Sexual conflict and the energetic costs of mating and mate choice in water striders.

Analyses of intersexual conflicts of interest over courtship, mating, or mate guarding require an understanding of the physiological costs of sexual interaction. Repeated respirometric measures of energetic expenditure were taken on female Aquarius remigis while unladen and while carrying a mating male, a small metal weight, or a euthanized male. Unladen "cruising" locomotion consumed an average of 334.6 microW of energy (82 J kg-1 m-1); this estimate of the cost of locomotion coincides with measures from voluntarily locomoting arthropods of similar mass and represents the first energetic measure of skating on a water surface. Cruising females carrying males or metal weights consumed 24% and 28% more energy than unladen females, respectively. Females engaged in "escape" locomotion consumed 43% more energy while carrying a male than while unladen. Further, our study shows that premating struggles, and therefore selective mating decisions, are energetically costly. Struggling females consumed an average of 936.6 microW, a 126% increase compared to cruising, nonstruggling females, and 64% more than mating females engaged in escape locomotion. We develop a quantitative model showing that at a certain harassment rate threshold, accepting superfluous matings becomes the "best of a bad job" for females.