Continuous variation in reproductive strategy as an adaptive response to population density in the moth Plodia interpunctella

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Abstract
Despite being variable, reproductive environments may be predictable. Because of this variability, individuals may maximize fitness through phenotypic plasticity. Here, I describe how males of the moth Plodia interpunctella are sensitive to population density during larval development (controlling for confounding effects of diet), and tailor a strategy which best suits the predicted reproductive environment. Such a strategy may be adaptive for this semelparous species, where all resources for reproduction are accrued during larval development and adults have a brief reproductive stage. Population density influences female mating pattern: at high densities, females mate more frequently, thus generating greater risks of sperm competition. Males reared at higher densities take longer to develop, despite achieving the same body mass. Adults from high densities have relatively larger abdomens and testes, produce greater numbers of sperm and live for shorter periods. At lower densities development time is reduced, and adult males have relatively larger heads and thoraxes, relatively smaller testes, ejaculate fewer sperm and live for longer periods. These results suggest that developing male P. interpunctella detect signals which reflect larval population structure, and hence eventual mating pattern. Males at low densities anticipate rare mate encounter and low risks of sperm competition, and therefore appear to invest in competence for migration and mate-searching. Males at higher densities anticipate frequent mate encounter and high risks of sperm competition, and therefore appear to invest in competence for mating and sperm competition. Such flexible male responses represent a `reaction norm' where male strategies vary as a continuous function of the predicted reproductive environment's signal.