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.

Keywords: acoustic allometry; honest signaling; larynx; pinniped; vocal tract.

Figure 1

Hemi-vocal (A) tract and hemi-larynx (B). VTL was measured as the distance TC in panel A. Trachea diameter was measured on the third tracheal ring, shown by the black arrows in panel A. VFL was measured as the distance CP in panel B. Notice that both our linear measure TC and the curved surface of the tongue above it are only approximations to the actual VTL. For alternatives see for example, Garcia et al. (2016). The scale shown in both figures corresponds to 1 mm (distance between any adjacent white line) and 1 cm (distance between the two longest white lines at the edges).

Figure 2

Plot of body length and VTL. The best fit line resulting from the significant correlation is included for purely illustrative purposes.

Figure 3

Correlation of body length and diameter of the trachea. The best fit line resulting from the significant correlation is included for purely illustrative purposes.

Figure 4

Plot of estimated age and vocal folds length.