Differential condition dependence among morphological traits inferred from responses to heat stress is predicted by sexual selection theory

Secondary sexual traits that are condition-dependent are expected to reveal the physiological state and/or genetic quality of individuals, and therefore, should more often be found to be under sexual selection than (1) secondary sexual traits not currently condition-dependent, and (2) nonsecondary sexual traits. In the present study, we contrasted the degree of condition dependence in three morphological traits of male Drosophila bipectinata: two secondary sexual traits (distinct components of the sex comb), one of which significantly predicts mating success in nature (segment 2), whereas the other does not (segment 1), and a nonsecondary sexual trait (sternopleural bristle number). As predicted, comb segment 2 decreased significantly in size, in response to increasing temperature during development, whereas comb segment 1 and sternopleural bristle number either did not change significantly, or increased with increasing temperature. These results support the hypothesis that condition-dependence, inferred from stress-induced reductions in trait expression, engenders a trait to sexual selection. Small-combed genotypes did not exhibit disproportionate reductions in larva-to-adult survivorship and adult body size compared to large-combed genotypes, suggesting that comb size does not reveal genotypic quality, at least as revealed by sensitivity in body size and juvenile survivorship to thermal stress.