How small could a pup sound? The physical bases of signaling body size in harbor seals

Vocal tract allometry in a mammalian vocal learner

Acoustic allometry occurs when features of animal vocalisations can be predicted from body size measurements. Despite this being considered the norm, allometry sometimes breaks, resulting in species sounding smaller or larger than expected for their size. A recent hypothesis suggests that allometry-breaking mammals cluster into two groups: those with anatomical adaptations to their vocal tracts and those capable of learning new sounds (vocal learners). Here, we tested which mechanism is used to escape from acoustic allometry by probing vocal tract allometry in a proven mammalian vocal learner, the harbour seal (Phoca vitulina). We tested whether vocal tract structures and body size scale allometrically in 68 young individuals. We found that both body length and body mass accurately predict vocal tract length and one tracheal dimension. Independently, body length predicts vocal fold length while body mass predicts a second tracheal dimension. All vocal tract measures are larger in weaners than in pups and some structures are sexually dimorphic within age classes. We conclude that harbour seals do comply with anatomical allometric constraints. However, allometry between body size and vocal fold length seems to emerge after puppyhood, suggesting that ontogeny may modulate the anatomy–learning distinction previously hypothesised as clear cut. We suggest that seals, and perhaps other species producing signals that deviate from those expected from their vocal tract dimensions, may break allometry without morphological adaptations. In seals, and potentially other vocal learning mammals, advanced neural control over vocal organs may be the main mechanism for breaking acoustic allometry.

Summary: Harbour seals are vocal learners that can escape acoustic allometry despite complying with anatomical allometric constraints. Advanced neural control over their vocal organs may allow them to break acoustic allometry.

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)'.

Laryngeal and soft palate valving in the harbour seal (Phoca vitulina)

Effective 'valving' in the upper aerodigestive tract (UAT) is essential to temporarily separate the digestive and respiratory pathways. Marine mammals are largely dedicated to feeding underwater, and in many cases swallowing prey whole. In seals, little work has been done to explore the anatomy and function of the UAT in the context of valving mechanisms that function to separate food and air pathways. Here we use videofluoroscopy, gross dissection, histology and computed tomography (CT) renderings to explore the anatomy of the larynx and soft palate in the harbour seal (Phoca vitulina), and generate models for how valving mechanisms in the head and neck may function during breathing, phonating, diving and swallowing. Harbour seals have an elevated larynx and the epiglottis may rise above the level of the soft palate, particularly in pups when sucking. In addition, the corniculate and arytenoid cartilages with associated muscles form most of the lateral margins of the laryngeal inlet and vestibule, and move independently to facilitate airway closure. The corniculate cartilages flex over the laryngeal inlet beneath the epiglottis to completely close the laryngeal vestibule and inlet. The vocal folds are thick and muscular and the medial margin of the folds contains a small vocal ligament. The soft palate has well-defined levator veli palatini muscles that probably function to elevate the palate and close the pharyngeal isthmus during feeding. Our results support the conclusion that harbour seals have evolved UAT valving mechanisms as adaptations to a marine environment that are not seen in terrestrial carnivores.

Acoustic allometry and vocal learning in mammals

Acoustic allometry is the study of how animal vocalizations reflect their body size. A key aim of this research is to identify outliers to acoustic allometry principles and pinpoint the evolutionary origins of such outliers. A parallel strand of research investigates species capable of vocal learning, the experience-driven ability to produce novel vocal signals through imitation or modification of existing vocalizations. Modification of vocalizations is a common feature found when studying both acoustic allometry and vocal learning. Yet, these two fields have only been investigated separately to date. Here, we review and connect acoustic allometry and vocal learning across mammalian clades, combining perspectives from bioacoustics, anatomy and evolutionary biology. Based on this, we hypothesize that, as a precursor to vocal learning, some species might have evolved the capacity for volitional vocal modulation via sexual selection for 'dishonest' signalling. We provide preliminary support for our hypothesis by showing significant associations between allometric deviation and vocal learning in a dataset of 164 mammals. Our work offers a testable framework for future empirical research linking allometric principles with the evolution of vocal learning.

Ontogeny of vocal rhythms in harbor seal pups: an exploratory study

Puppyhood is a very active social and vocal period in a harbor seal’s life Phoca vitulina. An important feature of vocalizations is their temporal and rhythmic structure, and understanding vocal timing and rhythms in harbor seals is critical to a cross-species hypothesis in evolutionary neuroscience that links vocal learning, rhythm perception, and synchronization. This study utilized analytical techniques that may best capture rhythmic structure in pup vocalizations with the goal of examining whether (1) harbor seal pups show rhythmic structure in their calls and (2) rhythms evolve over time. Calls of 3 wild-born seal pups were recorded daily over the course of 1–3 weeks; 3 temporal features were analyzed using 3 complementary techniques. We identified temporal and rhythmic structure in pup calls across different time windows. The calls of harbor seal pups exhibit some degree of temporal and rhythmic organization, which evolves over puppyhood and resembles that of other species’ interactive communication. We suggest next steps for investigating call structure in harbor seal pups and propose comparative hypotheses to test in other pinniped species.

Spontaneous rhythms in a harbor seal pup calls

Objectives

Timing and rhythm (i.e. temporal structure) are crucial, though historically neglected, dimensions of animal communication. When investigating these in non-human animals, it is often difficult to balance experimental control and ecological validity. Here I present the first step of an attempt to balance the two, focusing on the timing of vocal rhythms in a harbor seal pup (Phoca vitulina). Collection of this data had a clear aim: To find spontaneous vocal rhythms in this individual in order to design individually-adapted and ecologically-relevant stimuli for a later playback experiment.

Data description

The calls of one seal pup were recorded. The audio recordings were annotated using Praat, a free software to analyze vocalizations in humans and other animals. The annotated onsets and offsets of vocalizations were then imported in a Python script. The script extracted three types of timing information: the duration of calls, the intervals between calls’ onsets, and the intervals between calls’ maximum-intensity peaks. Based on the annotated data, available to download, I provide simple descriptive statistics for these temporal measures, and compare their distributions.

Comment on "Temporal and spatial variation in harbor seal (Phoca vitulina L.) roar calls from southern Scandinavia" [J. Acoust. Soc. Am. 141, 1824-1834 (2017)]

In their recent article, Sabinsky and colleagues investigated heterogeneity in harbor seals' vocalizations. The authors found seasonal and geographical variation in acoustic parameters, warning readers that recording conditions might account for some of their results. This paper expands on the temporal aspect of the encountered heterogeneity in harbor seals' vocalizations. Temporal information is the least susceptible to variable recording conditions. Hence geographical and seasonal variability in roar timing constitutes the most robust finding in the target article. In pinnipeds, evidence of timing and rhythm in the millisecond range-as opposed to circadian and seasonal rhythms-has theoretical and interdisciplinary relevance. In fact, the study of rhythm and timing in harbor seals is particularly decisive to support or confute a cross-species hypothesis, causally linking the evolution of vocal production learning and rhythm. The results by Sabinsky and colleagues can shed light on current scientific questions beyond pinniped bioacoustics, and help formulate empirically testable predictions.