Overcoming bias in the comparison of human language and animal communication.

Human language is a powerful communicative and cognitive tool. Scholars have long sought to characterize its uniqueness, but each time a property is proposed to set human language apart (e.g., reference, syntax), some (attenuated) version of that property is found in animals. Recently, the uniqueness argument has shifted from linguistic rules to cognitive capacities underlying them. Scholars argue that human language is unique because it relies on ostension and inference, while animal communication depends on simple associations and largely hardwired signals. Such characterizations are often borne out in published data, but these empirical findings are driven by radical differences in the ways animal and human communication are studied. The field of animal communication has been dramatically shaped by the "code model," which imagines communication as involving information packets that are encoded, transmitted, decoded, and interpreted. This framework standardized methods for studying meaning in animal signals, but it does not allow for the nuance, ambiguity, or contextual variation seen in humans. The code model is insidious. It is rarely referenced directly, but it significantly shapes how we study animals. To compare animal communication and human language, we must acknowledge biases resulting from the different theoretical models used. By incorporating new approaches that break away from searching for codes, we may find that animal communication and human language are characterized by differences of degree rather than kind.

A robust tool kit: First report of tool use in captive crested capuchin monkeys (Sapajus robustus).

Primate tool use is of great interest but has been reported only in a limited number of species. Here we report tool use in crested capuchin monkeys (Sapajus robustus), an almost completely unstudied robust capuchin species. Crested capuchins and their sister species, the yellow-breasted capuchin, diverged from a common ancestor over 2 million years ago, so this study fills a significant gap in understanding of tool use capacity and variation within the robust capuchin monkey radiation. Our study group was a captive population of seven individuals at the Santa Ana Zoo in California. The monkeys were given no prior training, and they were provided with a variety of enrichment items, including materials that could be used as tools as well as hard-to-access resources, for open-ended interactions. In 54 observation hours, monkeys performed eleven tool use actions: digging, hammering, probing, raking, sponging, striking, sweeping, throwing, waving, wedging, and wiping. We observed tool modification, serial tool use, and social learning opportunities, including monkeys' direct observation of tool use and tolerated scrounging of foods obtained through tool use. We also observed significant individual skew in tool use frequency, with one individual using tools daily, and two individuals never using tools during the study. While crested capuchins have never been reported to use tools in the wild, our findings provide evidence for the species' capacity and propensity for tool use, highlighting the urgent need for research on this understudied, endangered primate. By providing detailed data on clearly identified S. robustus individuals, this study marks an effort to counteract the overgeneralization in the captive literature in referring to any robust capuchins of unknown provenance or ancestry as Cebus apella, a practice that obfuscates potential differences among species in tool use performance and repertoire in one of the only species-rich tool-using genera in the world.

Just kidding: the evolutionary roots of playful teasing.

Accounts of teasing have a long history in psychological and sociological research, yet teasing itself is vastly underdeveloped as a topic of study. As a phenomenon that moves along the border between aggression and play, teasing presents an opportunity to investigate key foundations of social and mental life. Developmental studies suggest that preverbal human infants already playfully tease their parents by performing 'the unexpected,' apparently deliberately violating the recipient's expectations to create a shared humorous experience. Teasing behaviour may be phylogenetically old and perhaps an evolutionary precursor to joking. In this review, we present preliminary evidence suggesting that non-human primates also exhibit playful teasing. In particular, we argue that great apes display three types of playful teasing described in preverbal human infants: teasing with offer and withdrawal, provocative non-compliance and disrupting others' activities. We highlight the potential of this behaviour to provide a window into complex socio-cognitive processes such as attribution of others' expectations and, finally, we propose directions for future research and call for systematic studies of teasing behaviour in non-human primates.

The gaits of marsupials and the evolution of diagonal-sequence walking in primates.

OBJECTIVES: Documenting the variety of quadrupedal walking gaits in a variety of marsupials (arboreal vs. terrestrial, with and without grasping hind feet), to aid in developing and refining a general theory of gait evolution in primates. MATERIALS AND METHODS: Video records of koalas, ringtail possums, tree kangaroos, sugar gliders, squirrel gliders, wombats, numbats, quolls, a thylacine, and an opossum walking on a variety of substrates were made and analyzed to derive duty factors and diagonalities for symmetrical walking gaits. The resulting distributions of data points were compared with published data and theories. RESULTS: Terrestrial marsupials' gaits overwhelmingly plot slightly below the theoretical "horse line" (Cartmill et al., Zoological Journal of the Linnean Society. 2002;136:401-420) typical of terrestrial mammals; arboreal marsupials' gaits overwhelmingly plot more decisively above it. Both distributions are roughly parallel to the horse line, but arboreal animals exhibit increased diagonality, so that their higher-speed walking gaits overlap with those of typical primates on the Hildebrand diagram of diagonality against duty factor. CONCLUSIONS: Quadrupeds avoid gaits lying exactly on the (theoretically optimum) horse line, to avoid fore/hind limb interference ("forging"). This can be accomplished by either a slight reduction in diagonality ("downshifting") or a more decisive increase ("upshifting"). Tree-dwellers adopt the second option to eliminate unilateral bipods of support from the gait cycle. The upshifted horse line represents an early phase in the evolution of primate-like diagonal-sequence gaits.

Developmental perspectives on primate gesture: 100 years in the making.

This paper is an introduction to the special issue entitled Evolving the study of gesture: evaluating and unifying theories of gesture acquisition in great apes. The gestures of great apes have been recorded in scientific literature for over 100 years, but the ways in which apes acquire their gestures remains a highly debated topic. Through this historical framework, we summarize and contextualize contemporary research on the development of ape gesture. We describe the papers presented in this special issue, grouping them into three themes: assessing theories, methodological innovation, and new empirical approaches. Each of the papers is a significant contribution to the literature on ape gesture, but the collection of work together represents a unique collaboration across labs, theories, and studied species. By considering the papers side-by-side, we hope that readers will see the authors as engaging in a true dialogue, one which will help the field of primate gesture research make significant advances in the years to come.

Gesturing towards the future: cognition, big data, and the future of comparative gesture research.

The field of ape gesture research has grown significantly in the past two decades, but progress on the question of gesture development has been limited by methodological and terminological disagreements, small sample sizes, and a lack of fine-grained longitudinal data. The main theories of gesture acquisition are often portrayed as mutually exclusive, but only some theories actually detail learning mechanisms, and differences in the level of analysis may help explain some of the apparent disagreements. Gesture research would benefit greatly from the articulation of more testable hypotheses. We propose two hypotheses that follow from dominant theories of gesture acquisition. We urge scholars to collect new data and leverage existing data in ways that maximize the potential for comparison across datasets and articulation with studies of other communicative modalities. Finally, we advocate for a transition away from using intentionality as a marker of the 'special status' of gesture, and towards using gesture as a window onto the lives and minds of apes.

Quality of early parent input predicts child vocabulary 3 years later.

Children vary greatly in the number of words they know when they enter school, a major factor influencing subsequent school and workplace success. This variability is partially explained by the differential quantity of parental speech to preschoolers. However, the contexts in which young learners hear new words are also likely to vary in referential transparency; that is, in how clearly word meaning can be inferred from the immediate extralinguistic context, an aspect of input quality. To examine this aspect, we asked 218 adult participants to guess 50 parents' words from (muted) videos of their interactions with their 14- to 18-mo-old children. We found systematic differences in how easily individual parents' words could be identified purely from this socio-visual context. Differences in this kind of input quality correlated with the size of the children's vocabulary 3 y later, even after controlling for differences in input quantity. Although input quantity differed as a function of socioeconomic status, input quality (as here measured) did not, suggesting that the quality of nonverbal cues to word meaning that parents offer to their children is an individual matter, widely distributed across the population of parents.

Embodying Similarity and Difference: The Effect of Listing and Contrasting Gestures During U.S. Political Speech.

Public speakers like politicians carefully craft their words to maximize the clarity, impact, and persuasiveness of their messages. However, these messages can be shaped by more than words. Gestures play an important role in how spoken arguments are perceived, conceptualized, and remembered by audiences. Studies of political speech have explored the ways spoken arguments are used to persuade audiences and cue applause. Studies of politicians' gestures have explored the ways politicians illustrate different concepts with their hands, but have not focused on gesture's potential as a tool of persuasion. Our paper combines these traditions to ask first, how politicians gesture when using spoken rhetorical devices aimed at persuading audiences, and second, whether these gestures influence the ways their arguments are perceived. Study 1 examined two rhetorical devices-contrasts and lists-used by three politicians during U.S. presidential debates and asked whether the gestures produced during contrasts and lists differ. Gestures produced during contrasts were more likely to involve changes in hand location, and gestures produced during lists were more likely to involve changes in trajectory. Study 2 used footage from the same debates in an experiment to ask whether gesture influenced the way people perceived the politicians' arguments. When participants had access to gestural information, they perceived contrasted items as more different from one another and listed items as more similar to one another than they did when they only had access to speech. This was true even when participants had access to only gesture (in muted videos). We conclude that gesture is effective at communicating concepts of similarity and difference and that politicians (and likely other speakers) take advantage of gesture's persuasive potential.

Joyful by nature: approaches to investigate the evolution and function of joy in non-human animals.

The nature and evolution of positive emotion is a major question remaining unanswered in science and philosophy. The study of feelings and emotions in humans and animals is dominated by discussion of affective states that have negative valence. Given the clinical and social significance of negative affect, such as depression, it is unsurprising that these emotions have received more attention from scientists. Compared to negative emotions, such as fear that leads to fleeing or avoidance, positive emotions are less likely to result in specific, identifiable, behaviours being expressed by an animal. This makes it particularly challenging to quantify and study positive affect. However, bursts of intense positive emotion (joy) are more likely to be accompanied by externally visible markers, like vocalisations or movement patterns, which make it more amenable to scientific study and more resilient to concerns about anthropomorphism. We define joy as intense, brief, and event-driven (i.e. a response to something), which permits investigation into how animals react to a variety of situations that would provoke joy in humans. This means that behavioural correlates of joy are measurable, either through newly discovered 'laughter' vocalisations, increases in play behaviour, or reactions to cognitive bias tests that can be used across species. There are a range of potential situations that cause joy in humans that have not been studied in other animals, such as whether animals feel joy on sunny days, when they accomplish a difficult feat, or when they are reunited with a familiar companion after a prolonged absence. Observations of species-specific calls and play behaviour can be combined with biometric markers and reactions to ambiguous stimuli in order to enable comparisons of affect between phylogenetically distant taxonomic groups. Identifying positive affect is also important for animal welfare because knowledge of positive emotional states would allow us to monitor animal well-being better. Additionally, measuring if phylogenetically and ecologically distant animals play more, laugh more, or act more optimistically after certain kinds of experiences will also provide insight into the mechanisms underlying the evolution of joy and other positive emotions, and potentially even into the evolution of consciousness.

Orangutans modify their gestural signaling according to their audience's comprehension.

When people are not fully understood, they persist with attempts to communicate, elaborating their speech in order to better convey their meaning [1]. We investigated whether captive orangutans (Pongo pygmaeus and Pongo abelii) would use analogous communicative strategies in signaling to a human experimenter, and whether they could distinguish different degrees of misunderstanding. Orangutans' behavior varied according to how well they had apparently been understood. When their aims were not met, they persisted in communicative attempts. However, when the interlocutor appeared partially to understand their meaning, orangutans narrowed down their range of signals, focusing on gestures already used and repeating them frequently. In contrast, when completely misunderstood, orangutans elaborated their range of gestures, avoiding repetition of failed signals. It is therefore possible, from communicative signals alone, to determine how well an orangutan's intended goal has been met. This differentiation might function under natural conditions to allow an orangutan's intended goals to be understood more efficiently. In the absence of conventional labels, communicating the fact that an intention has been somewhat misunderstood is an important way to establish shared meaning.

Semantics of primate gestures: intentional meanings of orangutan gestures.

Great ape gesture has become a research topic of intense interest, because its intentionality and flexibility suggest strong parallels to human communication. Yet the fundamental question of whether an animal species' gestures carry specific meanings has hardly been addressed. We set out a systematic approach to studying intentional meaning in the gestural communication of non-humans and apply it to a sample of orangutan gestures. We propose that analysis of meaning should be limited to gestures for which (1) there is strong evidence for intentional production and (2) the recipient's final reaction matches the presumed goal of the signaller, as determined independently. This produces a set of "successful" instances of gesture use, which we describe as having goal-outcome matches. In this study, 28 orangutans in three European zoos were observed for 9 months. We distinguished 64 gestures on structural grounds, 40 of which had frequent goal-outcome matches and could therefore be analysed for meaning. These 40 gestures were used predictably to achieve one of 6 social goals: to initiate an affiliative interaction (contact, grooming, or play), request objects, share objects, instigate co-locomotion, cause the partner to move back, or stop an action. Twenty-nine of these gestures were used consistently with a single meaning. We tested our analysis of gesture meaning by examining what gesturers did when the response to their gesture did not match the gesture's meaning. Subsequent actions of the gesturer were consistent with our assignments of meaning to gestures. We suggest that, despite their contextual flexibility, orangutan gestures are made with the expectation of specific behavioural responses and thus have intentional meanings as well as functional consequences.

The development of iconicity in children's co-speech gesture and homesign.

Gesture can illustrate objects and events in the world by iconically reproducing elements of those objects and events. Children do not begin to express ideas iconically, however, until after they have begun to use conventional forms. In this paper, we investigate how children's use of iconic resources in gesture relates to the developing structure of their communicative systems. Using longitudinal video corpora, we compare the emergence of manual iconicity in hearing children who are learning a spoken language (co-speech gesture) to the emergence of manual iconicity in a deaf child who is creating a manual system of communication (homesign). We focus on one particular element of iconic gesture - the shape of the hand (handshape). We ask how handshape is used as an iconic resource in 1-5-year-olds, and how it relates to the semantic content of children's communicative acts. We find that patterns of handshape development are broadly similar between co-speech gesture and homesign, suggesting that the building blocks underlying children's ability to iconically map manual forms to meaning are shared across different communicative systems: those where gesture is produced alongside speech, and those where gesture is the primary mode of communication.

Perceiving referential intent: Dynamics of reference in natural parent-child interactions.

Two studies are presented which examined the temporal dynamics of the social-attentive behaviors that co-occur with referent identification during natural parent-child interactions in the home. Study 1 focused on 6.2 h of videos of 56 parents interacting during everyday activities with their 14-18 month-olds, during which parents uttered common nouns as parts of spontaneously occurring utterances. Trained coders recorded, on a second-by-second basis, parent and child attentional behaviors relevant to reference in the period (40 s) immediately surrounding parental naming. The referential transparency of each interaction was independently assessed by having naïve adult participants guess what word the parent had uttered in these video segments, but with the audio turned off, forcing them to use only non-linguistic evidence available in the ongoing stream of events. We found a great deal of ambiguity in the input along with a few potent moments of word-referent transparency; these transparent moments have a particular temporal signature with respect to parent and child attentive behavior: it was the object's appearance and/or the fact that it captured parent/child attention at the moment the word was uttered, not the presence of the object throughout the video, that predicted observers' accuracy. Study 2 experimentally investigated the precision of the timing relation, and whether it has an effect on observer accuracy, by disrupting the timing between when the word was uttered and the behaviors present in the videos as they were originally recorded. Disrupting timing by only ±1 to 2 s reduced participant confidence and significantly decreased their accuracy in word identification. The results enhance an expanding literature on how dyadic attentional factors can influence early vocabulary growth. By hypothesis, this kind of time-sensitive data-selection process operates as a filter on input, removing many extraneous and ill-supported word-meaning hypotheses from consideration during children's early vocabulary learning.

Pointing and naming are not redundant: children use gesture to modify nouns before they modify nouns in speech.

Nouns form the first building blocks of children's language but are not consistently modified by other words until around 2.5 years of age. Before then, children often combine their nouns with gestures that indicate the object labeled by the noun, for example, pointing at a bottle while saying "bottle." These gestures are typically assumed to be redundant with speech. Here we present data challenging this assumption, suggesting that these early pointing gestures serve a determiner-like function (i.e., point at bottle + "bottle" = that bottle). Using longitudinal data from 18 children (8 girls), we analyzed all utterances containing nouns and focused on (a) utterances containing an unmodified noun combined with a pointing gesture and (b) utterances containing a noun modified by a determiner. We found that the age at which children first produced point + noun combinations predicted the onset age for determiner + noun combinations. Moreover, point + noun combinations decreased following the onset of determiner + noun constructions. Importantly, combinations of pointing gestures with other types of speech (e.g., point at bottle + "gimme" = gimme that) did not relate to the onset or offset of determiner + noun constructions. Point + noun combinations thus appear to selectively predict the development of a new construction in speech. When children point to an object and simultaneously label it, they are beginning to develop their understanding of nouns as a modifiable unit of speech.

Ontogenetic ritualization of primate gesture as a case study in dyadic brain modeling.

This paper introduces dyadic brain modeling - the simultaneous, computational modeling of the brains of two interacting agents - to explore ways in which our understanding of macaque brain circuitry can ground new models of brain mechanisms involved in ape interaction. Specifically, we assess a range of data on gestural communication of great apes as the basis for developing an account of the interactions of two primates engaged in ontogenetic ritualization, a proposed learning mechanism through which a functional action may become a communicative gesture over repeated interactions between two individuals (the 'dyad'). The integration of behavioral, neural, and computational data in dyadic (or, more generally, social) brain modeling has broad application to comparative and evolutionary questions, particularly for the evolutionary origins of cognition and language in the human lineage. We relate this work to the neuroinformatics challenges of integrating and sharing data to support collaboration between primatologists, neuroscientists and modelers that will help speed the emergence of what may be called comparative neuro-primatology.

A word in the hand: action, gesture and mental representation in humans and non-human primates.

The movements we make with our hands both reflect our mental processes and help to shape them. Our actions and gestures can affect our mental representations of actions and objects. In this paper, we explore the relationship between action, gesture and thought in both humans and non-human primates and discuss its role in the evolution of language. Human gesture (specifically representational gesture) may provide a unique link between action and mental representation. It is kinaesthetically close to action and is, at the same time, symbolic. Non-human primates use gesture frequently to communicate, and do so flexibly. However, their gestures mainly resemble incomplete actions and lack the representational elements that characterize much of human gesture. Differences in the mirror neuron system provide a potential explanation for non-human primates' lack of representational gestures; the monkey mirror system does not respond to representational gestures, while the human system does. In humans, gesture grounds mental representation in action, but there is no evidence for this link in other primates. We argue that gesture played an important role in the transition to symbolic thought and language in human evolution, following a cognitive leap that allowed gesture to incorporate representational elements.