The Galapagos archipelago has been colonized by two species of otariid, the fur seal, Arctocephalus galapagoensis, and the sea lion, Zalophus californianus wollebaeki. The former probably arrived from South America and the latter from North America, both by way of periodic incursions of colder water forming the east Pacific corridor. The terrestrial behaviour of both these otariids is affected by the high ambient temperatures in the Galapagos Patterns of breeding behaviour of otariids which lead to intense polygyny and sexual dimorphism appear to be modified. The habit of prolonging lactation, widespread in otariids, is carried to an extreme in the Galapagos fur seal. The reason for this is unclear.