The brain of fur seals, seals, and walrus (Pinnipedia): A comparative anatomical and phylogenetic study of cranial endocasts of semiaquatic mammals

Abstract

Pinnipeds are semiaquatic carnivorans that spend most of their lives in water. Although there is currently a better under‑ standing of the phylogenetic relationships among pinniped species, there is a lack of neurobiological studies of their brains conducted in an evolutionary framework. We reconstructed virtual 3D brain endocast of 29 skulls corresponding to adult and subadult specimens, both male and female, belonging to 18 species of pinnipeds. Data collection involved mostly newly acquired computer tomography scan and μCT scan data. In the Otariidae species studied, the olfactory bulbs are proportion‑ ally more developed than in other pinnipeds. Individuals of these species spend longer periods on land than phocids and have a social structure in which olfactory information/cues play an important role. The olfactory bulb of phocids is smaller than that of the otariids, with the exception of Hydrurga leptonyx and the males of Mirounga leonina, in which they significantly even larger than those of otariids. The frontal cortex is more extended in otariids and odobenids than in phocids, probably due to a functional relation to a complex social structure. Concerning the temporal cortex, which is related to hearing/ori‑ entation, we recorded an enlargement of the posterior zone in phocids, probably related to the need for orientation at great depths in the absence of light, whereas in otariids and odobenids this extension is similar to that of terrestrial carnivorans. The occipital cortex, linked to vision, is well developed in Ommatophoca rossii and Hydrurga leptonyx, and slightly less so in Mirounga leonina and Leptonychotes weddelli