Ejaculate expenditures of male crickets in response to varying risk and intensity of sperm competition: not all species play games

Costs incurred in the manufacture of ejaculates may constrain the number of sperm that males can produce, so males should show some economy in their allocation of sperm across multiple matings. In species in which females mate with multiple males and are capable of storing sperm for extended periods, sperm allocation of males should be tailored to the risk of sperm competition. Recent game theory predicts that males should transfer the least sperm when there are no other rivals, and the most sperm when only one other rival is likely to inseminate the female. However, as the numbers of competitors increases beyond two, the models predict a corresponding decrease in ejaculate expenditure. We tested these predictions in three cricket species, Gryllodes sigillatus, Gryllus veletis, and Gryllus texensis, assessing the sperm allocation of males held under three levels of apparent interrival competition: no rivals, one rival and six rivals. Sperm allocation of G. veletis varied according to theory: males increased their sperm allocation with an increased risk of sperm competition (no rivals vs. one), but decreased their allocation with an increased intensity of sperm competition (one rival vs. six). Sperm allocation of male G. texensis showed no significant response to the density of rivals, and sperm allocation in G. sigillatus was influenced by an unexpected interaction between treatment density and the order in which males experienced the three treatments. The observed interspecific variation in facultative sperm allocation may be due to interspecific differences in population density, rearing environment, or female mating behavior.