Primate communication tends to be divided into four different modalities of behavioural communication – olfactory communication, gesture, facial expression and vocalization – which span four different sensory channels (olfaction, vision, touch and audition). We use the term modality to refer to each type of behavioural communication in this way, but others have used modality to refer specifically to the sensory mode of the stimulus (e.g. Partan and Marler, 1999). We use our broader definition of modality for three reasons. First, one of our aims is to review and examine the classic scientific distinctions made between communicative domains. Second, some types of communication can use more than one sense: for example, gesture can be visual, tactile or auditory. Third, different cognitive mechanisms are thought to underlie both the production and perception of facial expressions, vocalizations, manual gestures and olfactory signals, and so treating them as different modes might be most productive for our purpose.

The aim of this chapter is not to provide a review of what is known about each modality, but instead to give an overview of the methods and general approaches employed. What follows is an outline of the scientific questions, methods for data collection and approaches to analysis within each area of primate communication research. For each modality, there is a section that refers to the brain mechanisms underlying its production and perception. However, it is important to emphasize that the focus here is on the main methods used to study the neural correlates of primate communication, while Chapter 3 discusses the corresponding brain areas and neural circuits in more detail. Importantly, it will soon become clear that the methods (and perhaps more importantly, the scientific questions) can differ between modalities.

The previous chapter examined the generation of new signals, the modification of the structure of existing signals and acquisition of signals through development. This chapter will build on this previous discussion of ontogenetic flexibility and examine the extent to which existing signals can be used in different ways, combined into sequences and comprehended flexibly.

Why is it interesting to investigate flexibility?

Many very effective communicative displays in the animal kingdom are involuntary, inflexible, reflexive responses to specific stimuli. For instance, in response to a rapidly advancing predator, a moth will rapidly open its bottom wings to reveal two large, intimidating eye spots. This display is successful in deterring predators, but in the presence of the correct category of stimuli (any rapidly advancing object in this case) the moth will invariably give this response. Such automatic, reflexive, stimulus-response behaviour is very effective in many instances, but it is unlikely to require complex cognition and thus it tells us very little about the mental capacities of the animal producing the display. For instance, human pupils tend to dilate in response to attractive potential mates and this in turn makes the producer of this display appear more attractive: it is an effective display, but one not under the conscious or voluntary control of the producer. If looking for cognitively complex communication, such displays are not particularly informative. In contrast, in a flexible communication system where there is not a one-to-one correspondence between the stimulus and response, it is likely that more complex cognition is required to operate the system. Although it is difficult to elucidate the cognitive mechanisms underlying successful communication, attribution of cognitive processes can sometimes be the most parsimonious explanation for behaviour. Thus, where multiple signals and responses can be produced, it may be more parsimonious to assume that cognitive processes underlie the system, rather than a large number of automated responses. For these reasons, flexibility is often seen as a key hallmark of cognitive complexity in a communicative system.

What is reference?

Humans are very adept at referring to events and objects in the external world, using both gestural and linguistic signals. A large number of human words are referential: they refer to specific entities in the world. In linguistic terms these words ‘stand for’ the external referent and the relation between the physical form of the word and the referent is arbitrary. In language there is usually a one-to-one mapping between a word and its culturally agreed referent(s). Although the referential specificity of words varies greatly (e.g. the number of referents associated with the sign can be small (e.g. banana cake) or large (e.g. food)), the referential meaning is stable and shared between speakers and listeners. In contrast to this, gestural reference, in the form of human pointing, has no one-to-one referential meaning and can only be successfully interpreted by integrating the signal with the shared common ground between signaller and receiver (Liebal et al., 2009; Tomasello, 2008; Tomasello, Carpenter and Liszkowski, 2007). Points direct the attention of the receiver spatially to a location in the immediate perceptual environment, but the referent and meaning of the point can only be decoded by understanding the communicative intent of the signaller. Although this may seem cognitively complex, this ability emerges very early in human development. Humans usually comprehend pointing gestures before their first birthday and start to produce pointing gestures before any spoken words (Bates et al., 1979; Liszkowski et al., 2004; Tomasello, Carpenter and Liszkowski, 2007). Pointing is thought to scaffold the emergence of spoken language in infants. Referential signals in both the linguistic and gestural domains allow humans to direct the attention of others to specific external entities or events and share attention, feelings or thoughts about them, which is fundamental to the complexity of human communication. The extent to which these abilities are uniquely human or shared with other primate species has been a central theme of animal communication research in recent decades.

Scientists who study primate communication, with the goal of understanding human communication, pursue two different, yet related, questions. Some scientists ask phylogenetic evolutionary questions (what was the historical pathway of a specific communicative ability?), and others ask functional evolutionary questions (what were the selection pressures that led to evolutionary changes in this domain?). These two types of question are both necessary to fully understand the evolution of communication, and one is not necessarily more important or useful than the other. Interestingly, integration between these two foci is rare, despite the potential benefits of integration. Integration between phylogenetic and functional questions could be highly informative when considering the evolution of communication, as understanding the reasons for change could help elucidate the specific process of change, and vice versa. Here, we argue that one way to bridge the gap between phylogenetic and functional questions could be to adopt a more multimodal approach to the study of primate communication, which is usually neglected in favour of a unimodal approach.

In this final chapter, we first summarize the general advantages of adopting a multimodal approach (see Chapter 5), regardless of whether the research questions are phylogenetic or functional. Second, we discuss the difference between phylogenetic and functional questions. Finally, we propose that integration between phylogenetic and functional questions would be helpful to move the field forward, and that a multimodal approach could be particularly useful in this endeavour (see also Waller et al., 2013a).

Like most mammals, primates possess the special senses of olfaction, taste (gustation), vision and hearing (audition) as well as the general sense of touch (somatosensation). These senses represent the interface between an animal and its environment. Indeed, the survival and fitness of an animal depend upon its ability to accurately and adequately gather sensory stimuli. Olfaction and taste are grouped together as chemical senses because the stimuli they detect are molecules or compounds that either travel through the air or are settled on a physical substrate. Hearing, vision and somatosensation are grouped together as physical senses because the stimuli they detect are physical forces (e.g. sound waves, light, pressure, temperature). While taste is a critical part of an animal’s ability to judge aspects of its environment (such as food quality), it does not play a direct role in social communication and thus will not be included in the ensuing discussion (Dominy, Ross and Smith, 2004).

While we discuss these senses separately from one another for the benefit of conceptualizing their roles in primate communication, we recognize that there is usually a mix of sensory modes that are locked together in any potential interaction. In the following sections, we describe the morphology of each of these sensory systems, how they function to gather and interpret external stimuli and how they are used in primate communication.