Body temperature variation in free-living and food-deprived yellow-necked mice sustains an adaptive framework for endothermic thermoregulation

Abstract

In many mammalian species, variation in body temperature (T b) exceeds the values suitable for defining homeothermy, making it justifiable and even necessary to resort to the term Bheterothermic^. However, T b data are only available for ca. 1% of extant mammalian species. We investigated variations in T b in wild free-living and experimentally food-deprived yellow-necked mice Apodemus flavicollis, during the temperate-zone autumn-winter period. In line with the adaptive framework for endothermic thermoregulation, we hypothesised that T b in the mice should be adjustable with the energetic cost-benefit trade-off associated with maintaining homeothermy. In laboratory conditions, mice clearly entered a state of daily torpor when food-deprived. Our study thus makes it clear that A. flavicollis is a heterothermic species in which food deprivation results in switching between endothermic and poikilothermic thermoregulation. We also assumed that, in free-living mice, heterothermy increases with elevated environmental challenges, e.g. when the ambient temperature (T a) decreases. Consistent with this was the inverse correlation noted between variation in T b in free-living mice and T a , with most individuals clearly becoming torpid when T a decreases below 0°C. It is the increased cost of food hoarding under cold conditions that most likely triggers a state of torpor as a last result. Overall, our study indicates that yellow-necked mice can provide a further example of species sustaining an adaptive framework for endothermic thermoregulation.

Seasonal Changes in Nycthemeral Availability of Sympatric Temperate Mixed Forest Rodents: The Predators’ Perspective

“…Apodemus mice possess several traits that enable them to evade diurnal activity. They dominate over competitors such as bank voles [60], consume and store high-energy food [31], and can slow down their metabolism under certain circumstances [61,62]. In addition, sharpened sensory capabilities make them well-equipped to survive nocturnal predation attempts [23].…”

Continuous growth through winter correlates with increased resting metabolic rate but does not affect daily energy budgets due to torpor use

“…To measure the metabolic rate and T b over a ∼24 h period (for details, see section “DEE during fasting”), we used another 30 mice (15 individuals from each year) captured in 20 traps set randomly during both years in mid-winter in the vicinity of the fixed 0.9 ha research plot (at a distance of ∼100–300 m). After ∼1 week of acclimation to laboratory conditions, those animals were intraperitoneally surgically implanted with miniaturized, paraffin wax-coated T b data loggers (procedure as described in Boratyński et al (2018 , 2019 )). The loggers (logger size = 14 mm × 14 mm, mass = 1.8 g, resolution = 0.0625°C) were set to record T b every 5 min.…”

“…In turn, during the autumn of the 2nd year, mice were maintained under laboratory conditions for ∼1 week following measurement. Once RMR had been recorded (within 1–2 days after capture), 6 of the mice in autumn of the 2nd year were implanted intraperitoneally with miniaturised paraffin wax-coated T b data-loggers, prior to release at the place of capture (for details, see Boratyński et al 2018 ).…”

“…Data for this study were collected as part of a larger study, which aimed mainly to measure heterothermy in yellow-necked mice (see Boratyński et al 2018 , 2019 ). Thus, there are some methodological discordances that may raise doubts and require better explanation.…”

Predation can shape the cascade interplay between heterothermy, exploration and maintenance metabolism under high food availability

“…A study of a variety of species indicated that hibernation (multi‐day torpor) and daily torpor (lasting <24 h) lower the energy expenditure associated with BMR by ~95% and ~65%, respectively (Ruf & Geiser, 2015 ). Several factors can trigger heterothermy use (Geiser & Brigham, 2012 ) but, the general function of torpor is to balance the energy budget, and individuals flexibly use this strategy in response to current energetic requirements (Boratyński et al., 2018 ; Boyles et al., 2007 ; Bozinovic et al., 2007 ; Eto et al., 2015 ; Fjelldal et al., 2021 ; Vuarin et al., 2013 ; Wojciechowski et al., 2007 ). Moreover, studies have found consistent among‐individual variation in torpor use, and less or more heterothermic animals co‐occur within populations (Boratyński et al., 2019 ; Dammhahn et al., 2017 ; Nespolo et al., 2010 ; Tapper et al., 2021 ), suggesting the existence of a continuum in thermoregulatory strategies (Angilletta et al., 2010 ; Boratyński et al., 2019 ; Boyles et al., 2013 ).…”