Behavioural plasticity buffers offspring from heat stress but imposes somatic costs on parents

Global warming makes behavioural plasticity a critical defence enabling animals to buffer reproduction against environmental stress. Yet how well such plasticity safeguards offspring under heat stress, and at what cost to parents, has rarely been quantified within a single system. Burying beetles ( Nicrophorus ) are an ideal model: parents bury small vertebrate carcasses underground and rear their young within, where burial depth can buffer offspring against elevated soil temperatures. In a laboratory experiment on Nicrophorus nepalensis , we compared heated with unheated treatments to ask whether burial depth buffers the brood, and how depth and heat jointly shape brood size, larval mass, and maternal mass loss. Deeper burials created cooler, more stable microclimates, confirming a thermoregulatory function. Rather than burying deeper, heat-exposed beetles buried carcasses shallower—consistent with a thermal constraint on burial efforts, since deeper burial yields progressively less thermal benefit while still costing energy. Where deeper burial was achieved under heat, it supported larger broods and heavier larvae, though larvae remained lighter than controls. These benefits came at parental expense: beetles burying deeper and those exposed to heat lost more body mass. Burial thus acts as a double-edged buffer, exposing hidden parental costs that resilience models must incorporate.