Sexual Dimorphisms in Metabolism, Organ Mass, and Reproductive Energetics in Pre-breeding American Toads (Anaxyrus americanus)

Sexual size dimorphisms are common in many species of anurans, and have often been ascribed to different selective forces that influence reproductive success in males and females. However, less is known about dimorphisms in whole-organism physiology or in internal morphology in anurans. In this study, we investigated potential differences in metabolism, visceral organ mass, and stored energy content between male and female American Toads (Anaxyrus americanus) collected en route to breeding areas during the spring. Female toads were both longer and more massive than males. Males demonstrated larger increases in metabolic rate, heart mass, liver mass, and nonpolar lipid content with increasing body size than did females, but kidney mass did not differ between the sexes. The ovaries of the females represented, on average, 38% of the total dry mass and 54% of the total caloric energy contained in their bodies. However, the body-size adjusted caloric content of the carcass did not differ between males and females. Our findings suggest that larger males are physiologically able to support greater levels of activity during the breeding season and thus will more likely engage in more strenuous activities such as calling, active seeking of females, and amplexus interference, compared to smaller males. Large females, on the other hand, appear to preferentially allocate resources to oogenesis. In addition, our findings suggest that although the overall energetic cost of reproduction may be greater for females, there may be other energetic constraints within or outside of the reproductive period that limit the ability of males to allocate resources to growth and storage.