Extreme ecological constraints lead to high degree of individual stereotypy in the vocal repertoire of the Cape fur seal (Arctocephalus pusillus pusillus)

The ability to recognize the identity of conspecifics is a key component for survival of many animal species and is fundamental to social interactions such as parental care, intra-sexual competition or mate recognition. In group-living species, the simultaneous co-existence of many individuals increases the number of interactions and reinforces the need for individual recognition. Acoustic signals are widely used by birds and mammals to communicate and to convey information about identity, but their use in very dense colonies becomes challenging due to the high level of background noise and the high risk of confusion among individuals. The Cape fur seal (CFS) is the most colonial pinniped species and one of the most colonial mammals in the world, with colonies of up to 210,000 individuals during the breeding season. Here, we investigated the individual stereotypy in vocalizations produced by pups, females and male CFS using Random Forests and index of vocal stereotypy (IVS). We thus compared IVS values of CFS to other pinniped species. Within CFS we identified individuality in all call types but the degree of individual stereotypy varies in regard to their social function: affiliative calls produced in a mother-pup reunion context and territorial calls produced by mature bulls holding harem were more individualized than vocalizations involved in agonistic interactions. Our inter-species comparisons among pinnipeds showed that CFS affiliative and territorial calls displayed higher degrees of individuality compared to other species with similar or lower ecological constraints (colony density and social structure). Significance statement Individual recognition is primordial for the establishment of social interactions and notably occurs through acoustic signals in many birds and mammalian species. Understanding the role of social complexity in the development of individual vocal recognition improves our general knowledge on acoustic communication in a social system. This study investigated the vocal stereotypy in the Cape fur seal (CFS), a species that is an extreme illustration of social complexity. The CFS is the most colonial pinniped species, and this results in high selective pressure for individual recognition during mother-pup reunions and male territorial behaviour. By comparing the degree of individuality contained in CFS vocalizations with other pinniped species, we can assess how ecological (environmental and social) constraints drive communication systems in vertebrates.

Automated summary

Individual recognition is crucial for social interactions and often occurs through acoustic signals in birds and mammals. The study on the Cape fur seal (CFS), one of the most colonial pinniped species, showed that affiliative and territorial vocalizations display higher degrees of individuality compared to other pinniped species, possibly due to higher ecological constraints. This research sheds light on how environmental and social factors influence communication systems in vertebrates.