We investigated differences in metabolism and metabolic substrates between male and female small-mouthed salamanders (Ambystoma texanum) collected from breeding sites. Resting O2 consumption rates (VO2) of gravid females at 15 C were roughly double those of males and postgravid females. Whole-body triglyceride and glycogen contents were similar among males, gravid females, and postgravid females, but free glucose contents were higher in both gravid and postgravid females than in males. Respiratory quotients for all groups were ∼0.9, suggesting a relatively high dependence on carbohydrates to fuel metabolism. Our findings suggest that females have a considerably higher metabolic cost of reproduction than do males. Such differences may be associated with aspects of life history and ecology such as sexual size dimorphism, age of sexual maturity, asynchronous arrival at breeding areas, and differential survival between the sexes.

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