Studies into bird twitter, monkey chat and manspeak

Most of the research on vocal learning in animals has concentrated on how flexible an animal's learning abilities are and whether they are limited to a certain time in its development. Social Influences on Vocal Development presents a slightly different approach by focusing mainly on how social interactions influence what an animal learns and from whom.

In a well-thought-out chapter, Douglas Nelson draws attention to the different forms of learning that can influence communication. He proposes that vocal learning in birds is usually limited to an early period in life, and that later in life birds only select from their large repertoires of songs. This learning of when to use a particular sound is very different from the original learning of how to produce a sound. He also cautions researchers not to over-interpret their data in favour of social influences; advice that not everyone follows in this book. Luis Baptista and Sandra Gaunt review evidence for the importance of social influences on vocal development. In a convincing direct reply to Nelson they also clarify why they think birds still learn to produce new sounds later in life. A further five chapters in this section concentrate on selected bird species, for instance Richard Zann's careful discussion of captive zebra finches compared to birds in the wild, an area usually glossed over.

Experimental work on large mammals is not quite so easy to conduct. Peter Tyack and Laela Sayigh present a valuable review of what is known about dolphin communication. They report a lot of case histories to demonstrate that vocal learning, known to occur in dolphins since the early seventies, can influence the development of individually specific signature whistles.

The apparent lack of vocal learning in non-human primates has puzzled a lot of investigators. Why do our closest relatives not show even one of the most basic prerequisites for spoken language while birds and other mammals show it so clearly?

Charles Snowdon, Margaret Elowson, and Rebecca Roush present a case where a change in the social environment of pygmy marmosets resulted in changes in certain call parameters; but it is not clear whether this is a case of vocal learning or of selecting different calls from a large repertoire as pointed out in Nelson's chapter. While Robert Seyfarth and Dorothy Cheney, in a very good overview of what is known about production, comprehension, and usage learning in monkeys, point out that even cross-fostering experiments have not led to changes in the calls of macaques and that there is little evidence for production learning. However, monkeys show great competence in the usage and comprehension of sounds.

In the final part on humans, the two chapters by John Locke and Catherine Snow and by Susan Goldin-Meadow are particularly fascinating. Locke and Snow stress that vocal accommodation, making one's calls similar to those of other group members, is necessary for group membership in humans and suggest that further studies on vocal learning in non-human primates need to concentrate on affiliative exchanges. Goldin-Meadow, in distinct contrast, demonstrates that specific gestural input is not required for deaf children to develop a gestural communication system. The last two chapters present a long-term study on the vocal development of one twin pair by Annick Jouanjean-L'Anto ne and a comparison of language use in girls and boys between 9 and 14 years of age by Marjorie Harness Goodwin.

A particular strength of this book is the combination of studies on humans and a variety of different animal species. However, the comparisons between groups are often not very critical, especially in the introductory chapter. Sweeping remarks about the lack of differences between non-human primates and humans, birds, and dolphins are not very convincing when there still is no conclusive evidence for production learning in non-human primates. Only when similarities as well as differences between animal taxa are considered can we hope to understand how the environment influences vocal development. One would like to have found a more general discussion of the adaptive value of these forms of learning. Nor was there any mention of quite a few other mammalian species in which production, usage, or comprehension learning of sounds has been demonstrated. But despite these few lapses the book gives a good overview of current opinions in the field to biologists as well as interested non-biologists. Though non-biologists may find some sections very technical, good summaries can be found at the end of each chapter.

Vincent M. Janik is a postdoctoral research fellow, Woods Hole Oceanographic Institute, United States.