Thermal fluctuations affect embryonic physiology in a tropical agamid lizard

Climate change is increasing both mean temperatures and thermal fluctuations, posing novel challenges for ectotherms. Reptilian embryos are vulnerable due to limited thermoregulation and lack of parental care. Tropical reptiles often face high nest temperatures and fluctuations in the wild, yet most incubation studies use constant temperatures that fail to reflect this thermal variation. We examined the effects of constant versus fluctuating temperatures on embryonic physiology, development, and hatchling size in the Indian rock agama, Psammophilus dorsalis. Eggs were incubated at constant 25°C, 30°C, or 35°C, or under fluctuating regimes F2 (25-30°C) and F3 (25-30-35°C). Constant 35°C caused complete mortality, while the warmest fluctuating regime (F3) resulted in dampened embryonic heart rate trajectories, faster egg mass gain, shorter development, lower hatching success, and smaller hatchlings. These findings highlight the costs of thermal load and the importance of using fluctuating regimes to assess climate vulnerability in tropical reptiles.