Insects need desiccation prevention at all times, but only need a sexual signal after reaching sexual maturity. CHCs are ever-present on the insect cuticle. When and how, then, are the costs of balancing attractiveness and desiccation paid in this trait? As opposed to song, which only becomes possible to produce and detect at adulthood (Alexander 1961), we know next to nothing about how CHC development changes over ontogeny, even though such information gives valuable insight into how selection acts on the trait. Since CHCs are ostensibly desiccation prevention first and sexual signals second, I test the hypothesis that desiccation risk determines when and how males incur the cost of investing in CHCs as sexual signals.
Methods: I will rear male crickets from Oahu (Hawai’i) and Mangaia (Cook Islands) in incubators set at 75% and 50% humidity. The 75% humidity treatment represents normal humidity conditions for the crickets, while 50% poses a desiccation risk. Although I saw no evidence of desiccation risk constraining sexual selection in Aim 1, humidity levels in the field never reached 50%, the supranatural stimulus I plan to administer. I will subsample 20 male CHCs (as described in Aim 1 methods) at the 3rd instar, penultimate instar, and 8 days after eclosion (final molt). I will identify instars by body length, coloration, and eventually ovipositor and wing bud development (Gurule-Small and Tinghitella). The 3rd instar represents an early life stage when males are juveniles. The penultimate instar is right before adulthood, when males may be transitioning into sexual maturity. Adults are sexually mature 6-10 days after eclosion, so I will sample at day 8, in between this period.
Expected Results: We do not know if the costs of CHC production start early and occur across a whole lifespan, or if short-chain CHCs only appear at adulthood. I predict that males reared in the high desiccation risk incubator (50% humidity) to have little to no attractive short chain CHCs until adulthood, at which point they accrue more short chains and begin to balance the tradeoff between attractiveness and desiccation. Conversely, I expect less variation in the abundance of short chain CHCs over the lifetime of crickets reared in a low desiccation risk incubator (75% humidity) and more attractive CHCs overall in this treatment. I am also comparing between Oahu and Mangaia colonies, the former of which comes from a population undergoing decades of strong selection pressure by the parasitoid. It is possible that regional differences may appear in CHC development; for instance, males from the parasitized Oahu, who may need to invest more in CHCs as sexual signals, could slowly build up proportions of short chain CHCs over development instead of incurring that cost entirely at adulthood. Comparatively, Mangaia males, who need fewer short-chain CHCs, may develop them only at adulthood. The regional comparison is secondary and not as robust, since lab populations are under no risk of parasitization.