Whereas some authors have proposed that sexual dimorphism in the immune response is fixed, others pose that it is dynamic and depends on diet. The aim of the present study was to explore the second hypothesis. Immunocompetence differences between females and males can be linked to resource availability. We tested this idea by providing a low or high quality diet to two groups of Gryllodes sigilatus during their developmental period. Then, at the adult phase half of each group was challenged with LPS from Serratia marcescens. The size, phenol oxidase (PO), and lytic activity were compared between groups according to diet, sex, and immune challenge. Results show that diet quality favor size in both sexes. However, the overall immune response did not seem to be significantly different based on diet, but instead on sex. Females showed greater phenol oxidase levels than males, but the opposite was found with lytic activity. Perhaps in G. sigillatus the differences in the pathogens commonly confronted by each sex in the distinct habitat of each could explain the differences on PO and lytic activity. 1. Introduction The understanding of sexual dimorphism (SD) is one of the central challenges in evolutionary biology. SD includes differences in behavior, morphology, and physiology, but perhaps why males are less resistant to parasites and pathogens than females is one of the most studied questions regarding this topic [1–3]. The underlying physiological mechanism of this dimorphic resistance could be that females are more immunocompetent than males, implying SD in the immune response. Hence, immunocompetence is defined as the magnitude and effectiveness of immune response [4]. To understand why females are more immunocompetent than males can be explained by two hypotheses: sexual selection [2, 5] and the immunocompetence handicap hypothesis [6]. The sexual selection hypothesis suggests that males in most species invest in their sexual traits at the expense of survival, meaning that fewer resources are invested in the immune response [2]. For example, males in Panorpa vulgaris provide nuptial gifts to attract a female but have the lower values of phagocytic and lysozyme activity [7]. The second hypothesis known as the immunocompetence handicap hypothesis considers that females invest in the immune response to favor longevity, which is positively correlated with fecundity [8]. However, there are no studies that have tested whether female longevity and egg production are positively correlated with the immune response [8]. The problem with the sexual selection and
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