INTRODUCTION

Darwin first wrote about sexual selection in 1871, but only since the 1970s has it become a topic of intense scientific study. The Göttinger Freilandtage (conference) on sexual selection in primates which resulted in this chapter, was held exactly 125 years after Darwin first published his paper on sexual selection on visual signals in primates (1876). How far have we come in understanding sexual selection of communication signals since Darwin's first introduction of the topic? We have had considerable advances in studying sexually selected vocalisations in anurans (e.g. Ryan & Rand, 1990) and birds (e.g. Searcy & Andersson, 1986); sexually selected visual traits in birds (e.g. Borgia, 1985; Petrie, 1994) and invertebrates (e.g. Wilkinson & Reillo, 1994); as well as chemical signals in invertebrates (e.g. Eisner & Meinwald, 1995). The major focus of most of these studies has been on variation in male traits that provide a basis for female choice.

As I have surveyed the primate literature relating communication signals to sexual selection, my assessment is that we have very little real understanding of the role of sexual selection on primate communication that is comparable to our knowledge of other taxa. I think there are several reasons for this lack of progress since Darwin. First, primatologists have lost track of the steps needed for a truly scientific validation of sexual selection, which often requires controlled experimentation. A clear understanding of sexual selection requires not only attempting to understand ultimate causes, but also close attention to proximate mechanisms.

INTRODUCTION

The study of vocal development in nonhuman animals has focused primarily on vocal production. However, vocal development involves not only production, but development of appropriate usage and appropriate responses to the calls of others. The relative neglect of usage and responses has led to a distorted view of vocal development as emphasized by Seyfarth & Cheney (Chapter 13) for monkeys and West et al. (Chapter 4) for birds. The argument is often made that nonhuman mammals differ fundamentally in vocal development from both birds and human beings but, as we have argued previously (Snowdon & Elowson 1992) and Seyfarth & Cheney (Chapter 13) argue, primates are similar to humans and birds if all three components of vocal development – production, usage and response – are evaluated.

In this chapter we argue that when functionally similar vocalizations are chosen from species that have similar social organization, then similar developmental processes will be found regardless of the taxon studied. We examine the idea that nonhuman primate vocal structures are fixed and review evidence that vocal plasticity may be quite common. We then provide three examples of phenomena from our research with marmosets and tamarins that illustrate how plasticity in pygmy marmoset trill vocalizations can be influenced by changes in social companions, how infant babbling in pygmy marmosets is a social interaction, and how the structure and usage of food-associated calls is acquired by cotton-top tamarins, and how social environments can inhibit the expression of these calls.

In the late 1960s a series of developments in linguistics, developmental psycholinguistics and animal communication led to a convergent model of vocal development in human and nonhuman species. As presented by Lenneberg (1967) and Marler (1970), the development of language and of bird song required exposure to species-specific codes during a sensitive period of development, after which subsequent learning was extremely limited. The amount of input required could be quite small, and this input could be effective regardless of social interactions. Both birds and humans needed intact hearing and an extensive time for practice (babbling for human infants, subsong and plastic song for birds) to acquire adult competence in vocal production. Subsequent to this practice crystallization occurred, and further changes in vocal structure were rare.

This paradigm has led to extremely productive research over the past 25 years, not only in the study of vocal development but also in the understanding of the neurological controls of vocal production. However, as researchers interested in the ontogeny of primate and avian vocal communication, we have become increasingly aware of the need to consider some modifications to this paradigm. Some forms of social stimulation can extend sensitive periods for song learning in birds, and songs and calls in some species can be modified throughout life, often in response to changes in social stimuli. Parrots, dolphins and great apes with exceptional training acquired codes with some similarities to human language. Yet at the same time there was little evidence of vocal plasticity in nonhuman primates, suggesting a gap in continuity of developmental processes in the evolution from birds to humans.