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Abstract The ability to quickly perceive others' rank minimizes costs by helping individuals behave appropriately when interacting with strangers. Indeed, humans and at least some other species can quickly determine strangers' rank or dominance based only on physical features without observing others' interactions or behavior. Nonhuman primates can determine strangers' ranks by observing their interactions, and some evidence suggests that at least some cues to dominance, such as facial width‐to‐height ratio (fWHR), are also present in other primates. However, it is unknown whether they can determine strangers' rank simply by looking at their faces, rather than observing their interactions. If so, this would suggest selective pressure across the primates on both cues to dominance and the ability to detect those cues accurately. To address this, we examined the ability of male and female tufted capuchin monkeys (Sapajus [Cebus] apella) to categorize images of the faces of unknown conspecifics (Sapajusfrom different colonies) and humans (computer‐generated and real) as dominant or nondominant based only on still images. Capuchins' categorization of unknown conspecific faces was consistent with fWHR, a cue to dominance, although there was a strong tendency to categorize strangers as dominant, particularly for males. This was true despite the continued correct categorization of known individuals. In addition, capuchins did not categorize human strangers in accordance with external pre‐ratings of dominance by independent human raters, despite the availability of the same cue, fWHR. We consider these results in the context of capuchin socio‐ecology and what they mean for the evolution of rapid decision‐making in social contexts. 

Abstract Human cooperation can be facilitated by the ability to create a mental representation of one’s own actions, as well as the actions of a partner, known as action co-representation. Even though other species also cooperate extensively, it is still unclear whether they have similar capacities. The Joint Simon task is a two-player task developed to investigate this action co-representation. We tested brown capuchin monkeys (Sapajus [Cebus] apella), a highly cooperative species, on a computerized Joint Simon task and found that, in line with previous research, the capuchins' performance was compatible with co-representation. However, a deeper exploration of the monkeys’ responses showed that they, and potentially monkeys in previous studies, did not understand the control conditions, which precludes the interpretation of the results as a social phenomenon. Indeed, further testing to investigate alternative explanations demonstrated that our results were due to low-level cues, rather than action co-representation. This suggests that the Joint Simon task, at least in its current form, cannot determine whether non-human species co-represent their partner’s role in joint tasks. 

Abstract Reciprocally patterned behavior is widespread in animals in the wild, but experimental evidence has been frustratingly inconsistent. Contrary to earlier contentions that this inconsistency is because reciprocity in non‐human species is a rare or fragile effect, recent authors have argued that the evidence suggests that reciprocity is widespread, that it often relies on cognitive mechanisms that are common across species, and is potentially an important factor in animals' daily lives. Another possible explanation for its apparent rarity, then, is that due to experimental studies' (intentionally) structured environment, they can lack the appropriate context to promote and support reciprocity. Focusing on outcomes from experimental reciprocal tasks in non‐human primates, I consider several factors that may be important, including the identity of the interactors and their relationship to one another, whether there is free choice of partners, whether the individuals are interacting directly, the timing of the interaction, the commodity involved, whether individuals have a reason to reciprocate, and the equity of the interaction. Clarifying the role of each of these factors will help improve experimental tasks and the social and ecological contexts that promote reciprocity. 

ABSTRACT Social norms – rules governing which behaviours are deemed appropriate or inappropriate within a given community – are typically taken to be uniquely human. Recently, this position has been challenged by a number of philosophers, cognitive scientists, and ethologists, who have suggested that social norms may also be found in certain non‐human animal communities. Such claims have elicited considerable scepticism from norm cognition researchers, who doubt that any non‐human animals possess the psychological capacities necessary for normative cognition. However, there is little agreement among these researchers about what these psychological prerequisites are. This makes empirical study of animal social norms difficult, since it is not clear what we are looking for and thus what should count as behavioural evidence for the presence (or absence) of social norms in animals. To break this impasse, we offer an approach that moves beyond contested psychological criteria for social norms. This approach is inspired by the animal culture research program, which has made a similar shift away from heavily psychological definitions of ‘culture’ to become organised around a cluster of more empirically tractable concepts of culture. Here, we propose an analogous set of constructs built around the core notion of anormative regularity, which we define asa socially maintained pattern of behavioural conformity within a community. We suggest methods for studying potential normative regularities in wild and captive primates. We also discuss the broader scientific and philosophical implications of this research program with respect to questions of human uniqueness, animal welfare and conservation. 

Abstract Observed behavior can be the result of complex cognitive processes that are influenced by environmental factors, physiological process, and situational features. Pressure, a feature of a situation in which an individual’s outcome is impacted by his or her own ability to perform, has been traditionally treated as a human-specific phenomenon and only recently have pressure-related deficits been considered in relation to other species. However, there are strong similarities in biological and cognitive systems among mammals (and beyond), and high-pressure situations are at least theoretically common in the wild. We hypothesize that other species are sensitive to pressure and that we can learn about the evolutionary trajectory of pressure responses by manipulating pressure experimentally in these other species. Recent literature indicates that, as in humans, pressure influences responses in non-human primates, with either deficits in ability to perform (“choking”) or an ability to thrive when the stakes are high. Here, we synthesize the work to date on performance under pressure in humans and how hormones might be related to individual differences in responses. Then, we discuss why we would expect to see similar effects of pressure in non-humans and highlight the existing evidence for how other species respond. We argue that evidence suggests that other species respond to high-pressure contexts in similar ways as humans, and that responses to pressure are a critical missing piece of our understanding of cognition in human and non-human animals. Understanding pressure’s effects could provide insight into individual variation in decision-making in comparative cognition and the evolution of human decision-making. 

Delayed matching-to-sample (DMTS) tasks are commonly used in the field of comparative cognition to study memory, including working memory. However, specific task demands vary across studies and species, and as such, DMTS tasks may engage different memory systems when features such as the available stimulus pool differ. Further, individual or species-wide differences in response to pressure to perform may increase variation within a species. We explored how task features, memory systems, and pressure interact in tufted capuchin monkeys (Sapajus [Cebus] apella) to influence performance on a DTMS task by varying the size of the possible stimulus pool across testing blocks. We also varied the amount of pressure within a testing block by training monkeys to associate a background color change with a more difficult, but more highly rewarded, trial, as we had done in previous work. In accordance with previous literature (Basile & Hampton, 2013), we found that performance greatly decreased when the possible stimulus pool was limited as compared to a large possible stimulus pool, likely because monkeys could not rely on passive familiarity memory to complete the task. However, we found no overall species tendency to fail under pressure in either the limited- or large-set conditions; instead, we found a surprising tendency to thrive under higher pressure. Taken together, our results further highlight the importance of considering DMTS task features when studying specific memory systems in non-human species and suggest that the DTMS task might not be the best paradigm for testing pressure effects without consideration of individual differences. 

A key goal of the field of endocrinology has been to understand the hormonal mechanisms that drive social behavior and influence reactions to others, such as oxytocin. However, it has sometimes been challenging to understand which aspects and influences of hormonal action are conserved and common among mammalian species, and which effects differ based on features of these species, such as social system. This challenge has been exacerbated by a focus on a relatively small number of traditional model species. In this review, we first demonstrate the benefits of using non-traditional models for the study of hormones, with a focus on oxytocin as a case study in adding species with diverse social systems. We then expand our discussion to explore differing effects of oxytocin (and its response to behavior) within a species, with a particular focus on relationship context and social environment among primate species. Finally, we suggest key areas for future exploration of oxytocin’s action centrally and peripherally, and how non-traditional models can be an important resource for understanding the breadth of oxytocin’s potential effects. 

The origins of evolutionary games are rooted in both economics and animal behaviour, but economics has, until recently, focused primarily on humans. Although historically, specific games were used in targeted circumstances with non-human species (i.e. the Prisoner's Dilemma), experimental economics has been increasingly recognized as a valuable method for directly comparing both the outcomes of economic decisions and their underlying mechanisms across species, particularly in comparison with humans, thanks to the structured procedures that allow for them to be instantiated across a variety of animals. So far, results in non-human primates suggest that even when outcomes are shared, underlying proximate mechanisms can vary substantially. Intriguingly, in some contexts non-human primates more easily find a Nash equilibrium than do humans, possibly owing to their greater willingness to explore the parameter space, but humans excel at more complex outcomes, such as alternating between two Nash equilibria, even when deprived of language or instruction, suggesting potential mechanisms that humans have evolved to allow us to solve complex social problems. We consider what these results suggest about the evolution of economic decision-making and suggest future directions, in particular the need to expand taxonomic diversity, to expand this promising approach. This article is part of the theme issue ‘Half a century of evolutionary games: a synthesis of theory, application and future directions'.