Individual variation in animal communication: from species averages to unique voices

The comparative study of the communicative behaviour of non-human animals, especially primates, has yielded crucial insights into the evolution of human language. This research, mostly focused on the species and population level, has helped to understand the various socio-ecological factors that shape communication systems. However, despite the inherent flexibility of human communication, the impact of individual variation on non-human communication systems has often been overlooked, as have its potential insights into the roots of human language. While the eco-evolutionary relevance of genetic and phenotypic differences between individuals is well established, animal communication studies traditionally focus on group means and treat outliers as noise. In this review, we address this gap by providing a comprehensive overview of the sources of individual variation in animal communicative behaviour (e.g. physiological, sociodemographic or personality traits) in numerous parameters such as signal forms, repertoires, and strategies of use. In particular, recent evidence from comparative work underscores the potential evolutionary implications of individual plasticity in communicative behaviour. Thus, we argue for an explicit focus on within-individual variation and propose a way to advance the study of animal communication through multi-level approaches that consider intrinsic, environmental as well as between- and within-individual variation together. Such approaches not only refine our perception of complexity in animal communication systems and implications for social evolution, but also help to trace the evolutionary trajectory of human language through comparative studies.