Do male crickets strategically adjust the number and viability of their sperm under sperm competition?

Abstract

Sperm competition theory predicts that males should strategically adjust sperm ejaculate expenditure according to the number of competing ejaculates, because sperm production is costly. That is, males should provide females with larger or higher-quality ejaculates when one rival male is present, but decrease ejaculate size or quality as the number of rivals exceeds one. We tested this hypothesis in the laboratory by subjecting male domestic crickets (Acheta domesticus) to increased sperm competition risk (one rival male) or intensity (two rival males) and then measuring total sperm number and viability (proportion of living sperm) of the ejaculate. In addition, we tested whether male ejaculate expenditure covaried with their own or their mate's phenotypic quality. Contrary to theoretical predictions, males did not prudently adjust the number of sperm they transferred to mates based on either sperm competition risk or intensity. We also found that smaller males had higher proportions of living sperm in their ejaculate relative to larger males, suggesting that male A. domesticus can adjust their ejaculate quality based on their perceived reproductive prospects.