Early-life exposure to heat wave induces carry-over effects on oxidative status and behaviour in an amphibian

Climate change is making heat waves (HW) more frequent and intense. These periods of abnormally elevated temperatures pose serious challenges to the life of organisms, particularly of those with limited thermoregulation abilities like amphibians. How early-life stress impacts the phenotype after metamorphosis remains a relatively understudied field of research. In this study, we experimentally investigated carry-over effects of HW exposure during embryo or larval stage on oxidative status and movement behaviour of toadlets of the European common toads (Bufo bufo). Embryos or tadpoles were exposed to simulated HWs (27 °C) for five consecutive days or control conditions (18 °C). We measured markers of oxidative status in neo-metamorphosed individuals, and we carried out open field tests to toadlets 15-20 days post metamorphosis. Tadpoles exposed to HW developed faster than those exposed to HW as embryos, or to controls. Toadlets that were exposed to HW as embryos showed higher activity of the antioxidant enzyme superoxide dismutase than controls, but only toadlets exposed to HW as tadpoles showed higher concentration of thiols, indicating upregulated endogenous antioxidant defenses. Furthermore, toadlets exposed to HW during the larval stage spent more time climbing the open-field arena walls suggesting higher dispersal tendency. Overall, our results indicate stronger physiological and behavioural carry-over effects on toadlets exposed to HW as larvae, compared to those exposed to HW as embryos. The exposure to HW during larval life might impact the local distribution of populations through fitness costs imposed by upregulated oxidative status and by increasing long-distance dispersal.