Individual adjustment of sperm expenditure accords with sperm competition theory.

Sperm competition theory predicts that males should strategically allocate their sperm reserves according to the level of sperm competition, defined as the probability that the sperm of two males compete for fertilizing a given set of ova. Substantial evidence from numerous animal taxa suggests that, at the individual level, sperm expenditure increases when the risk of sperm competition is greater. In contrast, according to the "intensity model" of sperm competition [Parker, G. A., Ball, M. A., Stockley, P. & Gage, M. J. G. (1996) Proc. R. Soc. London Ser. B 263, 1291-1297], when more than two ejaculates compete during a given mating event, sperm expenditure should decrease as the number of competing males increases. Empirical evidence supporting this prediction, however, is still lacking. Here we measured sperm expenditure in two gobiid fishes, the grass (Zosterisessor ophiocephalus) and black goby (Gobius niger), in which up to six sneakers can congregate around the nest of territorial males and release their sperm when females spawn. We show that, in accordance with theory, sneaker males of both species release fewer sperm as the number of competitors increases.