Male sperm expenditure under sperm competition risk and intensity in quacking frogs

Abstract

In the frog Crinia georgiana, reproductive behavior comprises a "guarding tactic," in which males defend spawn sites and attract females by calling, and a "sneak tactic," in which males join spawning pairs. The aims of the present study were to (1) relate ejaculate expenditure by "guarding" and "sneak" males to their probability of mating with other males present (sperm-competition risk), and (2) determine if males adjust their ejaculate expenditure according to the number of males involved in a spawning (sperm-competition intensity). Theory predicts that because sneak males always mate with other males present, they will experience a higher sperm-competition risk and should release larger ejaculates relative to that of guarding males. However, as the proportion of sneaks in a population increases so does the risk of sperm competition to guarders, so expenditure by each tactic should move toward equality. Given that the incidence of sneak behavior is high in C. georgiana, guarders and sneaks were expected to experience similar risks of sperm competition and show similar investment in spermatogenesis. Comparison of testes size and ejaculate size showed no difference between tactics. Models of sperm-competition intensity predict that males should increase their ejaculate size when spawning in the presence of one other male but decrease their ejaculate size when spawning in the presence of multiple males. Here, males maintained a constant sperm number irrespective of whether a mating involved one, two, or three males. This result suggests that male C. georgiana do not facultatively adjust ejaculate investment in response to fluctuating intensities of sperm competition.