Mating-dependent lifespan cost of sterol depletion in male Drosophila melanogaster

Trade-offs between lifespan and reproduction are a central theme in evolution and ecology, often attributed to physiological constraints on the allocation of limiting resources. However, recent work suggests such trade-offs may depend on macro- and micro-nutritional context. We examined how macronutrient balance and cholesterol availability shape lifespan and reproductive performance in male Drosophila melanogaster, explicitly testing both unmated males and those permitted to mate freely across their lives—a design rarely applied. We found that cholesterol deprivation induced a substantial reduction in lifespan, but only in frequently mating males, indicating that sterols are continuously invested in reproduction, consistent with their known role in sperm and seminal fluid. Conversely, dietary cholesterol reduced lifespan in unmated males maintained on low protein–to–carbohydrate diets, suggesting that male reproductive investment is sufficiently high to generate sterol-dependent costs to the soma. Lifespan was maximised at higher protein–to–carbohydrate ratios than previously estimated, while reproductive performance in aged males was favoured by carbohydrate-biased diets, demonstrating a nutritional trade-off. Males preferentially consumed macronutrient ratios that promoted reproductive success rather than longevity, and discriminated the presence of cholesterol in food. Together, these findings reveal sterols as a key mediator of male longevity, with effects that depend critically on both diet and reproductive context. More broadly, by integrating sterols, macronutrient balance, and sustained reproductive investment, our study challenges the long-standing assumption that male reproduction is “cheap,” and refines our understanding of the evolutionary basis of dietary trade-offs.