The multi-dimensional nature of vocal learning

Abstract

How learning affects vocalizations is a key question in the study of animal communication and human language. Parallel efforts in birds and humans have taught us much about how vocal learning works on a behavioural and neurobiological level. Subsequent efforts have revealed a variety of cases among mammals in which experience also has a major influence on vocal repertoires. Janik and Slater (Anim. Behav.60, 1-11. (doi:10.1006/anbe.2000.1410)) introduced the distinction between vocal usage and production learning, providing a general framework to categorize how different types of learning influence vocalizations. This idea was built on by Petkov and Jarvis (Front. Evol. Neurosci.4, 12. (doi:10.3389/fnevo.2012.00012)) to emphasize a more continuous distribution between limited and more complex vocal production learners. Yet, with more studies providing empirical data, the limits of the initial frameworks become apparent. We build on these frameworks to refine the categorization of vocal learning in light of advances made since their publication and widespread agreement that vocal learning is not a binary trait. We propose a novel classification system, based on the definitions by Janik and Slater, that deconstructs vocal learning into key dimensions to aid in understanding the mechanisms involved in this complex behaviour. We consider how vocalizations can change without learning, and a usage learning framework that considers context specificity and timing. We identify dimensions of vocal production learning, including the copying of auditory models (convergence/divergence on model sounds, accuracy of copying), the degree of change (type and breadth of learning) and timing (when learning takes place, the length of time it takes and how long it is retained). We consider grey areas of classification and current mechanistic understanding of these behaviours. Our framework identifies research needs and will help to inform neurobiological and evolutionary studies endeavouring to uncover the multi-dimensional nature of vocal learning. This article is part of the theme issue 'Vocal learning in animals and humans'.

Keywords: behaviour; cognition; evolution; language; songbird; vocal learning.

Figure 1.

The multi-dimensional nature of vocal production learning. Vocal production learning is the production of modified or novel vocalizations, as a result of learning from the experience of the acoustic signals of others. In this process, auditory input leads to the formation of a memory, to which the vocal output is compared. Examples of avian and mammalian vocal production learning species are represented across the dimensions of vocal production learning outlined in §4. The Horsfield's bronze cuckoo is also included based on our discussion of its inclusion in the wider definition of vocal production learning (see §5). This is not an exhaustive list, rather a selection of species with shared and contrasting abilities. Categorizations are made for each species using discrete or continuous scales, as indicated in the legend. This schema can be used to take an in depth look at the vocal learning properties of an individual species (by looking down a column) or to compare abilities within a single dimension of vocal learning (by looking along a row). Categorizations are intended to indicate what each species has been observed to be capable of doing, and may not reflect their behaviour in their normal environment, or the full extent of their capacity. For example, Asian elephants have been demonstrated to have a high breadth of learning largely based on an Asian elephant in human care that learned human speech-like sounds [34]. However, this behaviour is not observed in normally reared Asian elephants in the wild. African elephants in human care are reported to imitate the sounds of Asian elephants and of trucks [81], but the breadth of mimicry for copying other sounds in elephants is not known. A question mark (?) indicates where an accurate categorization cannot be made based on current published work and further research is needed. In the case of the Horsfield's bronze cuckoo, learning does not appear to employ auditory input of a model sound and as such, the other categories within that dimension are labelled not applicable (N/A). All classifications are made based on current available knowledge (see reference numbers within circles for evidence used for classifications), and should be updated as new information comes to light.