- Northern sea lions (Eumetopias lubatus) were studied at three sites (Rogue Reef, Oregon, Marmot Island, Alaska, and T.Ugamak Island,
Alaska) during the 1982, 1983, 1985, and 1986 breeding seasons. Data were collected on female activity budgets, attendance patterns, births, copulations, population structure, and patterns of seasonal and daily abundance. Females spent most of their time on land in rest, maternal care, and comfort behaviors. Little time was devoted to sexual, movement, and agonistic behaviors. Significant differences were
observed in behaviors between females of different maternal status, at different sites, and at different times-of-day. Females without young rested more, were less aggressive, and spent more time at sea than those with young. Females with pups and older young spent similar amounts of time in nursing, but only those with pups gave
other care to their young. Females with pups spent the most time ashore. These differences are due to the necessity of females to attend to their young. Rogue Reef females were the least active and least aggressive. Females at Marmot Island were more active, more aggressive, and showed the least change in behavior by time-of-day of any site. Ugamak Island females were intermediate in behavior to the other sites, but showed the strongest time-of-day effects. These differences appear related to thermal, tidal, and substrate differences among the three sites. Females with pups were the largest group of animals on the
rookeries. This group's reproductive cycle and behavior shaped the demographic patterns of primary rookeries, and as a result primary
rookeries had many features in common. These included high pupping rates, low pup mortality, low numbers of juveniles, maximum
abundance in late June to early July, and low animal abundance during the remainder of the year. Rookeries with few pups (secondary rookeries) had earlier peaks in seasonal abundance, greater pup mortality, and greater variation in juvenile numbers.
Variability in the behavior of females and differences in rookery population structure require care to be exercised when
observations are generalized from one site to the population as a whole.