Functional relationship between muscles of mastication and the skull with enlarged orbit in the Baikal seal (Phoca sibirica).

We expected that the enlarged area of zygomatic arch, one of some skull characters adapted for enlarged eye, has an influence on form and function of muscles of mastication in the Baikal seal (Phoca sibirica). So, in this species, the Mm, masseter, temporalis, pterygoidei, digastricus were observed in the macroscopic level. The skull characters related to these muscles were also compared between the Baikal seal and a close-related species, the ringed seal (Phoca hispida). The Mm, masseter and temporalis were well-developed using the enlarged attachment area of zygomatic arch. In contrast, the M. digastricus is suggested to be not so important in the Baikal seal, because the temporal bone is not so developed as in the ringed seal. It is suggested that the Baikal seal has especially developed the Mm. temporalis and masseter using an enlarged area of zygomatic arch among Pusa species. We also suggest that the robust temporal bone is equipped to have the M. digastricus developed as a main retractor of mandibular body in the ringed seal.