Growth variation in skull morphology of Kuril harbor seals (Phoca vitulina stejnegeri) and spotted seals (Phoca largha) in Hokkaido, Japan.

We examined morphological growth variations in skull features between the Kuril harbor seal and the spotted seal in Hokkaido, Japan. Skulls from 80 Kuril harbor seals and 41 spotted seals were collected, and we measured 29 metric and 6 non-metric cranial characteristics. Three growth classes were defined according to the postnatal developmental stage: pups (0 year), subadults (1-4 years old) and adults (more than 5 years old). We detected sexual dimorphism in Kuril harbor seal pups, subadults, and adults. Although interspecies differences were detected in each growth class, Kuril harbor seals were larger and more massive than spotted seals; this feature was already detectable in pups. We did not detect certain cranial characteristics with which to identify the two species, but it was possible to identify any unknown specimens to their species, sex, and growth class using the cranial data generated in this study. Using 6 non-metric cranial characteristics, we identified significant interspecies differences with regard to the shape of the temporozygomatic suture and the extent of the nasal-incisive suture; the shape of the temporozygomatic suture and the shape of the nares were indicators of growth class in Kuril harbor seals. Although non-metric cranial characteristics have a lower discriminating power than metric characteristics, they are easy to use in the field even by inexperienced researchers.