INTRODUCTION

This chapter considers cognition in great apes as integrated systems that orchestrate the many abilities that great apes express, systems for which satisfactory characterizations remain elusive. In part, difficulties owe to research trends. Empirical studies have been guided by diverse and sometimes contradictory models, questions, measures, tasks, and living conditions. Performance levels have proven inconsistent across individuals, rearing conditions, and testing conditions, and evidence is patchy across species for virtually any facet of cognition. Evidence on wild great apes, the most important from an evolutionary perspective, is especially patchy because research has favored captives; much of what is available was collected for other purposes, so it was neither described nor analyzed with cognition in mind. The issues at stake are also hard–felt ones that touch on the human–nonhuman boundary, so entrenched beliefs infect how the literature is interpreted and even what of it is read.

Attempts have none the less been made to develop an integrated model of great ape cognition using available evidence. They include both edited survey volumes (Matsuzawa 2001a; Parker, Mitchell & Miles 1999; Russon, Bard & Parker 1996) and integrative reviews, three of the latter as major books (Byrne 1995 (RWB), Parker & McKinney 1999 (P&M); Tomasello & Call 1997 (T&C)) and others as articles (e.g., Byrne 1997; Suddendorf & Whiten 2001; Thompson & Oden 2000; Whiten & Byrne 1991). My aim is not to analyze this terrain, yet again, in detail, but to offer a compact mise à date to ground evolutionary reconstruction.

INTRODUCTION

This chapter will focus on aspects of great ape gestural communication that have implications for cognition and human language. From an evolutionary perspective, gestures may provide an important link across primate species in communicative systems. As a researcher interested in the development of language in human infants, and its precursors, I will stress similar developments, or their absence, in great apes. These developments will include communicative gestures, symbolic gestures, sign acquisition, and sign combinations.

Gesture has a very broad usage; here it will be restricted to movements of the hand, arm, head, and body with communicative functions. Postural and tactile gestures, as well as facial expressions, although often communicative, are not a focus because their implications for language are unclear. It must be kept in mind, however, that visual communication is a complex, redundant system in which these various components are usually combined (Marler 1965).

I will begin with a brief review of communicative gestures according to their functions and then focus on specific gestures involved in exchange, requests involving cognizance of agency, and pointing, as well as on the degree to which great ape gestures are intentional and inventive. These are the kinds of gestures and their characteristics that are related to the development of language in human infants. I then discuss the extent to which apes display symbolic gestures, both in captivity and in the wild, because such gestures in human infants emerge during the transition to language.

INTRODUCTION

Is the intelligence of monkeys and apes primarily a “Machiavellian” adaptation for social life, an ecological adaptation for resource exploitation, or both a social and an ecological adaptation? How can we evaluate these competing models of intelligent adaptation? Are great apes really smarter than monkeys? How can we assess the cognitive complexity of social life? Are the social organizations and/or social roles of great apes more complex than those of monkeys? If they are, do different developmental and acquisition processes underlie that greater complexity, and which models are useful for studying those processes? Similarly, how is socioecological knowledge distributed and transmitted from one generation to the next, and what models are useful for studying this process? All these questions plague efforts to characterize great ape cognition and to reconstruct its evolution.

Systematic evaluation of social and/or ecological hypotheses about primate cognition calls for an integrated framework for comparing the sophistication of social organization and socialization in wild primates. Despite keen interest in specific topics in social cognition (e.g., social learning, social communication), a comprehensive ecologically based framework for comparing primate social cognition is currently lacking.

In this chapter, I propose such a framework. Specifically, this comparison can be made in terms of (1) the number, function, and composition of activity subgroups within primate range groups; (2) the social roles the members of these activity subgroups play; and (3) the typical routines or scripts in which these roles are played.

INTRODUCTION

Unlike the majority of the larger mammals, which are terrestrial herbivores, omnivores, or insectivores, nonhuman primates have created unique niches as arboreal insectivores, frugivores, or folivores. Primates now play important roles as fruit consumers and seed dispersers in tropical forests (Gautier–Hion et al. 1985; Terborgh 1986). However, food is still the primary limiting factor of primate populations because of its sparse distribution, physical protection (hard shells, spines, etc.), and toxic secondary compounds (Feeny 1976; Freeland & Janzen 1974; Milton 1984). Primates have evolved different strategies to cope with these dietary difficulties, and their specializations have influenced both anatomy and behavior.

Primates have evolved various features of gastrointestinal anatomy and the digestive system to cope with such dietary constraints. Leaves in particular are high in structural carbohydrates and are difficult to digest. Folivorous primates need more time to digest and absorb important food components to satisfy nutritional requirements. Specialization in gut morphology has raised the capacity of some primates to consume structural carbohydrates and detoxify secondary compounds (Kay & Davies 1994; Milton 1986). For example, the Colobinae have evolved a sacculated fermenting chamber in the stomach in which microbial fermentation precedes digestion and absorption (Bauchop & Martucci 1968; Chivers & Hladik 1980). Some secondary compounds are degraded during fermentation in the alkaline stomach environment before absorption. As an alternative strategy, a number of more folivorous primates, including howler monkeys, gorillas, bamboo lemurs, and sportive lemurs, have evolved an enlarged caecum or colon in which bacterial fermentation is activated (Stevens & Hume 1995).

ENVIRONMENTS OF NATURAL SELECTION

Understanding the evolution of great ape cognition depends on identifying past adaptive settings and the factors that influenced early ape cognitive responses. Reconstructing past environments is not sufficient for developing and testing evolutionary arguments. It is the sequence of selective environments in which ancestral apes lived that is essential to determining how great ape mental abilities evolved. This requires us to assess the ways in which environmental settings (i.e., specific habitat reconstructions), trends (e.g., cooling, drying), and variability (e.g., seasonality and long-term oscillation) affected the resources and survival regimes of ancestral great apes – and thus posed adaptive problems in the places where they lived and the time periods when they evolved. This chapter investigates the evolutionary adaptability of great apes in light of local, regional, and global paleoenvironments; the geographic patterns of ape evolution; and the cognitive, social, environmental, and dietary characteristics of living great apes.

PRIOR HYPOTHESES OF GREAT APE COGNITIVE EVOLUTION

Although attempts to define unique aspects of great ape cognition have generated much debate (e.g., Tomasello & Call 1997), great apes appear to have achieved levels of cognitive sophistication and flexibility unknown in other nonhuman primates.