Biotic and abiotic factors can both impact communication. In many species, accurate communication is crucial: it can increase potential mating opportunities, the probability of escaping from a predator, and the speed of social learning. This article is part of the theme issue ‘Voice modulation: from origin and mechanism to social impact (Part I)’. Our data represent a relatively rare case in mammalian neonates, and have implications for the evolution of vocal plasticity and vocal learning across species, including humans. This lowering of f 0 is unusual, as most animals commonly display no such f 0 shift.

Our relatively low noise levels may have favoured f 0 modulation while inhibiting amplitude adjustments. However, one seal showed two characteristics of the Lombard effect known for human speech in noise: significant increase in call amplitude and flattening of spectral tilt. Noise did not seem to affect call amplitude. In addition, higher levels of noise induced less dispersion around the mean f 0, suggesting that pups may have actively focused their phonatory efforts to target lower frequencies. This modulation was precise and adapted to the particular experimental manipulation of the noise condition. Unlike most mammals, pups modified their vocalizations by lowering their f 0 in response to increased noise. We exposed individual pups to low- and high-intensity bandpass-filtered noise, which spanned-and masked-their typical range of f 0 simultaneously, we recorded pups' spontaneous calls. We tailored noise playbacks to this species and age to induce seal pups to shift their fundamental frequency ( f 0), rather than adapt call amplitude or temporal characteristics.

To evaluate early mammalian vocal development, we tested 1–3 weeks-old seal pups. Here, we investigate the vocal plasticity of harbour seals ( Phoca vitulina), a species with vocal learning abilities observed in adulthood but not puppyhood. Pinnipeds are one of the few mammalian groups capable of vocal learning, and are therefore relevant to understanding the evolution of vocal plasticity in humans and other animals. Vocal plasticity can occur in response to environmental and biological factors, including conspecifics' vocalizations and noise.

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