Vocal production learning in mammals revisited

Abstract

Vocal production learning, the ability to modify the structure of vocalizations as a result of hearing those of others, has been studied extensively in birds but less attention has been given to its occurrence in mammals. We summarize the available evidence for vocal learning in mammals from the last 25 years, updating earlier reviews on the subject. The clearest evidence comes from cetaceans, pinnipeds, elephants and bats where species have been found to copy artificial or human language sounds, or match acoustic models of different sound types. Vocal convergence, in which parameter adjustments within one sound type result in similarities between individuals, occurs in a wider range of mammalian orders with additional evidence from primates, mole-rats, goats and mice. Currently, the underlying mechanisms for convergence are unclear with vocal production learning but also usage learning or matching physiological states being possible explanations. For experimental studies, we highlight the importance of quantitative comparisons of seemingly learned sounds with vocal repertoires before learning started or with species repertoires to confirm novelty. Further studies on the mammalian orders presented here as well as others are needed to explore learning skills and limitations in greater detail. This article is part of the theme issue 'Vocal learning in animals and humans'.

Keywords: Cetacea; Chiroptera; Pinnipedia; elephants; primates; vocal communication.

Figure 1.

Different forms of vocal production learning. Vocal production learning is not a dichotomous trait but arranged on a continuum [13]. Manifestations of vocal production learning range from subtle modifications of existing call or song types to the imitation of vocalizations of other species or novel sounds. Sketches provide graphic references to mammalian vocal production learners covered in our review, and their position on the continuum represents our evaluation of their vocal production learning abilities. Three domains of vocal production learning (respiratory, phonatory and filter learning), their association with the sound producing apparatus, and the resulting signal characteristics are depicted as well. Most of the examples covered in our review concern phonatory learning. Note that different mammals can have vastly different sound production mechanisms (the human apparatus serves as a familiar example). Figure modified with permission from Scharff et al. [14]. (Online version in colour.)

Figure 2.

Waveforms and spectrograms of vocalizations produced by species with different capacities for vocal production learning. Call types from three taxa were selected to illustrate the degree of structural variability of acoustic signals covered in this review. (a) Strikingly different signature whistles from two common bottlenose dolphins, Tursiops truncatus. (b) Moderately different isolation calls from two greater sac-winged bats, Saccopteryx bilineata. (c) Subtly different grunts from two male Guinea baboons, Papio papio (courtesy of J. Fischer). Note that different taxa have different sound production mechanisms. Spectrograms were generated with a 1024-point FFT and a Hamming Window with 75% (b) or 87.5% (a,c) overlap, a sampling rate of 80 (a), 300 (b) or 41 (c) kHz and a resolution of 16 bits.