Visual Orientation and Feeding in Seals and Sea Lions

Abstract

AT least some, and perhaps most, pinnipeds of the families Phocidae and Otariidae are nocturnal feeders1. The eyes of these predators, being sensitive to very low light intensities2, are suited for use at night and in deep water. During the day the visual acuity of the California sea lion (Zalophus californianus) allows this animal to discriminate a size difference ratio as small as 1.06 : 1 (ref. 3). A great deal of vocalizing occurs among pinnipeds, both above and below the surface of the water1,4–6. This fact, along with experimental work such as that of Mohl with the harbour seal (Phoca vitulina)7, indicates that hearing is generally well developed. Poulter suggested that the California sea lion, and probably other species, is able to capture prey in darkness by utilizing an echo-ranging sense similar to that known to occur among cetaceans4. However, subsequent work5,6 has failed to support Poulter's theory. Nevertheless, as “individuals totally blind but in a good state of health have been reported”1, the possibility that such a sense may bo developed when needed should not be dismissed lightly.