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When were the weaners weaned? Identifying the onset of Australian sea lion nutritional independence

ABSTRACT

Weaning in mammals is typically thought of as the transition from reliance on maternal milk to feeding independently. Current theory suggests a complex process involving mothers imparting enough resources to offspring as to ensure survival without compromising both prior and future reproductive efforts, and the demands of offspring whose primary concern is survival. Otariid seals are a suitable group to study this given the morphological and behavioral similarities across species of the primary care giver, adult females. At higher latitudes, the duration of maternal care is short and tightly linked to seasonal productivity of the marine environment, punctuated by a predictable migration of mothers away from breeding sites. In contrast, nonmigratory temperate latitude otariid species have a much wider range of lactation periods, with mothers prolonging maternal support in relation to seasonal unpredictability of food resource. Prolonging care into the subsequent reproductive effort will likely have profound effects on the survival of the younger offspring. The Australian sea lion Neophoca cinerea has broken the phylogenetic constraint of a 12-month breeding cycle, which may reflect an alternate strategy to reduce the fitness costs of prolonged support by providing up to 18 months nutritional support to offspring. We use stable isotope analysis of temporally matched whisker sections combined with telemetry data on nutritionally dependent Australian sea lion pups to determine the weaning process and characterize the transition to nutritional independence. Using changes in isotopic nitrogen (15N) over time, pups undergo a gradual transition to independent foraging during a 3- to 6-month period before the onset of the next reproductive effort. Telemetry data supported this conclusion, indicating benthic foraging of weaned pups in areas consistent with adult female foraging.