Despite having relatively little dimorphism in external morphology, male and female Ambystoma maculatum have substantial differences in both short-term (e.g., resting metabolic rate) and long term (e.g., growth rate) physiological processes. In this study, I investigated differences in the mass of reproductive (gonads and proximal reproductive tract) and somatic (liver and heart) organs, and in hematological parameters related to oxygen carrying capacity (erythrocyte count, hematocrit, and hemoglobin concentration) and osmotic balance (plasma osmolality) between males and females before and after breeding. Pre-breeding females had substantially larger ovaries and oviducts compared to post-breeding females, whereas males had similar reproductive organ masses before and after breeding. Females had smaller livers than males, but similar heart masses. Males had similar erythrocyte counts to females, but higher hematocrits, hemoglobin concentrations, and plasma osmolalities. The internal anatomical and physiological differences between males and females suggest greater investment in gametogenesis in females and higher performance in aquatic environments in males. These findings highlight the importance of physiological dimorphisms in organisms where there is little apparent difference in external morphology between the sexes.
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