Cranial ontogeny of otariid seals

Abstract

Over 2000 otariid skulls were measured for a morphometric study of cranial ontogeny in fur seals and sea lions. Few interspecific differences in cranial ontogeny were observed in the Otariidae, with only minor differences in rates of growth. Sexual dimorphism was significant in all otariids but was more apparent in the larger species. Female otariids of each species showed monophasic development in all characteristics, whereas males expressed monophasic growth for some characters and biphasic growth for others. Biphasic development in skulls of male otariids occurred well after physical maturity had been reached, usually at a suture index of 27. The rate of development varied between skull characters; components relating to the nervous system completed growth well before the rest of the skull, whereas those related to feeding, respiration and vocalisation developed in synchrony with the overall growth of total skull length. Sutures of the calvaria, or braincase, were the first to show partial closure and those uniting the facial bones were usually the last to fuse. As with the calvaria, the orbits and otic capsules developed quickly, suggesting a need for good hearing and vision early in independent postnatal life. Development of the rostral and palatal regions required significantly longer to complete growth.