in the use of protruding body parts as directional signals.

Jackdaws respond to human attentional states and communicative cues in different contexts

Humans communicate their intentions and disposition using their eyes, whereas the communicative function of eyes in animals is less clear. Many species show aversive reactions to eyes, and several species gain information from conspecifics' gaze direction by automatically co-orienting with them. However, most species show little sensitivity to more subtle indicators of attention than head orientation and have difficulties using such cues in a cooperative context. Recently, some species have been found responsive to gaze direction in competitive situations. We investigated the sensitivity of jackdaws, pair-bonded social corvids that exhibit an analogous eye morphology to humans, to subtle attentional and communicative cues in two contexts and paradigms. In a conflict paradigm, we measured the birds' latency to retrieve food in front of an unfamiliar or familiar human, depending on the state and orientation of their eyes toward food. In a cooperative paradigm, we tested whether the jackdaws used familiar human's attentional or communicative cues to locate hidden food. Jackdaws were sensitive to human attentional states in the conflict situation but only responded to communicative cues in the cooperative situation. These findings may be the result of a natural tendency to attend to conspecifics' eyes or the effect of intense human contact during socialization.

Comparative social cognition

Theory of mind is said to be uniquely human. Is this statement justified? Thirty years of research on a variety of species has produced differences in opinion, from unequivocal positive evidence to no evidence at all for mental attribution in animals. Our review concludes that animals are excellent ethologists, but on the whole, poor psychologists. Those studies that we believe present a good case for mental attribution all possess high ecological validity, including studies on food competition by chimpanzees and cache-protection strategies by corvids. Even though the current focus of research on prediction rather than explanation may be misplaced, we believe the field is now in a strong position to discover what animals really know about their fellow beings, be it based on simple associations, behavior reading, mind reading, or something else.

A natural history of the human mind: tracing evolutionary changes in brain and cognition

Since the last common ancestor shared by modern humans, chimpanzees and bonobos, the lineage leading to Homo sapiens has undergone a substantial change in brain size and organization. As a result, modern humans display striking differences from the living apes in the realm of cognition and linguistic expression. In this article, we review the evolutionary changes that occurred in the descent of Homo sapiens by reconstructing the neural and cognitive traits that would have characterized the last common ancestor and comparing these with the modern human condition. The last common ancestor can be reconstructed to have had a brain of approximately 300-400 g that displayed several unique phylogenetic specializations of development, anatomical organization, and biochemical function. These neuroanatomical substrates contributed to the enhancement of behavioral flexibility and social cognition. With this evolutionary history as precursor, the modern human mind may be conceived as a mosaic of traits inherited from a common ancestry with our close relatives, along with the addition of evolutionary specializations within particular domains. These modern human-specific cognitive and linguistic adaptations appear to be correlated with enlargement of the neocortex and related structures. Accompanying this general neocortical expansion, certain higher-order unimodal and multimodal cortical areas have grown disproportionately relative to primary cortical areas. Anatomical and molecular changes have also been identified that might relate to the greater metabolic demand and enhanced synaptic plasticity of modern human brain's. Finally, the unique brain growth trajectory of modern humans has made a significant contribution to our species' cognitive and linguistic abilities.

Development of using experimenter-given cues in infant chimpanzees: longitudinal changes in behavior and cognitive development

The use of gaze shifts as social cues has various evolutionary advantages. To investigate the developmental processes of this ability, we conducted an object-choice task by using longitudinal methods with infant chimpanzees tested from 8 months old until 3 years old. The experimenter used one of six gestures towards a cup concealing food; tapping, touching, whole-hand pointing, gazing plus close-pointing, distant-pointing, close-gazing, and distant-gazing. Unlike any other previous study, we analyzed the behavioral changes that occurred before and after choosing the cup. We assumed that pre-choice behavior indicates the development of an attentional and spatial connection between a pointing cue and an object (e.g. Woodward, 2005); and post-choice behavior indicates the emergence of object permanence (e.g. Piaget, 1954). Our study demonstrated that infant chimpanzees begin to use experimenter-given cues with age (after 11 months of age). Moreover, the results from the behavioral analysis showed that the infants gradually developed the spatial link between the pointing as an object-directed action and the object. Moreover, when they were 11 months old, the infants began to inspect the inside of the cup, suggesting the onset of object permanence. Overall, our results imply that the ability to use the cues is developing and mutually related with other cognitive developments. The present study also suggests what the standard object-choice task actually measures by breaking the task down into the developmental trajectories of its component parts, and describes for the first time the social-physical cognitive development during the task with a longitudinal method.

Comprehension of human pointing gestures in horses (Equus caballus)

Twenty domestic horses (Equus caballus) were tested for their ability to rely on different human gesticular cues in a two-way object choice task. An experimenter hid food under one of two bowls and after baiting, indicated the location of the food to the subjects by using one of four different cues. Horses could locate the hidden reward on the basis of the distal dynamic-sustained, proximal momentary and proximal dynamic-sustained pointing gestures but failed to perform above chance level when the experimenter performed a distal momentary pointing gesture. The results revealed that horses could rely spontaneously on those cues that could have a stimulus or local enhancement effect, but the possible comprehension of the distal momentary pointing remained unclear. The results are discussed with reference to the involvement of various factors such as predisposition to read human visual cues, the effect of domestication and extensive social experience and the nature of the gesture used by the experimenter in comparative investigations.