Modelling mammalian energetics: the heterothermy problem

Abstract

Global climate change is expected to have strong effects on the world's flora and fauna. As a result, there has been a recent increase in the number of meta-analyses and mechanistic models that attempt to predict potential responses of mammals to changing climates. Many models that seek to explain the effects of environmental temperatures on mammalian energetics and survival assume a constant body temperature. However, despite generally being regarded as strict homeotherms, mammals demonstrate a large degree of daily variability in body temperature, as well as the ability to reduce metabolic costs either by entering torpor, or by increasing body temperatures at high ambient temperatures. Often, changes in body temperature variability are unpredictable, and happen in response to immediate changes in resource abundance or temperature. In this review we provide an overview of variability and unpredictability found in body temperatures of extant mammals, identify potential blind spots in the current literature, and discuss options for incorporating variability into predictive mechanistic models.

Does metabolism constrain bird and mammal ranges and predict shifts in response to climate change?

“…The Scholander‐Irving model we employ provides a tractable approximation of metabolic constraints, but we highlight ways that refining metabolic estimates could improve upon the analyses. We estimate metabolic costs assuming homeothermy, but many studies highlight that endotherms exhibit a continuum of heterothermy (Boyles et al, ; Levesque, Nowack, & Stawski, ). Consideration of the occurrence of torpor/hibernation in the present study only partially accounted for deviations from thermoregulation due to T b variation.…”

Revisiting concepts of thermal physiology: Predicting responses of mammals to climate change

“…We take the view of Porter and Kearney () and of Levesque et al. () that TNZ has no direct ecological meaning. That animals thrive at ambient temperatures above the UCT indicate that life outside the TNZ is sustainable energetically (e.g.…”

“…Traditionally, CT min and CT max are the ambient temperatures at which ectotherms either become immobile, according to some researchers, or die, according to others. However, even for ectotherms (Sinclair et al., ), and certainly for large mammals, there will not be a single ambient lower or upper temperature at which an animal will perish (Levesque et al., ). There will be a trade‐off between the duration of exposure to the heat load and the intensity of the load, as there is for ectotherms (Rezende, Castañeda, & Santos, ; Terblanche et al., ), so that an ambient temperature that is not immediately lethal may lead to death with sustained exposure.…”

Hibernation in bats (Mammalia: Chiroptera) did not evolve through positive selection of leptin

“…Pteronotus davyi , Nyctimene major , Macroglossus minimus , and Syconyteris australis all represent reversals back to daily heterothermy after their lineages evolved homeothermy. However, due to inconsistent methods for measuring T b and the setting of arbitrary thresholds to determine a torpid state (Levesque et al, ), some of the bats categorized as homeothermic in our study may in fact be heterothermic. This may alter the interpretation of reversals back to heterothermy.…”

“…However, this binary categorization of heterothermy has been debated, with some authors arguing that hard boundaries between hibernation and daily torpor do not exist (Boyles et al, ; Canale, Levesque, & Lovegrove, ; Geiser & Ruf, ; Ruf & Geiser, ; van Breukelen & Martin, ). As new methods are developed, support for a continuum of heterothermic abilities has been growing but a consensus has not been reached (Boyles, Bennett, Mohammed, & Alagaili, ; Levesque, Nowack, & Stawski, ).…”