Three questions about the eco-physiology of overwintering underground.

In cold environments ectotherms can be dormant underground for long periods. In 1941 Cowles proposed an ecological trade-off involving the depth at which ectotherms overwintered: on warm days, only shallow reptiles could detect warming soils and become active; but on cold days, they risked freezing. Cowles discovered that most reptiles at a desert site overwintered at shallow depths. To extend his study, we compiled hourly soil temperatures (5 depths, 90 sites, continental USA) and physiological data, and simulated consequences of overwintering at fixed depths. In warm localities shallow ectotherms have lowest energy costs and largest reserves in spring, but in cold localities, they risk freezing. Ectotherms shifting hourly to the coldest depth potentially reduce energy expenses, but paradoxically sometimes have higher expenses than those at fixed depths. Biophysical simulations for a desert site predict that shallow ectotherms have increased opportunities for mid-winter activity but need to move deep to digest captured food. Our simulations generate testable predictions to eco-physiological questions but rely on physiological responses to acute cold rather than to natural cooling profiles. Furthermore, natural-history data to test most predictions do not exist. Thus, our simulation approach uncovers knowledge gaps and suggests research agendas for studying ectotherms overwintering underground.