When rivals appear or disappear, do male fruit flies adjust their mating behavior in ways that produce more offspring?

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Behavioral plasticity—the ability to change behavior—is fundamentally important in variable environments, especially when the social and sexual environment changes rapidly. Males can respond adaptively to changes in the overall level of reproductive competition. What remained unknown was how flexible an individual's behavior could be, and how closely rapid behavioral changes would match fitness outcomes.

The researchers experimentally exposed individual Drosophila melanogaster males to up to three episodes with rivals or without rivals, testing all combinations. They determined each male's behavioral and fitness profiles, measuring mating duration as behavior and offspring number as the fitness outcome.

Mating duration was remarkably sensitive to the level of competition and fully reversible. This suggested that expressing even highly flexible behavior in the wrong circumstances can carry substantial costs. However, changes in mating duration matched offspring outcomes only in scenarios where males experienced no competition followed by high competition.

After competition was removed, mating duration decreased below control levels, but offspring production did not. This indicates that the benefit of increasing reproductive investment when rivals appear may exceed the benefit of decreasing investment when rivals disappear. The researchers expect these asymmetric fitness benefits, and the mismatches between behavioral responses and fitness outcomes, to exert strong selection on the evolution of plasticity.