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Size- and age-specific physiological constraints coupled with inexperience make the transition to independent foraging in juvenile divers a crucial period with important consequences for survival, dispersal and future reproduction. Variation in environmental conditions and associ- ated changes in food availability can additionally constrain the juvenile's development to inde- pendence. Juvenile Galapagos sea lions Zalophus wollebaeki (GSL) represent an extreme exam- ple of these constraints owing to a low growth rate, seasonal variation in food abundance and unpredictable declines in food availability during El Nino events. Time−depth recorder (TDR) data and stable isotope data of δ 15 N and δ 13 C of known age juvenile GSL were analysed to inves- tigate the development of diving and foraging, and a regular year was compared with an El Nino year to determine the effect of environmental change on the foraging behaviour of juvenile and adult GSL. GSL juveniles started diving activity at 12 mo of age and showed distinct juvenile- specific diving strategies that suggest there is an ontogenetic niche shift between juvenile and adult GSL foraging. Successful independent foraging, indicated by a strong decrease in δ 15 N ratio, started considerably later (at 18 mo of age) and was further delayed in the El Nino warm season, indicating there is an even longer dependency on maternal milk during environmentally adverse conditions. Our results suggest that prolonged maternal energetic investment buffers GSL juveniles against the constraints of diving and foraging under environmentally variable conditions and is linked to the low reproductive rate of this species.

Age, body mass and environmental variation shape the foraging ontogeny of Galapagos sea lions