Abstract
Endotherms regulate their core body temperature by adjusting metabolic heat production and insulation. Endothermic body temperatures are therefore relatively stable compared to external temperatures. The thermal sensitivity of biochemical reaction rates is thought to have co-evolved with body temperature regulation so that optimal reaction rates occur at the regulated body temperature. However, recent data show that core body temperatures even of non-torpid endotherms fluctuate considerably. Additionally, peripheral temperatures can be considerably lower and more variable than core body temperatures. Here we discuss whether published data support the hypothesis that thermal performance curves of physiological reaction rates are plastic so that performance is maintained despite variable body temperatures within active (non-torpid) endotherms, and we explore mechanisms that confer plasticity. There is evidence that thermal performance curves in tissues that experience thermal fluctuations can be plastic, although this question remains relatively unexplored for endotherms. Mechanisms that alter thermal responses locally at the tissue level include transient potential receptor ion channels (TRPV and TRPM) and the AMP-activated protein kinase (AMPK) both of which can influence metabolism and energy expenditure. Additionally, the thermal sensitivity of processes that cause post-transcriptional RNA degradation can promote the relative expression of cold-responsive genes. Endotherms can respond to environmental fluctuations similarly to ectotherms, and thermal plasticity complements core body temperature regulation to increase whole-organism performance. Thermal plasticity is ancestral to endothermic thermoregulation, but it has not lost its selective advantage so that modern endotherms are a physiological composite of ancestral ectothermic and derived endothermic traits.
Highlights
The basic principles of thermodynamics dictate that the rates of physiological functions in both endotherms and ecotherms are sensitive to changes in temperature (Landeira-Fernandez et al, 2012; Tattersall et al, 2012; Arcus et al, 2016; Else, 2016)
The notion that optimal physiological reaction rates of endotherms have evolved to be fixed within a narrow range of regulated body temperatures is questionable
Endothermic thermoregulation is distinct from that of ectotherms, but thermal plasticity of physiological reaction rates can be as advantageous in endotherms as in ectotherms
Summary
The basic principles of thermodynamics dictate that the rates of physiological functions in both endotherms and ecotherms are sensitive to changes in temperature (Landeira-Fernandez et al, 2012; Tattersall et al, 2012; Arcus et al, 2016; Else, 2016). Thermal performance curves represent the change in a physiological reaction rate across a range of acute temperatures (Figure 1A). Thermal plasticity in response to a (non-acute) chronic change in body temperature may manifest as a horizontal shift in the performance curve, so that maximal performance (mode) occurs at the new temperature.
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