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

Abstract

There is increasing recognition that rather than being fully homeothermic, most endotherms display some degree of flexibility in body temperature. However, the degree to which this occurs varies widely from the relatively strict homeothermy in species, such as humans to the dramatic seasonal hibernation seen in Holarctic ground squirrels, to many points in between. To date, attempts to analyse this variability within the framework generated by the study of thermal performance curves have been lacking. We tested if frequency distribution histograms of continuous body temperature measurements could provide a useful analogue to a thermal performance curve in endotherms. We provide examples from mammals displaying a range of thermoregulatory phenotypes, break down continuous core body temperature traces into various components (active and rest phase modes, spreads and skew) and compare these components to hypothetical performance curves. We did not find analogous patterns to ectotherm thermal performance curves, in either full datasets or by breaking body temperature values into more biologically relevant components. Most species had either bimodal or right-skewed (or both) distributions for both active and rest phase body temperatures, indicating a greater capacity for mammals to tolerate body temperatures elevated above the optimal temperatures than commonly assumed. We suggest that while core body temperature distributions may prove useful in generating optimal body temperatures for thermal performance studies and in various ecological applications, they may not be a good means of assessing the shape and breath of thermal performance in endotherms. We also urge researchers to move beyond only using mean body temperatures and to embrace the full variability in both active and resting temperatures in endotherms.

Assessing responses to heat in a range-shifting, nocturnal, flying squirrel

“…We currently lack a comprehensive, comparative data set of mammalian body temperature rhythms. However, the difference between the modal active (39.4 °C) and resting (37.3 °C) body temperatures observed in this study (2.1 °C) was higher than most of the 14 species analyzed in a recent review ( Levesque et al 2021 ). What is perhaps more surprising is that both the resting and active modal temperatures are in the upper percentiles for mammals with the resting temperatures near the 70th percentile and active temperatures falling above the 80th percentile in the range of supraendotherms (sensu Lovegrove 2012 ).…”

Physiological Performance Curves: When Are They Useful?

“…We partitioned curve shapes, curve shifts, and functional boundaries into distinct subsections for simplicity, but we acknowledge that biological systems are rarely so defined. The complexity of natural systems is reflected by the studies included in this special issue, where comparisons within and between individuals, populations, endotherms and ectotherms ( Levesque et al, 2021 ), and phylogenetic differences between species highlight the complex interactions that exist between evolutionary forces and epigenetic mechanisms underlying plasticity. Focusing on multiple traits across functional boundaries can also help elucidate mechanisms that underlie animal responses to environmental change and make predictions about the impacts of lower-level responses on higher levels of biological organization.…”