Examining the Influence of Mating Systems on Testes Size in Salamanders

Sperm competition theory predicts that relatively larger testes sizes evolve in animals with polygamous mating systems compared to those in monogamous mating systems due to sperm competition. Whereas intensity of sperm competition is the significant predictor of testes sizes in other taxa such as mammals, frogs, birds, insects, and fish, in salamanders the intensity of male-male competition in the transfer of spermatophores to females is predicted to be a critical factor. This is because males have to deposit more spermatophores to secure reproductive pay-off under higher intensity of male-male competition. I hypothesized that salamander species that breed explosively as groups possess increased proportional testes mass than those breeding in less competitive environments. I measured snout-vent length, body mass, and testes mass of Ambystoma maculatum (n=15), A. opacum (n=15), A. texanum (n=10), A. tigrinum (n=12), P. glutinosus (n=15) and Notopthalmus v. viridescens (n=14). I selected these species because they represented a variety of mating strategies with varied intensities of male-male competition. Accordingly, I predicted a gradient of proportional testes masses with A. maculatum having the greatest testes mass and A. opacum having the least. Testes were also examined microscopically, and the stage of spermatogenesis was classified to account for seasonal change in testes mass. The variables that I tested in relation to testes mass were species, body mass, stage of spermatogenesis, and their interactive effects. The best-fit generalized linear model was based on AICC and BIC. The results supported the hypothesis that increased male-male competition results in increased testes size, but other factors such as breeding season duration may also have an important effect on testes size