INTRODUCTION

Vocal learning involves the ability to modify and acquire new signals in an organism's vocal repertoire through the use of auditory information and feedback. Humans and many avian species, particularly songbirds, have demonstrated similarities and analogous patterns in the vocal acquisition of their respective repertoires. These similarities include the importance of auditory input, feedback, and social influences on vocal structure and acquisition, and stages of developmental overproduction, selective attrition, and vocal babbling/subsong (for reviews, see Kroodsma 1982; Pepperberg & Neapolitan 1988; Locke 1990, 1993a,b). Finding such parallels in phylogenetically distinct species is striking and suggests a convergence in strategies of vocal learning.

Evidence for vocal learning in other species is rare. Studies of vocal learning in nonhuman primates have suggested that learning plays a role in vocal development of contextual use and comprehension (Seyfarth et al. 1980; Cheney & Seyfarth 1982; Seyfarth 1986; Hauser 1988; Gouzoules & Gouzoules 1989) but clear evidence for the learning of vocal repertoires by nonhuman primates has been slow to emerge. However, recent results of studies of nonhuman primates (Elowson & Snowdon 1994; Snowdon et al., Chapter 12; Mitani & Brandt 1994) and birds (Brown & Farabaugh, Chapter 7) suggest greater vocal plasticity than was previously described and point to an importance of social factors on vocal structure and acoustic variability of calls. Therefore, to more clearly elucidate the phenomenon of vocal learning it is important to make a distinction between vocal learning (the ability to acquire new elements in one's vocal repertoire) and vocal plasticity (the ability to modify signal structure due to social or environmental conditions).