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

The comparative study of communicative behaviour in 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 improved our understanding of 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, along with its potential to shed light on the roots of human language. While the eco-evolutionary relevance of genetic and phenotypic differences between individuals is well established, animal communication studies have traditionally focused on group averages and treated 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), across parameters such as signal forms, repertoires, and usage strategies. In particular, recent evidence from comparative work underscores the potential evolutionary significance of individual plasticity in communicative behaviour. We argue for an explicit focus on within-individual variation and propose advancing the study of animal communication through multi-level approaches that integrate intrinsic and environmental factors, as well as between- and within-individual variation. Such approaches not only refine our view of complexity in animal communication systems and their implications for social evolution, but also help trace the evolutionary trajectory of human language through comparative studies.