Vocal plasticity can occur in response to environmental and biological factors, including conspecifics' vocalisations and noise. 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. Here, we investigate the vocal plasticity of harbour seals (Phoca vitulina), a species with vocal learning abilities attested in adulthood but not puppyhood. To zoom into early mammalian vocal development, we tested 1-3 weeks old seal pups. We tailored noise playbacks to this species and age to induce seal pups to shift their fundamental frequency (F0), rather than adapt call amplitude or temporal characteristics. We exposed individual pups to bandpass-filtered noise, which purposely spanned - and masked - their typical range of F0s, and simultaneously recorded pups' spontaneous calls. Seals were able to modify their vocalisations quite unlike most mammals: They lowered their F0 in response to increased noise. This modulation was punctual and adapted to the particular noise condition. In addition, higher noise induced less dispersion around the mean F0, suggesting that pups may have been actively focusing their phonatory efforts to target lower frequencies. Noise masking did not seem to affect call amplitude. 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. Our relatively low noise levels may have favoured F0 shifts while inhibiting amplitude adjustments. This lowering of F0 is quite unusual, as other animals commonly display no F0 shift independently of noise amplitude. Our data represents a relatively rare case in mammalian neonates, and may have implications for the evolution of vocal plasticity across species, including humans.