This chapter reviews current topics within the subfield of perception with an emphasis on nonhuman primates. We review the psychophysical approach to the study of perception and misperception, including its application to the study of visual illusions and perceptual completion. Geometric illusions emerge when a target stimulus is embedded in an illusory-inducing context and include size illusions such as the Ebbinghaus–Titchener and Delboeuf illusions as well as line-length illusions such as the Ponzo illusion and Müller–Lyer illusion. We review differential perception of these illusions by primates and other species to understand better the role of perceptual processing mode and experimental design in the emergence of illusory experiences. Additionally, perceptual completion has contributed to our knowledge of the mechanisms underlying primate perception and includes studies on amodal completion and illusory contours. Current topics concerning these areas of research are emphasized, such as grouping mechanisms and other mechanisms of perceptual processing.

Although commonly treated as two separate areas of study in primate cognition, inferential reasoning and problem solving share two key features. They involve going “beyond the information given,” and they compete with associative accounts to explain observable behavior. Despite these commonalities, the study of inferential reasoning and problem solving differ in non-trivial ways from both a methodological and conceptual perspective. They use different setups and use different concepts to investigate how individuals innovate when faced with novel challenges. However, these differences, I will argue, are far less substantial than their commonalities, especially when contrasted with competing frameworks such as associative or perceptual-based accounts of behavior. In this chapter, I will review some of the most relevant empirical studies in primates on inferential reasoning and problem solving. In general, studies on inferential reasoning entail choosing from two or more alternatives to locate a hidden food item (e.g., object permanence) whereas problem-solving studies require individuals to overcome some obstacle that is blocking their access to a visible food item (e.g., tool use). I will then attempt to synthesize this information to extract the key theoretical constructs, paying particular attention to the commonalities and differences between them. Finally, I will contrast the “inferential” approach to other competing approaches (associative, perceptual) in an attempt to strengthen the ties between inferential reasoning and problem solving and propose ways to foster progress in the coming years.

Visual information is important for many aspects of primate social life, including social learning, social relationships, and mate choice. Analyzing the attentional patterns of primates can provide key insights into the mechanisms underlying social interactions. Historically, primate visual attention was studied using live or videotaped looking-time paradigms, potentially prone to human error and providing only rough measures of attentional preferences. However, the application of advanced non-invasive eye-tracking methods is now gaining traction in nonhuman primates. This technology opens doors for conducting novel comparative social cognition research with greater precision than ever before, and allows us to better explore social cognition within and across species. In this chapter, we provide a brief review of previous studies of visual attention both in the field and the laboratory. We then examine ways that eye tracking has elucidated social cognitive processes in primates, with a focus on a comparative social memory paradigm used in human infants, gorillas, chimpanzees, and capuchins. We conclude by highlighting several fruitful directions for future comparative research.

Whether great apes possess the capacity to acquire elements of human language is an enduring scientific question. Over the last 50 years, results from laboratories using either American Sign Language or an artificial symbol-based communication system suggested that core capacities for language acquisition and comprehension are present in apes. After the completion of these projects, newer approaches examining properties of great ape vocalizations and referential gestures have taken up the question. Results from ape language research challenge the claim that human language is a uniquely derived evolutionary specialization, but we are far from reaching consensus on this point. Through these language studies, apes have demonstrated socio-cognitive abilities crucial for the development of language skills, such as joint attention, intersubjectivity, and processing abilities that include referentiality and use of top-down processing for speech restoration. In this chapter, we review ape language projects, the additional work they inspired, and how the results of these studies offer insight into the evolution of language-related cognitive capabilities. We also discuss the effects of enculturation on language acquisition and ethical quandaries that stem from raising apes in homes and laboratories to test hypotheses about the evolution of cognition and language.

We adapt a method from mammalian comparative biology to study spatial cognition in relation to lifestyle. We compare members of the family Pitheciidae (titi monkeys, sakis, bearded sakis, and uakaris) to one another and to two cebid relatives, squirrel monkeys and owl monkeys. We review experimental studies that directly compare titis and squirrel monkeys in spatial tasks and social settings. Titis occupy small, defended home ranges and live in small groups comprising an adult male-female pair and young. In contrast, bearded sakis, uakaris, and squirrel monkeys, occupy large, undefended home ranges, move rapidly, and live in large groups. White-faced sakis illustrate an intermediate condition. Lab studies show that titis and squirrel monkeys differ in their use of visual information in travel tasks and in responsiveness to environmental novelty. Proximate sources of titis’ cautious, sedentary lifestyle include attention to contextual detail, preferences for familiar pathways and areas, behavioral inhibition, parasympathetic dominance, and adult heterosexual attachment bonds. A speculative scenario for the evolution of titis within the Pitheciidae is offered, in which spatial cognition is included as a factor. Further potential applications of this approach within the primate order are considered. We maintain that a holistic, biological, and evolutionary methodology is most likely to elucidate the underpinnings and form of complex cognition.

The intentional communication of affective states is a central part of human sociality and cognition. Although nonhuman primates (henceforth primates) also signal intentionally, there is a perceived chasm between their intentional versus affective forms of communication. Whereas primate vocalizations and facial expressions are traditionally viewed as involuntary “read-outs” of affective states, gestures are considered as products of intentional control. However, this traditional view is increasingly contentious, given recent evidence of intentional signal production of primate vocalizations and facial expressions, as well as the general void of arousal-based explanations in gesture research. In this chapter, we challenge the perceived dichotomy between affective and intentional communication in primates and propose a dimensional approach, whereby primate signals can be both affective and intentional, regardless of signal modality (tactile, audible, visible) or component (gesture, facial expression, vocalization). We argue that a dimensional approach, which incorporates both affective and intentional components, would improve our knowledge on how affective and cognitive processes have jointly shaped the evolution of primate communication.

Traditionally, primate cognition research has been conducted by independent teams on small populations of a few species. Such limited variation and small sample sizes pose problems that prevent us from reconstructing the evolutionary history of primate cognition. In this chapter, we discuss how large-scale collaboration, a research model successfully implemented in other fields, makes it possible to obtain the large and diverse datasets needed to conduct robust comparative analysis of primate cognitive abilities. We discuss the advantages and challenges of large-scale collaborations and argue for the need for more open science practices in the field. We describe these collaborative projects in psychology and primatology and introduce ManyPrimates as the first, successful collaboration that has established an infrastructure for large-scale, inclusive research in primate cognition. Considering examples of large-scale collaborations both in primatology and psychology, we conclude that this type of research model is feasible and has the potential to address otherwise unattainable questions in primate cognition.

How is number represented without language? Research on numerical cognition in humans and nonhuman primates suggests two possible shared systems of processing. The most widely demonstrated system comes from research with humans, nonhuman primates, and a wide variety of other species. This system, known as the approximate number system, is characterized by ratio-dependent representation of a wide range of numerical values. A smaller body of research points to a second system, the object file system, which represents small numerical values with precision. In addition to reviewing this literature, we will also attend to the role of training in numerical discrimination, the use of number when other continuous variables are present, and the arithmetic abilities of nonhuman primates.

There are surprisingly few experimental studies directly comparing the cognition of primate species representing distinct phylogenetic groupings, specialized foraging ecologies, or unique social structures. Although researchers have focused on the role of foraging and social ecology in predicting cognition, they have examined social and foraging strategies in a nuanced fashion that would permit an understanding of how specific aspects of a species’ natural environment might sculpt the evolution of specific forms of cognition. In the absence of such studies, and a clear consensus as to whether cognition should best be viewed as domain-general or domain-specific suites of abilities, it is challenging to draw conclusions as to (1) cognitive differences between primate families or (2) selection pressures responsible for shaping differences. We conclude, based on paltry but accumulating evidence, that there is little utility in postulating separate physical and social domains. In addition, we see little evidence that group-living species are cognitively advantaged compared to primates that exhibit other social structures. Lastly, we advocate for greater attention to reproductive and parental strategies and individual differences in ontogenetic experiences that may color species-level comparisons.

As the breadth and scope of primate cognition research continues to evolve, it remains essential that the ethical considerations of such work do so as well. The evaluation of ethics is shaped by time and place and centers on a variety of factors, including the questions being asked, the methods used, the setting, and the species studied. Here, we take a pragmatic approach in examining ethical considerations as they relate to cognitive research with primates in both captive and wild settings. We encourage primatologists to consider how primates’ lives are impacted prior to, during, and following the research. In addition, we highlight the importance of considering how such research activities interface with the people who work or live alongside the primates. Thus, we aim to help guide those studying and working with primates to plan and conduct ethically sound research.

Categorization – assimilating objects to psychological equivalence classes – is a crucial cognitive capacity that has always enhanced vertebrate fitness. This chapter reviews from a primate perspective the state of knowledge in comparative categorization’s subdomains: prototypes, exemplars, rules, and abstractions. Primate studies have made a profound contribution to the prototype-exemplar debate – essentially resolving it. They have illuminated the evolutionary emergence of a cognitive capacity for category rules, illuminating also the emergence of humans’ explicit-declarative cognition. In this area, primates appear as a pivotal transitional form. In the literature on abstract concepts (e.g., Same-Different), primate studies highlight the differences in cognitive capacities across vertebrate lines. The review will demonstrate the crucial role of a fitness/ecological perspective in understanding categorization as an adaptive, information-processing capability. It will raise important questions about the similarity structure of natural (and unnatural) kinds and categories. It will show strong continuities between human and animal cognition, but important discontinuities as well. In all the subdomains, the primates have been extraordinary behavioral ambassadors to the broader field of categorization.

Although cooperation is widespread from amoebas to humans, the underlying mechanisms are still not well understood, which precludes a full understanding of how cooperation evolved, particularly the complex forms seen in both nonhuman and human primates. The diversity of forms and expressions of cooperation seen across species complicates this, a challenge that has been addressed empirically with studies of cooperation into the lab, where similar methods can be used across species, allowing us to determine what mechanisms are, or are not, shared across species. In the case of cooperation, these methods include joint-action tasks (such as the cooperative barpull) and economic games. With data from standardized lab tests, we can make predictions about how each species should respond in more species-typical, natural contexts. This process allows us to understand not only when mechanisms are shared that might not be obvious (i.e., because they manifest in different ways), but when similar outcomes are underpinned by dissimilar mechanisms. For instance, many primates coordinate, but results from economic games suggest that they do so using a variety of different mechanisms. In addition, we can use these results to identify situations in which cognitive abilities are present, but may not manifest, and to look for the environmental pressures that may inhibit their expression. For example, chimpanzees show evidence of many of the mechanisms necessary for trade and barter, but they do not manifest in all contexts, possibly due to the absence of third-party enforcement mechanisms. Ultimately, understanding cooperation requires recognizing the interplay between cognitive mechanisms and ecology, such that we identify not only how and in what contexts other primates cooperate, but also those situations in which primates do not cooperate, but might be expected to. In so doing, we also move closer to understanding both how humans cooperate, and why it sometimes breaks down so spectacularly.

Metacognition, or awareness of one’s cognition, involves several different but overlapping cognitive abilities, such as working memory, explicit memory, monitoring, and control. These processes are guided by multiple internal or external signals, including memory signals in the case of metamemory. Primate species including apes and rhesus monkeys have demonstrated that they can respond to both internal and external signals, and like humans, these signals can be additive and fallible. In the past few decades, there have been about five dozen studies published on nonhuman animal metacognition and while robust results have been obtained, rigorous experimental paradigms have been employed, and general progress has been made, there is a still a lot we do not know. For example, there are only a few species whose metacognitive abilities have been relatively well characterized, and even in those species significant open questions remain. After an introduction, what is known and what is not known will be explored. Similarities and differences among different primate species will be highlighted. As the chapter is comparative in nature, disparities in behavioral findings across apes and monkeys, New World and Old World monkeys, as well as primates and non-primate species will be explored. The extent to which methods can or cannot be standardized across species will be discussed, with special consideration of species’ ecological niches and experimental methods typically employed. Limitations in nonhuman metacognition research will also be considered, including the fact that most metacognition studies focus on just one species. Finally, possibilities for promising future directions in research will be offered.