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.

Social life demands complex strategies for coordinating and competing with others. In humans, these strategies are supported by rich cognitive mechanisms, such as theory of mind. Theory of mind (i.e., mental state attribution, mentalizing, or mindreading) is the ability to track the unobservable mental states, like desires and beliefs, that guide others’ actions. Deeply social animals, like most nonhuman primates, would surely benefit from the adept capacity to interpret and predict others’ behavior that theory of mind affords. Yet, after forty years of investigation, the extent to which nonhuman primates represent the minds of others remains a topic of contentious debate. In the present chapter, we review evidence consistent with the possibility that monkeys and apes are capable of inferring others’ goals, perceptions, and beliefs. We then evaluate the quality of that evidence and point to the most prominent alternative explanations to be addressed by future research. Finally, we take a more broadly phylogenetic perspective, to identify evolutionary modifications to social cognition that have emerged throughout primate evolutionary history and to consider the selective pressures that may have driven those modifications. Taken together, this approach sheds light on the complex mechanisms that define the social minds of humans and other primates.

The seminal work on mirror self-recognition, theory of mind, and ape-language abilities beginning in the 1960s has stimulated a recent, significant body research on the cognitive abilities of animals. Because of their greater genetic, morphological, and neuroanatomical similarities with humans, research on cognition in nonhuman primates has held a particular fascination from scientific and public perspective. In this chapter, we present a summary of recent studies by our research group on the general intelligence of chimpanzees. We further present data on (1) the contribution of genetic and non-genetic factors in explaining individual variation in cognitive performance in the chimpanzees and (2) phenotypic, genetic, and environmental associations found between chimpanzee cognition and neuroanatomical organization. We end by discussing limitations in the study of cognition and emphasize the need to include individual as well as grouped data in the reporting of results. We also offer some suggestions for future research that would provide new insight into the evolution of human unique cognitive abilities.

Mental time travel involves remembering personal past events (i.e., episodic memory) and thinking about future ones (i.e., future thinking). Despite empirical evidence showing that animals might be capable of mental time travel, some still remain skeptical about this issue. The aim in this chapter will be to reflect on the concept of episodic memory and future thinking as well as on the experimental approaches used in comparative psychology to study these abilities. A critical analysis of both the conceptualization of mental time travel and the experimental paradigms will be provided. I will finish by questioning the extent to which the sense of past has been addressed in this type of research and by suggesting lines of future research.

Replication is an important tool used to test and develop scientific theories. Areas of biomedical and psychological research have experienced a replication crisis, in which many published findings failed to replicate. Following this, many other scientific disciplines have been interested in the robustness of their own findings. This chapter examines replication in primate cognitive studies. First, it discusses the frequency and success of replication studies in primate cognition and explores the challenges researchers face when designing and interpreting replication studies across the wide range of research designs used across the field. Next, it discusses the type of research that can probe the robustness of published findings, especially when replication studies are difficult to perform. The chapter concludes with a discussion of different roles that replication can have in primate cognition research.

The difference between humans and other primates has often been attributed to humans’ unique ability to learn language and more specifically, represent complex sequential and grammatical structures. Even in the case of language-trained apes, the animals are severely limited in how they put together strings of their learned symbols. This led to theories that this limitation is due to animals’ inability to represent the complex sequential and grammatical patterns needed for language. However, work testing the types of sequences and artificial grammars that nonhuman primates can represent has come a long way. Studies have shown that like humans, nonhuman primates can represent adjacent dependencies, ordinal sequences, and algebraic patterns. Until recently, the types of sequential structures attested in nonhuman animals have been limited to these linear sequences. However, recent work has shown that even in some of the most complex forms of grammatical structures, long-distance dependence and recursive sequences are within the limits of the nonhuman mind.

When George John Romanes published his book Animal Intelligence in 1882, marking for many the beginning of the study of comparative cognition, he devoted chapter 17 to the mental life of “Monkeys, Apes, and Baboons.” His subsequent experiments on the mental powers of zoo chimpanzees would be published in Science (Romanes, 1889) and Nature (Romanes, 1890). Around the turn of the last century, Edward Thorndike of Columbia University and A. J. Kinnaman of Clark University conducted laboratory studies on the intelligence of capuchin and rhesus monkeys, respectively. Thus, from psychology’s earliest days as an experimental science, nonhuman primate cognition was a focus of interest and investigation. Many of the best-known and most influential psychologists in history – names like Watson, Yerkes, Köhler, Harlow, Lashley, Hebb, Premack, Rumbaugh, and countless others – made significant contributions to our understanding of learning and cognition through studies of monkeys and apes. Although the history of cognitive research with nonhuman primates is too long and too rich to be reviewed exhaustively in one chapter, the present introduction will highlight significant research themes and important milestones across the history of primate cognition research. The goal of this review is to show how contemporary comparative cognition research is intricately connected to its history, and how our understanding of primate cognition shows cumulative progress across the last 140 years of inquiry.