From first to second: Does thermal stress shape reproductive investment decisions?

Climate change is rapidly reshaping thermal environments with major consequences for animal fitness. Although the lethal effects of heat are well documented, sublethal thermal stress may influence reproductive investment, parental behaviour, and ultimately population persistence. Here, we tested whether elevated temperature alters parental investment decisions between current and future reproduction across successive breeding attempts in the burying beetle Nicrophorus vespilloides. Breeding pairs were maintained under control (20 °C) or warm (23 °C) conditions and monitored across two breeding bouts. We quantified parental behaviour (i.e., care duration and carcass preparation), somatic investment, reproductive output, and offspring fitness to determine whether warming modifies reproductive strategies. Elevated temperature did not interact with breeding bout to alter investment trajectories; instead, temperature and breeding bout exerted independent effects on reproductive strategies. Across thermal treatments, parents invested more in reproduction during the second bout, producing larger and heavier broods while gaining less mass themselves, consistent with increased reproductive allocation. Warming reduced reproductive success and offspring fitness, lowering brood size, brood mass, offspring survival, and female mass gain, but it did not affect parental care. These results indicate that chronic warming constrains reproductive efficiency without altering parental investment strategies, highlighting the need to distinguish intrinsic life-history dynamics from environmental constraints when forecasting animal responses to climate warming.