Abstract
Hormones mediate the physiological responses of animals to environmental changes. Consequently, hormones can be responsible of trade offs between different life history traits. Juvenile hormone (JH) is thought to mediate resource allocation in insects: specifically, it is thought to enhance the expression of condition-related traits like sexual signals, whilst reducing immune responsiveness. Here, we experimentally test whether a JH analog (JHa) had an effect on immunity of male dragonflies Celithemis eponina, and if such effects are translated into faster growth or development of a natural parasite (water mite). We also tested the effects of JHa on host condition (muscular mass and fat reserves) of mature male dragonflies. Mites from JHa treated dragonflies grew faster than mites from control dragonflies receiving just an acetone carrier. However, there was no effect of JHa on measures of host immune response (melanization of a nylon implant) or condition of mature males. We suggest that better parasite growth in JHa treated males does not result from the JH immunosuppressive function, but instead it appears that parasites receive hormone signals from the host and alter their development without affecting host condition measurably. Our work highlights the importance of measuring both immune parameters and response to real parasites when studying evolutionary trade offs.
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