The transfer of male cuticular hydrocarbons provides a reliable cue of the risk and intensity of sperm competition in decorated crickets

Theoretically, males should increase their ejaculate expenditure when the probability of sperm competition occurring (or risk) is high but decrease ejaculate expenditure as the number of competing ejaculates (or intensity) increases. Here we examine whether male decorated crickets (Gryllodes sigillatus) use cuticular hydrocarbons (CHCs) transferred to females by rival males at mating to assess the risk and intensity of sperm competition and adjust their ejaculate accordingly. Unmated females and those perfumed with CHCs extracted from one, three or five males could be distinguished chemically, providing a reliable cue of the risk and intensity of sperm competition. In agreement with theory, males mating to these females increased sperm number with the risk of sperm competition and decreased sperm number with the intensity of sperm competition. Similarly, as the risk of sperm competition increased, males produced a larger and more attractive spermatophylax (an important non-sperm component of the ejaculate) but there these traits did not vary with the intensity of sperm competition. Our results therefore demonstrate that both sperm and non-sperm components of the male ejaculate respond to the risk and intensity of sperm competition in different ways and that CHCs provide males with an important cue to strategically tailor their ejaculate.

Methods
In Experiment 1, we collected the cuticular hydrocarbon (CHC) profiles of virgin females that had been perfumed with the CHCs of one male, three males and five males, as well as virgin males and virgin females that were not perfumed. We determined the CHC of crickets in these treatments using established GC-MS protocols for this species (Gryllodes sigillatus). This experiment showed that the CHC profile of crickets in these treatments could be statistically distinguished and therefore provide a reliable cue for the risk and intensity of sperm competition.

In Experiment 2, we applied these same perfuming treatments to females (virgin female - control, one male, three males and five males) and then allowed them to mate with a naive virgin male. We measured each males sperm count using microscopy. We also measure the dry weight of the spermatophylax (using an electronic balance) and the attractiveness of the spermatophylax based on the free amino acid composition. We determined the free amino acid composition of the spermaotphylax using established GC-MS protocols for this species.