Summary
Growing juvenile animalsu ndergom any morphologicalp, hysiological, a nd behaviouracl hangesth at influencet heir energetic requirementsp, atternso f energyu se,a nd ultimately,t heir survival and reproductives uccessW. e examinedc hangesin mass loss and body composition of juvenile southern elephant seals (1- and 2-yr-olds) during their two annual haul-outs. At the start and end of the midyear and molt haul-outs, we caught, weighed,a nd measured4 1 and 14 sealsr,e spectivelyW. e measured blubber depth using ultrasound to estimate body composition (lean and adipose tissue mass). Using energy densities of the adipose and lean tissue, we calculated total, lean, and adiposem assc hangesa nd energye xpenditureW. hile molting, juvenile sealsu sedm ore energyt han during the midyearw, hich is relatedt o the increasedu se of lean tissuef or hair and skin regeneration.T he amount of energyu sedi ncreasesw ith mass as individuals mature. We found sexuald ifferencesin energy use where femalesr etained greaterf at reservesth an malesb y utilizing more lean tissue. These differences are most likely related to haul-out function and behavior, growth, and earlier development of females toward sexual maturity.