Variation in reproductive investment increases body temperature amplitude in a temperate passerine

Jan-Åke Nilsson,Andreas Nord

doi:10.1007/s00442-021-05026-2

Abstract

Many birds and mammals show substantial circadian variation in body temperature, which has been attributed to fluctuations in ambient temperature and energy reserves. However, to fully understand the variation in body temperature over the course of the day, we also need to consider effects of variation in work rate. We made use of a dataset on body temperature during the resting and active periods in female marsh tits (Poecile palustris) that bred in a temperate area and were subjected to experimental changes in reproductive investment through brood size manipulations. Furthermore, the amplitude increased with daytime, but were unaffected by nighttime, ambient temperature. Amplitudes in females with manipulated broods were 44% above predictions based on inter-specific allometric relationships. In extreme cases, amplitudes were > 100% above predicted values. However, no individual female realised the maximum potential amplitude (8.5 °C, i.e. the difference between the highest and lowest body temperature within the population) but seemed to prioritise either a reduction in body temperature at night or an increase in body temperature in the day. This suggests that body temperature amplitude might be constrained by costs that preclude extensive use of both low nighttime and high daytime body temperatures within the same individual. Amplitudes in the range found here (0.5–6.7 °C) have previously mostly been reported from sub-tropical and/or arid habitats. We show that comparable values can also be found amongst birds in relatively cool, temperate regions, partly due to a pronounced increase in body temperature during periods with high work rate.

Highlights

During recent decades, an increasing number of studies have shown that body temperature of endothermic homeotherms, i.e. birds and mammals, varies much more than previously appreciated (Angiletta et al 2010)
The fact that benign ambient temperatures and ample food availability reduce the decrease in nighttime body temperature (Nord et al 2009, 2011), and that readily available water reduces the extent of hyperthermia (Hetem et al 2010), suggests that there are physiological costs for deviating from normothermia (Angiletta et al 2010; Boyles et al 2011)
The body temperature amplitude increased with increasing maximum daytime ambient temperature (Table 1; Fig. 2) but nighttime minimum temperature did not affect the amplitude (Table 1)

Summary

Introduction

An increasing number of studies have shown that body temperature of endothermic homeotherms, i.e. birds and mammals, varies much more than previously appreciated (Angiletta et al 2010). Many endotherms reduce body temperature during the night, presumably to save energy (McKechnie and Lovegrove 2002; Ruf and Geiser 2015). Endotherms may let body temperature increase above normothermia (Hetem et al 2010; Nilsson et al 2016) to save water and energy from reduced cooling demands. Endothermic homeotherms may face a trade-off between maintenance of normothermic body temperature and the use of limited energy and water resources for activities other than thermoregulation

Methods

Results

Conclusion