Quantitative tools for comparing animal communication systems: informationtheory applied to bottlenose dolphin whistle repertoires

Comparative analysis of nonhuman animal communication systems and their com plexity, particularly in comparison to human language, has been generally h ampered by both a lack of sufficiently extensive data sets and appropriate analytic tools. Information theory measures provide an important quantitati ve tool for examining and comparing communication systems across species. I n this paper we use the original application of information theory, that of statistical examination of a communication system's structure and organiza tion. As an example of the utility of information theory to the analysis of animal communication systems, we applied a series of information theory st atistics to a statistically categorized set of bottlenose dolphin, Tursiops truncatus, whistle vocalizations. First, we use the first-order entropic r elation in a Zipf-type diagram (Zipf 1949, Human Behavior and the Principle of least Effort) to illustrate the application of temporal statistics as c omparative indicators of repertoire complexity, and as possible predictive indicators of acquisition/learning in animal,vocal repertoires. Second, we illustrate the need for more extensive temporal data sets when examining th e higher entropic orders, indicative of higher levels of internal informati onal structure, of such vocalizations, which could begin to allow the stati stical reconstruction of repertoire organization. Third,;we propose using ' communication capacity' as a measure of the degree of temporal structure an d complexity of statistical correlation, represented by the values of entro pic order, as an objective tool for interspecies comparison of communicatio n complexity. In doing so, we introduce a new comparative measure, the slop e of Shannon entropies, and illustrate how it potentially can be used to co mpare the organizational complexity of vocal repertoires across a diversity of species. Finally, we illustrate the nature and predictive application o f these higher-order entropies using a preliminary sample of dolphin whistl e vocalizations. The purpose of this preliminary report is to re-examine th e original application of information theory to the field of animal communi cation, illustrate its potential utility as a comparative tool for examinin g the internal informational structure of animal vocal repertoires and thei r development, and discuss its relationship to behavioural ecology and evol utionary theory. (C) 1999 The Association for the Study of Animal Behaviour .