Abstract
Ectotherms in cold environments often spend long winters underground. In
1941 Raymond Cowles proposed a novel ecological trade-off involving
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 then simulated consequences of overwintering at
fixed depths. In warm localities shallow ectotherms have low energy
costs and largest reserves in spring; but in cold localities, shallow
ectotherms risk freezing. Ectotherms shifting to the coldest depth
potentially reduce energy expenses, but paradoxically sometimes have
higher expenses than those at fixed depths. Biophysical simulations for
one desert site predict that shallow ectotherms should have elevated
opportunities for mid-winter activity but may need to move deep to
digest captured food. Our simulations generate testable
eco-physiological predictions but rely on physiological responses to
acute cold rather to natural cooling profiles. Furthermore, testing
ecological predictions requires natural-history data that do not exist.
Thus, our simulation approach uncovers “unknown unknowns” and suggests
research agendas for studying ectotherms overwintering underground.