Raw data from Vocal plasticity in harbour seal pups

Vocal plasticity can occur in response to environmental and biological factors, including conspecifics' vocalizations 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 observed in adulthood but not puppyhood. To evaluate 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 low- and high-intensity bandpass-filtered noise, which spanned—and masked—their typical range of f0; simultaneously, we recorded pups' spontaneous calls. Unlike most mammals, pups modified their vocalizations by lowering their f0 in response to increased noise. This modulation was precise and adapted to the particular experimental manipulation of the noise condition. In addition, higher levels of noise induced less dispersion around the mean f0, suggesting that pups may have actively focused their phonatory efforts to target lower frequencies. Noise 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 modulation while inhibiting amplitude adjustments. This lowering of f0 is unusual, as most animals commonly display no such f0 shift. 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 article is part of the theme issue ‘Voice modulation: from origin and mechanism to social impact (Part I)’.

发声可塑性（vocal plasticity）可响应环境与生物因素而产生，包括同种个体的叫声与噪声。鳍足类（Pinnipeds）是少数具备发声学习能力的哺乳动物类群之一，因此对探究人类及其他动物的发声可塑性演化具有重要意义。本研究针对斑海豹（Phoca vitulina）的发声可塑性展开调查，该物种在成年个体中已被观测到具备发声学习能力，但幼龄阶段尚无相关记录。为评估早期哺乳动物的发声发育情况，我们对1至3周龄的斑海豹幼崽进行了实验测试。我们针对该物种及其所处发育阶段定制了噪声回放方案，以诱导幼崽改变其基频（fundamental frequency, f0），而非调整叫声振幅或时域特征。实验中，我们让单只幼崽分别暴露于低强度与高强度带通滤波噪声中，该噪声覆盖并掩蔽了幼崽典型的基频范围；同时记录幼崽的自发叫声。与多数哺乳动物不同，本研究中的斑海豹幼崽会通过降低基频来响应噪声强度的提升，该调制过程精准且适配实验中的噪声条件操控。此外，更高强度的噪声会使基频均值周围的离散程度降低，表明幼崽可能主动将发声调控聚焦于更低的频率区间。噪声似乎并未对叫声振幅产生影响，但有一头斑海豹表现出人类在噪声环境中发声时已知的两项隆巴德效应（Lombard effect）特征：叫声振幅显著提升，以及频谱倾斜度趋于平缓。我们采用的相对较低的噪声水平可能更利于基频调制，同时抑制了振幅调整行为。这种基频降低的现象并不常见，因为多数动物通常不会出现此类基频偏移。我们的数据在哺乳动物新生幼崽中属于较为罕见的案例，可为包括人类在内的多个物种的发声可塑性与发声学习演化提供研究启示。本文属于主题刊“语音调制：从起源、机制到社会影响（第一部分）”的一部分。