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.

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.

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.

Dental hard tissues contain periodic incremental markings that can be used as an absolute temporal archive to reconstruct their growth and can incorporate trace elements into their chemical structure during formation that reflect diet, the environment, metabolism, and health. The growth history can be recovered from teeth using virtual tooth histology to resolve a continuous record of trace element composition within each of the mineralized dental tissues, to further our understanding of past life history events in extant and extinct taxa. While acellular cementum forms slowly and regularly and is ideal for recording annual increments, compensatory cellular cementum can mirror this regular growth through gradual physiological changes in anterior tooth inclination. Here, we document this growth through microstructural elemental mapping using non-destructive synchrotron x-ray fluorescence on chimpanzee dental thin sections. We evidence clear seasonal mineral fluctuations, and match the narrower, brighter incremental markings visible in TLM with peaks in mineral concentration in zinc and strontium.

Social learning, a type of information transmission in which individuals gain information by observing or interacting with another animal or the products of another animal’s actions, is an extensively studied subject in a wide array of species. Of particular interest is the ability of chimpanzees (Pan troglodytes) to learn socially, especially given their extensive sociality and fission–fusion dynamics, which provides many opportunities for individuals to learn from each other in different contexts. Using observational and experimental approaches, researchers have explored how faithfully chimpanzees copy others, the type of information conveyed between individuals, and the extent to which social learning is influenced by external factors. In this chapter we review what is currently known about the mechanisms by which chimpanzees socially learn and the strategies they may employ when doing so. We also discuss the much-debated topic of chimpanzee "culture," and how this compares to our own culture. Last, we provide a comparative perspective for social learning in chimpanzees with other species, and discuss how understanding chimpanzee social learning can be useful in their captive care and aiding their conservation in the wild.

Great apes deploy gestures as flexible communicative strategies in many social contexts. For decades, studies on chimpanzee gestures were biased towards captive settings. Despite well-documented intersite variation in chimpanzee behaviour, studies on their gestural communication in the wild have mainly focused on single communities. We investigated gestural usage and development in two chimpanzee communities and subspecies (Pan troglodytes schweinfurthii at Kanyawara, Uganda; P.t. verus at Taï, Côte d’Ivoire). Studies focused on interactions between infants and their social partners in three contexts: joint travel, play solicitation and food sharing. Overall, gestural usage was strongly affected by infants’ age, kin relationship with recipients and previous interactions with non-maternal conspecifics. While repertoires largely overlapped, gesturing rates and intentional signal use seemed to differ between the communities. We speculate that variation in gregariousness and differentiated social relationships could be driving this difference. We discuss our results in light of the social negotiation hypothesis and the relevance of social exposure for communicative development.

A variety of studies have investigated endocrinological aspects of ecology, cooperation and immune system activation in Taï chimpanzees, making use of the ever-growing number of validated biomarkers used on non-invasive samples by the endocrinology laboratory at the MPI EVAN. In particular, the measurement of urinary oxytocin allows for insights into benefits of food-sharing, bonds and the potential physiological mechanism behind cooperation. Measures of urinary cortisol in combination with creatinine and C-peptide allow for the investigation of causes of seasonal variation in stress levels while patterns of immune system activation are monitored by the measurement of urinary neopterin. Future studies will profit from combined analysis of endocrine and immune parameters in relation to behaviour and reproductive success to investigate life-history trade-offs. Across-site comparisons of behaviour and endocrine patterns will help us to understand how variation in ecological and physiological parameters form the social setting of a population which leads to relatively low intergroup hostility, low leverage of males over females and relatively high levels of female sociality at Taï.

After observing wild chimpanzees in Taï National Park, Côte d’Ivoire, for years, Professor Christophe Boesch and collaborators noticed behaviours indicating that the chimpanzees knew where they were heading hours before arrival. One can assume that animals living in such complex environments should possess developed cognitive abilities. To study their intelligence in the wild, we followed adult chimpanzees for long consecutive periods and collected detailed behavioural data including measurements of travel direction and distances, and feeding tree properties. We examined their spatial orientation abilities and found they travelled in a goal-directed manner, using different directions to reach the same feeding tree rather than using the same paths, which was consistent with the use of a Euclidean map. The content of their spatiotemporal memory, and revisiting and ranging behaviour, revealed that chimpanzees remembered the characteristics of the feeding trees and feeding experiences, and that the nutrient content of fruit impacted their travel path. Our investigations revealed that chimpanzees used many food-finding strategies, adding pieces to the puzzle of our closest relatives.

Social network analysis is a widely applied tool to study the social organization of different species. Only few studies have investigated social network changes over time. A previous study of the North Group chimpanzees in Taï National Park indicated network structure changes with decreasing community size. We investigated how two natural ‘knockouts’, i.e. sudden large-scale declines in community size, affected association networks. Network dynamics following the 1992 knockout, during which eight individuals died, showed a delayed response: in the first months, association rates were lower than expected, and only after seven months did they converge to expected community-size–specific level. In contrast, network dynamics following the 1994 knockout, during which 12 individuals died, showed no deviation from expected community-size–specific levels. Thus, chimpanzee association networks can react differently to large-scale declines in community size. Differences might be related to the identity of deceased individuals and knockout amplitude. The patterns of observed changes did not support the idea that large-scale knockouts lead to strong disturbances of chimpanzee communities.

The chimpanzees of the Taï Forest, Côte d’Ivoire, crack highly nutritious Coula edulis nuts using anvils and hammers. While using tools to access encased food items provides obvious benefits, the energetic gain of tool-assisted foraging can be further increased by optimal selection of tools. Previous studies of animal tool selection often relied on implicit assumptions or theoretical arguments about how tool features would influence foraging efficiency, and comprehensive measures of actual efficiency are still missing. We used field observations of nut-cracking efficiency and previously published estimates of energetic costs to investigate the rate of net energy intake as a function of hammer weight and hammer material. While stones allowed for a generally more efficient performance, nut-cracking efficiency depended on an interaction of hammer weight and material. Relative performance of stones and wood varied according to the ripeness of the nuts. Chimpanzees’ tool selection tends to optimize nut-cracking in many respects. Nonetheless, we also observed a few mismatches between efficiency and selection, some of which may be explained on cognitive, motivational or cultural grounds.