Nutritional insensitivity to mating in male fruit flies

Animals can adjust their consumption of different nutrients to adaptively match their current or expected physiological state. Changes in diet preference can arise from social and sexual experience. For example, in female Drosophila melanogaster fruit flies, a single mating triggers a behavioural switch in diet choice towards increased protein intake and total food consumption, which supports offspring production. In contrast, male diet choice appears to be unaffected by a single mating. However, one mating may not fully capture the impact of mating on male feeding behaviour. Males can often mate multiply in natural settings, and the costs of ejaculate production and energetic courtship may be cumulative, such that males might experience increased nutritional demands only after multiple matings. In this study we tested this idea by measuring the effect of multiple matings on the diet choice of male D. melanogaster fruit flies. Males were assigned to one of three mating treatments - unmated, mated once or mated five times consecutively - and then allowed to feed freely on chemically-defined diets of protein and carbohydrate. In contrast to the prediction, we found that males that mated five times did not alter the amount of food, nor the proportion of protein and carbohydrate consumed, when compared with unmated or once-mated males. This absence of a feeding response occurred despite substantial ejaculate depletion from multiple matings: males sired fewer offspring in each consecutive mating. These results reveal a lack of plasticity in male feeding behaviour according to mating status, despite substantial potential physiological costs, and highlight the remarkably distinct nutritional ecologies of males versus females.