Optionality in animal communication: a novel framework for examining the evolution of arbitrariness

Abstract

A critical feature of language is that the form of words need not bear any perceptual similarity to their function - these relationships can be 'arbitrary'. The capacity to process these arbitrary form-function associations facilitates the enormous expressive power of language. However, the evolutionary roots of our capacity for arbitrariness, i.e. the extent to which related abilities may be shared with animals, is largely unexamined. We argue this is due to the challenges of applying such an intrinsically linguistic concept to animal communication, and address this by proposing a novel conceptual framework highlighting a key underpinning of linguistic arbitrariness, which is nevertheless applicable to non-human species. Specifically, we focus on the capacity to associate alternative functions with a signal, or alternative signals with a function, a feature we refer to as optionality. We apply this framework to a broad survey of findings from animal communication studies and identify five key dimensions of communicative optionality: signal production, signal adjustment, signal usage, signal combinatoriality and signal perception. We find that optionality is widespread in non-human animals across each of these dimensions, although only humans demonstrate it in all five. Finally, we discuss the relevance of optionality to behavioural and cognitive domains outside of communication. This investigation provides a powerful new conceptual framework for the cross-species investigation of the origins of arbitrariness, and promises to generate original insights into animal communication and language evolution more generally.

Keywords: animal communication; arbitrariness; language evolution; language origins; optionality.

Fig. 1

Abstract examples of optionality corresponding to each dimension discussed in this review. These examples are not exhaustive, and other ways in which associations between form and function may exist, be created or be altered are discussed herein. Each of these may also be viewed from the perspective of receivers, representing our fifth aspect, ‘signal perception optionality’.

Fig. 2

(A) Spectrum of optionality with examples of each degree. The four degrees highlighted are not intended to be a prescriptive, quantitative evaluation of points on this spectrum but rather an aid for conceptualising our proposed framework. (B) Illustration of five dimensions of communicative optionality represented as separate axis on a radar plot. This plot represents a hypothetical species with a high degree of signal usage, perception and adjustment optionality but low degrees of signal production and combinatorial optionality.