Ravignani, A., Gross, S., Garcia, M., Rubio-Garcia, A., & De Boer, B.
(2017). How small could a pup sound? The physical bases of signaling body size in harbor seals. Current Zoology, 63(4), 457-465. doi:10.1093/cz/zox026.
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.