In comparison to other homeotherms, young recently weaned marine mammals in high latitudes face exceptional energetic demands when foraging and thermoregulating. Lipids are an important source of energy and a major component of insulation that allow them to meet these demands. To examine the role of lipid stores in a high-latitude pinniped, the body composition and thermoregulatory capabilities of Antarctic fur seal (Arctocephalus gazella) pups and yearlings were measured via flow-through respirometry and hydrogen isotope dilution. From these data we constructed a model to examine the importance of post-weaning fasting capability in free-ranging, young fur seals. Resting metabolic rates were different for pups and yearlings measured in 0.6 ºC water, 10.3 ºC water, and ambient air; however, mass and percent lipid as covariates accounted for the different metabolic responses in pups and yearlings for all treatments. The estimated lower critical temperature for combined pups and yearlings was 14.4 ºC, 10 to 15 ºC above water temperatures normally experienced by Antarctic fur seals. Modeling predicted that a weaned fur seal pup would survive at-sea from 9.8 to 36.2 days before succumbing to starvation. The most likely maximum travel distance within this time constraint suggests food resources close to the natal rookery are important to first year survival for this species.