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

Abstract

Vocal communication is a crucial aspect of animal behavior. The mechanism which most mammals use to vocalize relies on three anatomical components. First, air overpressure is generated inside the lower vocal tract. Second, as the airstream goes through the glottis, sound is produced via vocal fold vibration. Third, this sound is further filtered by the geometry and length of the upper vocal tract. Evidence from mammalian anatomy and bioacoustics suggests that some of these three components may covary with an animal’s body size. The framework provided by acoustic allometry suggests that, because vocal tract length (VTL) is more strongly constrained by the growth of the body than vocal fold length (VFL), VTL generates more reliable acoustic cues to an animal’s size. This hypothesis is often tested acoustically but rarely anatomically, especially in pinnipeds. Here, we test the anatomical bases of the acoustic allometry hypothesis in harbor seal pups Phoca vitulina. We dissected and measured vocal tract, vocal folds, and other anatomical features of 15 harbor seals post-mortem. We found that, while VTL correlates with body size, VFL does not. This suggests that, while body growth puts anatomical constraints on how vocalizations are filtered by harbor seals’ vocal tract, no such constraints appear to exist on vocal folds, at least during puppyhood. It is particularly interesting to find anatomical constraints on harbor seals’ vocal tracts, the same anatomical region partially enabling pups to produce individually distinctive vocalizations.

Spontaneous rhythms in a harbor seal pup calls

“…I recorded a female harbor seal pup. The seal was born in the wild and brought into rehabilitation at the Sealcentre Pieterburen, The Netherlands [ 21 , 22 ], at the estimated age of 7 days [ 14 , 22 ]. The animal was individually housed in a pool situated in a 1-room cabin.…”

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

“…We recorded 3 wild-born harbor seal pups. A female pup (tagged 292) was brought in for rehabilitation at the Sealcentre Pieterburen, The Netherlands (Ravignani et al. 2017), at the estimated age of 7 days (Ravignani 2018b).…”

“…This means that the mother must adapt to her pup’s calls as it continues to grow. Indeed, the resonant properties of the vocal tract change as the pup grows (Ravignani et al. 2017).…”

Animal vocal communication: function, structures, and production mechanisms

“… Ravignani et al (2017) investigate the anatomical determinants of sound production in harbor seal pups. By contrasting their results with acoustic data available on the study species, this article highlights the potential importance of the source- and filter-related components of seals vocalizations for transmitting information about body size (i.e., provide insight into the allometric relationship of harbor seals body size and calls).…”