Sperm depletion may not limit male reproduction in a capital breeder

Male reproduction is not only constrained by the number of encountered females but also by physiological limitations, including sperm production and the ability to sustain courtship and mating. Over a breeding season, sperm stores may drop in tandem with male energetic reserves or motivation, confounding the constraints imposed by sperm quantity with other correlated physiological limitations. We used wood frogs (Lithobates sylvaticus) to test whether explosive capital breeders are functionally limited by sperm depletion. We paired males with four conspecific females in succession and counted all of the fertilized and unfertilized eggs that each pair produced. In general, males did not experience a progressive decrease in fertilization rates with each mating (which is characteristic of sperm depletion). Instead, a few males experienced complete fertilization failure during their third and/or fourth mating, perhaps as a result of physical exhaustion, loss of motivation or hormonal changes. This pattern suggests that, even if male wood frogs do become increasingly sperm depleted over a breeding season, they are not necessarily limited by sperm. Understanding how and why fertilization rates change over multiple matings (i.e. incremental decrease vs. all-or-nothing) will help clarify the role of sperm depletion in limiting reproduction.