A New Comparative Metric for Estimating Heterothermy in Endotherms

Abstract

A major focus in the study of endothermic thermoregulation has been the description of thermoregulatory patterns used by various species and/or populations. Compared with ectotherms, relatively few attempts have been made to study the thermoregulation of endotherms in an adaptive framework. We believe that one of the main factors limiting this area of research has been the lack of an appropriate metric to directly compare body temperature (T b ) variation across all endothermic species. Thus, we present a simple comparative metric, the heterothermy index (HI), to quantify the expression of heterothermy by endotherms during a given time frame. Key advantages of HI are that (1) it represents a new analytical technique that has different strengths than the metrics commonly used to describe variation in T b , (2) it allows for evaluation of nonenergetic costs and benefits that affect the expression of heterothermy, and (3) it has the potential to unify research on homeotherms and heterotherms through quantitative comparative analyses that examine the entire continuum of thermoregulatory patterns. In short, we suggest that our metric provides a means to overcome one of the hurdles presently slowing the advancement of research on endothermic thermoregulation beyond the simple description of thermoregulatory patterns.

Torpor in Dark Times: Patterns of Heterothermy Are Associated with the Lunar Cycle in a Nocturnal Bird

“…1.) for each individual (Boyles et al 2011;McKechnie et al 2007) and then estimated the extent of heterothermy for each bird on each night using a simple metric called the Heterothermy Index (HI):…”

“…The HI gives an estimate of variation in T skin around the modal T skin and does not rely on an arbitrary cut-off for defining torpor bouts as has been used in most studies (Boyles et al 2011). We calculated HI values for the period between 19h00 and 7h00, which was when sampling was most intensive, and ensured that we did not include morning torpor bouts into calculations of nocturnal heterothermy.…”

Adaptive thermoregulation during summer in two populations of an arid‐zone passerine

“…We followed Boyles et al (2011b) and assumed that modal body temperature (T mod , defined as the most frequently measured body temperature to the nearest 0.18C), measured at temperatures when the birds were active but unlikely to be experiencing thermal stress, approximated active-phase optimal, or set-point, T b . We determined T mod for each bird by pooling all T b measurements made between 06:00 and 10:00 hours.…”

“…The random effects (fitted through model selection) of individual and date were included in a nested design because T b measurements for individuals were repeated on 18-28 days. We investigated the extent of daily heterothermy (i.e., T b variation across the circadian cycle) expressed by each individual by calculating a heterothermy index (HI; Boyles et al 2011b), using 24-h T b measurements (midnight to midnight). The HI value quantifies deviation of T b away from modal T b , and provides a quantitative index of heterothermy.…”

Specialist-generalist model of body temperature regulation can be applied on the intraspecific level

“…In total, we analyzed T b recordings from six individuals measured at each T a . Following Boyles et al (2011b), we used the heterothermy index (HI), which is a statistical description of T b variations that can be used for all thermoregulating animals:…”