Sex-specific nutritional requirements of mating in insects with contrasting mating systems

Fitness of most animals is affected by the amount and ratio of nutrients they consume. Therefore, maximizing fitness relies on consumers fine-tuning their intake towards a specific nutritional target. However, mating might alter this target because the nutrient ratio that maximizes reproductive investment often differs from ratios that elevate the expression of other fitness traits, e.g. survival and immunity. Therefore, consumers may be under selection to shift their intake towards nutrient ratios that promote reproductive success only when the likelihood of mating is high or after mating activity. Here, we tested how mating affects total macronutrient intake and the protein-to-carbohydrate ratio consumed by males and females given a dietary choice. Three insect species, namely Australian field crickets, Teleogryllus commodus , decorated crickets, Gryllodes sigillatus , and cockroaches, Nauphoeta cinerea , were studied. Males in these species differ in the traits they use to attract females and in postcopulatory sexual selection, while females differ in the timing and magnitude of offspring investment. Despite these differences, mating triggered increased macronutrient intake in females across all species, while male intake remained unchanged. This elevated consumption indicates that mating increases the energetic demands of females more than males. Neither sex altered the nutrient ratio consumed after mating, despite nutrient ratios mediating trade-offs between aspects of reproduction, e.g. sexual display versus sperm production, and other life-history traits, e.g. survival, in these species. We speculate that this is because selection skews nutrient regulation strategies towards ratios that promote reproductive success, and mating does not trigger deviation from these relatively fixed courses. In addition, the magnitude and direction of sex differences in protein and carbohydrate intake as well as how tightly each sex regulates their macronutrient intake, differed between species. We discuss what this suggests about species-specific physiology and the costs of reproduction.