Heterothermy in large mammals: inevitable or implemented?

• Published in: Biological reviews of the Cambridge Philosophical Society Vol. 91; no. 1; pp. 187 - 205
• Main Authors: Hetem, Robyn S., Maloney, Shane K., Fuller, Andrea, Mitchell, Duncan
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.02.2016
• Subjects: Adaptation, Physiological; Animals; biologging; body size; Body Size - physiology; Body temperature; Body Temperature Regulation - physiology; Habitats; Herbivores; Herbivory - physiology; heterothermy index; Homeostasis; hyperthermia; hypothermia; Mammals; Mammals - physiology; body size; heterothermy index; hyperthermia; body temperature; hypothermia; biologging
• ISSN: 1464-7931; 1469-185X
• DOI: 10.1111/brv.12166

ABSTRACT Advances in biologging techniques over the past 20 years have allowed for the remote and continuous measurement of body temperatures in free‐living mammals. While there is an abundance of literature on heterothermy in small mammals, fewer studies have investigated the daily variability of body core temperature in larger mammals. Here we review measures of heterothermy and the factors that influence heterothermy in large mammals in their natural habitats, focussing on large mammalian herbivores. The mean 24 h body core temperatures for 17 species of large mammalian herbivores (>10 kg) decreased by ∼1.3°C for each 10‐fold increase in body mass, a relationship that remained significant following phylogenetic correction. The degree of heterothermy, as measured by the 24 h amplitude of body core temperature rhythm, was independent of body mass and appeared to be driven primarily by energy and water limitations. When faced with the competing demands of osmoregulation, energy acquisition and water or energy use for thermoregulation, large mammalian herbivores appear to relax the precision of thermoregulation thereby conserving body water and energy. Such relaxation may entail a cost in that an animal moves closer to its thermal limits for performance. Maintaining homeostasis requires trade‐offs between regulated systems, and homeothermy apparently is not accorded the highest priority; large mammals are able to maintain optimal homeothermy only if they are well nourished, hydrated, and not compromised energetically. We propose that the amplitude of the 24 h rhythm of body core temperature provides a useful index of any compromise experienced by a free‐living large mammal and may predict the performance and fitness of an animal.