| Summary: | Many polygynous, capital breeders exhibit sexual dimorphism with respect to body size and composition. This dimorphism is often facilitated by sex differences in foraging behavior, growth rates and patterns of nutrient deposition during development. In species that undergo extended fasts during development, metabolic strategies for fuel use have the potential to influence future reproductive success by directly impacting somatic growth and acquisition of traits required for successful breeding. My aim was to investigate sexual dimorphism associated with the metabolic strategies for fasting in developing northern elephant seals. Thirty one juvenile seals of both sexes were sampled over extended fasts during autumn haul-outs. Field metabolic rate (FMR) and protein catabolism were estimated from changes in mass and body composition over 22.6 ± 4.6 (SD)days of fasting. Protein catabolism was assessed directly in a subset of animals based on urea flux at the beginning and end of the fast. Regulatory hormones and blood metabolites measured included growth hormone (GR), cortisol, insulin, glucagon, testosterone, estradiol, glucose, urea and ~-hydroxybutyrate. Males exhibited higher rates of energy expenditure during the fast but spared body protein stores more effectively than females. Rates of protein catabolism were strongly influenced by GR levels which varied between the sexes. Estimates of protein catabolism from urea flux were significantly lower than those from mass loss. These data suggest that sex differences in fuel metabolism and energy expenditure during fasting arise early in juvenile development. These differences may play an important role in the development of adult traits associated with reproductive success. Kelso, Elizabeth J. 2011. Sex Differences in Fuel Use and Metabolism During Development in Fasting Juvenile Northern Elephant Seals (Mirounga angustirostris). Department of Biology, Sonoma State University.
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