The role of sex and temperature in melanin-based immune function

Sex differences in immunity have been observed across a wide range of species. Still, it remains unclear how sex-specific interactions with the environment are linked to sex differences in immunity. We studied the plasticity of immunological sex differences by focusing on melanin-based traits in the Pacific field cricket (Teleogryllus oceanicus (Le Guillou, 1841)). Insects rely on the pigment melanin for both immune function and coloration of the cuticle; therefore, changes in melanin production for one of these traits may indirectly affect the other. Male crickets use melanized wing structures to chirp. These cuticular structures are missing in females and a songless male morph. Given that the thermal environment influences cuticle melanization, we investigated the interactive effects of sex and developmental temperature on melanin-based immunity. Both immunity and wing cuticle melanism were reduced in individuals that developed under warmer temperatures. Rearing temperature also mediated the extent to which the sexes differed in immune traits. Males had darker cuticles, whereas females expressed greater immune activity, suggesting that sex-specific investment in melanin corresponds with sex differences in immunity. However, the lack of immunological differences between the two male morphs does not support the hypothesis that investment in cuticle melanism affects investment in immunity.