Do endotherms have thermal performance curves?

Abstract

Temperature is an important environmental factor governing the ability of organisms to grow, survive and reproduce. Thermal performance curves (TPCs), with some caveats, are useful for charting the relationship between body temperature and some measure of performance in ectotherms, and provide a standardized set of characteristics for interspecific comparisons. Endotherms, however, have a more complicated relationship with environmental temperature, as endothermy leads to a decoupling of body temperature from external temperature through use of metabolic heat production, large changes in insulation and variable rates of evaporative heat loss. This has impeded our ability to model endothermic performance in relation to environmental temperature as well as to readily compare performance between species. In this Commentary, we compare the strengths and weaknesses of potential TPC analogues (including other useful proxies for linking performance to temperature) in endotherms and suggest several ways forward in the comparative ecophysiology of endotherms. Our goal is to provide a common language with which ecologists and physiologists can evaluate the effects of temperature on performance. Key directions for improving our understanding of endotherm thermoregulatory physiology include a comparative approach to the study of the level and precision of body temperature, measuring performance directly over a range of body temperatures and building comprehensive mechanistic models of endotherm responses to environmental temperatures. We believe the answer to the question posed in the title could be ‘yes’, but only if ‘performance’ is well defined and understood in relation to body temperature variation, and the costs and benefits of endothermy are specifically modelled.

Antagonistic effects of long- and short-term environmental variation on species coexistence

“…From a biological perspective, our model can be applied to a variety of species for which thermal performance can be quantified; the performance component can be the survival of the species, its reproduction or other fitness components. Although in the literature the thermal performance curve is most often applied to the performance of ectotherms at different body temperatures [36,37], the concept of the thermal performance curve has been extended to the performance of different fitness components of organisms at different environmental or body temperatures [34,38,39]. As a result, our model can be applied to both endotherms and ectotherms.…”

Body Temperature Frequency Distributions: A Tool for Assessing Thermal Performance in Endotherms?

“…Although the general idea should be transferable from ectotherms to endotherms, the specifics will necessarily be different because body temperature is highly modulated by endotherms through enhanced physiological thermoregulation. In practice, physiological thermoregulation has made measuring thermal performance curves in endotherms exceedingly difficult ( Levesque and Marshall, 2021 ). A few researchers have managed to describe a thermal performance curve for a specific tissue group either in vitro ( James et al, 2015 ; Seebacher and Little, 2017 ) or in vivo ( Rummel et al, 2018 , 2019 ).…”

“…A few researchers have managed to describe a thermal performance curve for a specific tissue group either in vitro ( James et al, 2015 ; Seebacher and Little, 2017 ) or in vivo ( Rummel et al, 2018 , 2019 ). Even fewer have successfully measured whole animal performance across temperatures in endotherms ( Seymour et al, 1998 ; Rojas et al, 2012 ; reviewed in Levesque and Marshall, 2021 ).…”

“…Because thermal performance is difficult to measure directly in endotherms, several authors (including us) have suggested that distributions of body temperature might serve as a proxy for thermal performance ( Angilletta et al, 2010 ; Boyles and Warne, 2013 ; Levesque and Marshall, 2021 ). This suggestion is based on the idea that thermoregulation and thermal sensitivity have coadapted in endotherms, as appears to be the case in ectotherms.…”

“…It is telling that no such analyses exist. In fact, although researchers have found some utility in using thermal performance curves to address theoretical aspects of thermoregulation in endotherms ( Boyles et al, 2011 ; Boyles and Warne, 2013 ; Levesque and Marshall, 2021 ), few researchers have used them in empirical research (but see Seebacher and Little, 2017 ). An inherent limitation of using body temperature distributions as a proxy for thermal performance is that the entire idea is based on an unverified and very difficult to test assumption proposed by Angilletta et al (2010 ) that thermoregulation and thermal sensitivity are coadapted in endotherms.…”