cal Pacific into the southern hemisphere some 5 million years ago; sea lions followed about 2 million years later (Wynen et al., 2001). Otariids have never occupied the North Atlantic Ocean, apparently because they were unable to breach the tropical Atlantic.

Traditionally the otariids have been separated into two subfamilies—the fur seals and sea lions. However, this classification is not supported by recent molecular phylogenetic analyses (Wynen et al., 2001). These analyses indicate that the northern fur seal (Callorhinus ursinus) has a basal relationship, diverging from a lineage leading to the remaining otariid species about 6 million years ago. Precise relationships among these species are unclear because various clades radiated rapidly at the same time they were diverging from one another. In particular, it is unclear whether the modern sea lions are mono- or multiphyletic.

Seventeen species of otariids are recognized (Gentry, 2002)—10 fur seals (2 genera: Arctocephalus and Callorhinus) and 7 sea lions (5 genera: Eumetopias, Neophoca, Otaria, Phocarctos, and Zalophus). Two species (Zalophus wollebaeki and Arctocephalus galapogoensis) are equatorial, occurring in the upwelled waters of the Galapagos; five species occur in the northern hemisphere (Z. californianus, Z. japonicus, E. jubata, C. ursinus, and A. townsendi) and the remaining 10 species occur in the southern hemisphere. Otariids vary greatly in overall abundance, from the comparatively rare (more than 7,000 individuals) Guadalupe fur seal (Arctocephalus townsendi) to the extremely abundant (about 3 million) Antarctic fur seal (Arctocephalus gazella). According to Gentry, only Steller sea lions and southern sea lions (Otaria flavescens) are in overall decline, although northern fur seals in the eastern North Pacific Ocean could be added to this list.

In contrast with the phocids, which display a wide array of foraging behaviors and mating systems (Riedman, 1990), both of these features are more highly conserved in the otariids. All species of otariids are sexually dimorphic, with males being larger than females, and all species have polygynous mating systems in which males aggressively defend breeding areas or groups of females. This extreme sexual dimorphism and polygyny probably arose from the combined selective effects of (1) high population densities in productive temperate seas; (2) a dependency on land for birth and reproduction; (3) the limited availability of suitable land-based habitats (e.g., predator-free islands) at temperate latitudes; (4) the resulting aggregation of individuals on land; and (5) the consequent ability of males to compete for and acquire large numbers of females (Bartholomew, 1970). The postpartum behavior of adult females is also highly stereotyped—during this period, all otariid species alternate between at-sea foraging and on-land suckling until their pups are weaned.