Postnatal craniomandibular growth in Harbor seals (Phoca vitulina vitulina)

As secondarily aquatic carnivores, seals display several phenotypic differences compared to their terrestrial relatives. Differences in the craniomandibular system are associated with aquatic feeding strategies and involve the absence of oral processing of prey. Since Harbor seal pups are particularly precocial, this raises the question of whether the cranium and mandible achieve adult morphology early during postnatal growth. Here, we took advantage of a unique sample of Phoca vitulina vitulina, quantifying the postnatal growth of the craniomandibular system in 152 specimens ranging from 4 months to 20 years. Our results show that craniomandibular growth follows the same general pattern as body length growth: rapid growth until the age of 5 years, followed by a deceleration and finally a stagnation between 15 and 20 years of age, which is well after reaching sexual maturity at around 4 years. Although the overall sexual dimorphism in P. v. vitulina is known to be low, we found significant differences in craniomandibular dimensions between males and females. Most of the craniomandibular postnatal growth patterns in P. v. vitulina are conservative, sharing allometric trends, except for the dimensions of the foramen magnum, interorbital width and palatal width. Calculations of the lever mechanics suggest differences in the masticatory muscles (M. masseter and M. temporalis) and a relatively slow increase in bite force with increasing age. This suggests that the function of the craniomandibular system in P. v. vitulina is achieved early and develops gradually during postnatal ontogeny. This differs from terrestrial carnivores, which experience a pronounced shift from milk suckling to active mastication.