Neonatal auditory input affects vocal development in harbour seals

Vocal individuality has important biological functions in mammals: at crucial stages of development, it ensures feeding and is a prerequisite for auditory-based mother-pup recognition. Is vocal individuality only shaped by maturation or also by the degree of conspecific acoustic input? Here, we test how the neonatal auditory environment shapes development and individualization of calls in harbour seal pups (Phoca vitulina). To simulate low- versus high-conspecific acoustic density, 18 pups heard playbacks of calls from either 2 or 30 conspecifics. We recorded calls before and after playback exposure and extracted 12 acoustic parameters. Supervised machine learning and discriminant function analyses showed greater individual distinctiveness in both groups after playback, indicating a developmental trend toward individualization. Notably, pups exposed to less-variable input showed higher individuality. Euclidean distances on call parameters showed that both groups diverged from the playback signals. Distances on within-pup and between-pups housed together revealed opposite trajectories in the two groups: after the exposure, less-variable auditory input determined steadier individual calls, while more-variable auditory input scattered calls across the acoustic space. Altogether, our findings indicate that auditory input modulates vocal development in pups, making harbour seals a promising model for unravelling how neonatal environment affects vocal plasticity in a non-human mammal. This article is part of the theme issue 'Mechanisms of learning from social interaction'.