Three questions about the eco‐physiology of overwintering underground

Abstract

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.

Snow modulates winter energy use and cold exposure across an elevation gradient in a montane ectotherm

“…Over the past decade, snowy years increased energy use and the probability of lethal energy depletion (Figures 4 and 5), particularly at low elevations, and the snowy plot in the field snow‐manipulation experiment (which was a low elevation site) caused almost complete (and possibly lethal) energy depletion. This addresses calls for a greater focus on the factors driving overwinter mortality (Huey et al, 2021), and adds to a growing body of evidence that snow can cause energy depletion in insects at low elevations (Irwin & Lee Jr, 2003; Marshall & Sinclair, 2012). Our energy use model suggests that this constraint is lifted at high elevations due to low energetic costs from lower winter air temperatures and snow buffering from warm spring temperatures that can drain energy stores.…”

“…Two of the most important traits determining winter performance and survival are cold tolerance and energetics (Huey et al, 2021), which influence winter fitness and set range limits for a wide range of species (Humphries et al, 2003;Marshall et al, 2020;Osland et al, 2021;Overgaard et al, 2014;Shuter & Post, 1990).…”

“…Two of the most important traits determining winter performance and survival are cold tolerance and energetics (Huey et al, 2021), which influence winter fitness and set range limits for a wide range of species (Humphries et al, 2003; Marshall et al, 2020; Osland et al, 2021; Overgaard et al, 2014; Shuter & Post, 1990). If microclimate temperatures drop below a population‐ or species‐specific cold tolerance threshold, which depends on the combination of exposure time and temperature, then mortality or sublethal cold injuries can result (Bale, 1996; Sinclair et al, 2003).…”

A hierarchical approach to understanding physiological associations with climate

“…As with T pref , our study suggests that climate drives the evolution of CT max indirectly through its effects on vegetation cover and hence shading and other buffering effects (Kellerman, Overgaard et al, 2012). Indeed, it has been demonstrated that lizard species from forested habitats have lower CT max (Huey et al, 2009(Huey et al, , 2021.…”

“…Across all spatial scales, minimum air temperatures were negatively associated with CT min , and at micro and biophysical scales CT min was also positively related to maximum air/ body temperatures. Ectotherms frequently experience body temperatures below the CT min in nature, mostly when they are inactive (Huey et al, 2021), and therefore unable to thermoregulate. Thus, the very general relationship we observed between CT min and our temperature metrics at all hierarchical scales is not unexpected.…”