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.
Growth models are a fundamental aspect of metabolic theory but remain controversial. It is a century since the first theoretical model of growth was put forward by Pütter. His insights were deep, but his model ended up being attributed to von Bertalanffy and his ideas largely forgotten. Here I review Pütter’s ideas and trace their influence on existing theoretical models for growth and other aspects of metabolism, including those of von Bertalanffy, the Dynamic Energy Budget (DEB) theory, the Gill-Oxygen Limitation Theory and the Ontogenetic Growth Model (OGM). I then synthesise, compare and critique the ideas of the two most comprehensive theories, DEB and the OGM, in relation to Pütter’s original ideas, and discuss how these theories have been used to explain ‘macrometabolic’ patterns including the scaling of respiration, the temperature size rule (first modelled by Pütter), and the connection to life history. Although theoretical work on growth and metabolism has generally proceeded in an un-coordinated and disconnected fashion, significant progress has been made and it has been built upon the original and fundamental insights of Pütter. What we need now is a coordinated empirical research program to test the existing ideas and motivate new theoretical directions.