Abstract
Mammalian heterotherms, species that employ short or long periods of torpor, are found in many different climatic regions. Although the underlying physiological mechanisms of heterothermy in species from lower latitudes (i.e. the tropics and southern hemisphere) appear analogous to those of temperate and arctic heterotherms, the ultimate triggers and resulting patterns of energy expenditure and body temperature are often noticeably different. Phenotypic flexibility in the patterns of thermoregulation in non-Holarctic species can be extensive (depending on body condition, environmental parameters and species competition) and the factors responsible for inducing heterothermy are more variable in non-Holarctic species. As well as being a regular adaptation to seasonality, heterothermy can also be employed as a response to unpredictability in environmental parameters and as a response to emergency situations. Non-Holarctic heterotherms also challenge the notion that regular inter-bout arousals during hibernation are obligatory and suggest all that is necessary to maintain proper functioning during hibernation is an occasional passive return to -or maintenance of - a relatively high body temperature. The study of non-Holarctic heterotherms has led to the conclusion that heterothermy must be defined on the basis of mechanistic, physiological parameters, and not solely by body temperature; yet we are still limited in our abilities to record such mechanistic parameters in the field. It is now believed that homeothermy in mammals evolved in hot climates via an ancestral heterothermic state. Similar to extant warm-climate heterotherms, early mammals could have relied mainly on passive body temperature regulation with a capacity for short- to longer-term up-regulation of metabolism when needed. Hibernation, as seen in temperate and arctic species may then be a derived state of this ancestral heterothermy, and the study of torpor in warm climates can provide potential models for the energetics of early mammals.
Highlights
Torpor in heterothermic endotherms is a controlled, reversible depression of metabolic rate, and active thermoregulation, well below the usual daily cycle
Torpor and hibernation patterns in non-Holarctic species are similar to those observed in temperate/arctic animals, albeit with some distinctive differences
They exhibit daily fluctuations in Tb and use torpor flexibly (Kuchel, 2003; Grigg et al, 2004; Nicol and Andersen, 2006). Depending on their habitat they increase torpor use during the cold period and either show long-term hibernation (e.g., Tasmania or Australian Alps: Augee and Ealy, 1968; Grigg et al, 1989; Nicol and Andersen, 2002) or prolonged torpor lasting for a few days (Brice et al, 2002; Kuchel, 2003; Western Australian Wheatbelt: Nowack et al, 2016). This form of continuous heterothermy, either via torpor or highly variable Tb, has to date predominantly been found in spiny, terrestrial insectivores such as tenrecs and echidnas, it may possibly exist in other groups
Summary
Torpor in heterothermic endotherms is a controlled, reversible depression of metabolic rate, and active thermoregulation, well below the usual daily cycle (sensu Geiser and Ruf, 1995). Over the last two to three decades, it has become apparent that torpid states in endotherms are employed in a wide range of ecological and physiological settings and under contrasting conditions (Cossins and Barnes, 1996; Geiser and Brigham, 2012; Boyles et al, 2013; Levesque et al, 2016; Nowack et al, 2017) This challenges the “traditional” view of torpor as essentially an adaptation to mismatches between energy supply and demand during cold seasonal northern hemisphere winters (Geiser, 2004b; Heldmaier et al, 2004; McKechnie and Mzilikazi, 2011). Torpor and hibernation patterns in non-Holarctic species are similar to those observed in temperate/arctic animals, albeit with some distinctive differences As far as it is known, the physiological basis seems analogous: metabolism (and other physiological variables, such as heart rate, respiration rate, etc.) is actively depressed to a fraction of euthermic levels, usually during the daily resting phase or at the end of the activity phase, Frontiers in Ecology and Evolution | www.frontiersin.org
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