Male Cockroaches Prefer a High Carbohydrate Diet That Makes Them More Attractive to Females: Implications for the Study of Condition Dependence

Abstract

Sexual selection is a major force driving the evolution of elaborate male sexual traits. Handicap models of sexual selection predict that male sexual traits should covary positively with condition, making them reliable indicators of male quality. However, most studies have either manipulated condition through varying diet quantity and/or caloric content without knowledge of specific nutrient effects or have correlated proxies of condition with sexual trait expression. We used nutritional geometry to quantify protein and carbohydrate intake by male cockroaches, Nauphoeta cinerea, and related this to sex pheromone expression, attractiveness, and dominance status. We found that carbohydrate, but not protein, intake is related to male sex pheromone expression and attractiveness but not dominance status. Additionally, we related two condition proxies (weight gain and lipid reserves) to protein and carbohydrate acquisition. Weight gain increased with the intake of both nutrients, whereas lipid reserves only increased with carbohydrate intake. Importantly, lipid accumulation was not as responsive to carbohydrate intake as attractiveness and thus was a less-accurate condition proxy. Moreover, males preferentially consumed high carbohydrate diets with little regard for protein content suggesting that they actively increase their carbohydrate intake thereby maximizing their reproductive fitness by being attractive.K E Y W O R D S : Diet choice, Nauphoeta cinerea, nutritional geometry, sex pheromone, sexual selection.

Protein and carbohydrate intake influence sperm number and fertility in male cockroaches, but not sperm viability

“…This ratio, however, contrasts the optima shown for male traits used in pre-copulatory sexual selection, such as nightly calling effort in the black field cricket (Teleogryllus commodus), which requires a much higher intake of C to fuel this energetically costly behaviour (P : C ratio ¼ 1 : 8 [53]). In N. cinerea, male-male competition and female mate choice are mediated by the same three male sex pheromones (3-hydroxy-2-butanone, 4-ethyl-2-methoxyphenol and 2-methylthiazolidine [54]), and the expression of these pheromones and subsequent attractiveness is also optimized at a P : C ratio of 1 : 8 [45] (see the electronic supplementary material, text S4 for a detailed comparison of our feeding methodologies [45]). Collectively, these findings have two important implications for the evolution of sperm number in N. cinerea.…”

“…The distribution of these diets in nutritional space can be seen in the electronic supplementary material, figure S1, and the composition of these diets in the electronic supplementary material, table S1. These represent the same 24 diets used by South et al [45].…”

“…We used a sequential model-building approach to determine whether the linear and nonlinear effects of P and C intake differed across our response variables (outlined in the electronic supplementary material, text S2) [45]. While this sequential approach provides a statistic test of the difference in magnitude of the linear and nonlinear gradients across response variables, it does not provide information on the direction of this difference in nutrient space.…”

“…We quantified the linear and nonlinear (quadratic and correlational) effects of P and C intake on sperm number, sperm viability and male fertility using a multivariate response surface approach (outlined in the electronic supplementary material, text S1) [45]. Non-parametric thin-plate splines were used to visualize the nutritional landscape for each response variable and were constructed using the Tps function in the 'FIELDS' package in R (version 2.13.0).…”

“…Diet consumption was calculated as the difference in dry weight of diet before and after feeding. This amount of consumed diet was converted to a weight of P and C ingested by multiplying by the proportion of these nutrients in the diet [45].…”