Temperature can reverse sexual conflict, facilitating population growth.

Sexual conflict frequently gives rise to adaptations that increase male reproductive success at the expense of harming females ("male harm") and decreasing population growth. Studying the ecology of male harm is paramount to understand how sexual conflict unfolds in nature, and its consequences for populations viability. Here, we used seed beetles (Callosobruchus maculatus), a species where males harm females via both harassment and traumatic male insemination, to study whether temperature (24, 28, or 32 °C) can modulate male harm. We disentangled temperature effects on male harm via pre- and postcopulatory mechanisms ("harassment and mating") vs. precopulatory mechanisms (i.e., "harassment" only; ablated males). These treatments were applied under different levels of sexual conflict, with females continuously exposed throughout their lifespan to either no males (control; "no harm"), 1 male (low sexual conflict), or 2 males (high sexual conflict). Constant exposure to males decreased female fitness at warmer environments, particularly at 28 °C and when females were subject to constant harassment and mating under high sexual conflict. In contrast, constant exposure to male harassment and mating increased female fitness at 24 °C, particularly under low sexual conflict (significant ~14% increase vs. control females). At the population level, not being exposed constantly to males resulted in higher net reproductive rates at 28 and 32 °C, whereas constant male-female cohabitation resulted in optimal net reproductive rates at 24 °C, rescuing estimated population growth rate and thus reversing the cost/benefit balance of exposure to males. Our findings show that, by dictating the outcome of female fitness under constant male exposure, temperature can modulate sexual conflict to the point of reversing it and facilitating population growth. Our results support the emerging notion that environmental variation can significantly decrease overall levels of sexual conflict in nature.