title: How low can you go? An adaptive energetic framework for interpreting basal metabolic rate variation in endotherms.
abstract: Adaptive explanations for both high and low body mass-independent basal metabolic rate (BMR) in endotherms are pervasive in evolutionary physiology, but arguments implying a direct adaptive benefit of high BMR are troublesome from an energetic standpoint. Here, we argue that conclusions about the adaptive benefit of BMR need to be interpreted, first and foremost, in terms of energetics, with particular attention to physiological traits on which natural selection is directly acting. We further argue from an energetic perspective that selection should always act to reduce BMR (i.e., maintenance costs) to the lowest level possible under prevailing environmental or ecological demands, so that high BMR per se is not directly adaptive. We emphasize the argument that high BMR arises as a correlated response to direct selection on other physiological traits associated with high ecological or environmental costs, such as daily energy expenditure (DEE) or capacities for activity or thermogenesis. High BMR thus represents elevated maintenance costs required to support energetically demanding lifestyles, including living in harsh environments. BMR is generally low under conditions of relaxed selection on energy demands for high metabolic capacities (e.g., thermoregulation, activity) or conditions promoting energy conservation. Under these conditions, we argue that selection can act directly to reduce BMR. We contend that, as a general rule, BMR should always be as low as environmental or ecological conditions permit, allowing energy to be allocated for other functions. Studies addressing relative reaction norms and response times to fluctuating environmental or ecological demands for BMR, DEE, and metabolic capacities and the fitness consequences of variation in BMR and other metabolic traits are needed to better delineate organismal metabolic responses to environmental or ecological selective forces.
authors.affiliation: Department of Biology, University of South Dakota, 414 East Clark Street, Vermillion, SD, 57069, USA. david.swanson@usd.edu.
authors.forename: David L
authors.identifier: http://orcid.org/0000-0003-2258-7335
authors.initials: DL
authors.lastname: Swanson
authors.affiliation: Department of Zoology and Entomology, DST-NRF Centre of Excellence at the Percy FitzPatrick Institute, University of Pretoria, Private Bag X20, Hatfield, 0028, South Africa.
authors.forename: Andrew E
authors.initials: AE
authors.lastname: McKechnie
authors.affiliation: Département de Biologie, Chimie et Géographie, Université du Québec à Rimouski, Rimouski, QC, Canada.|Groupe de recherche sur les environnements nordiques BORÉAS, Centre d'Études Nordiques, Centre de la Science de la Biodiversité du Québec, Rimouski, QC, Canada.
authors.forename: François
authors.initials: F
authors.lastname: Vézina
mesh_terms: D000222:Adaptation, Physiological; D000818:Animals; D001481:Basal Metabolism; D006801:Humans; D012641:Selection, Genetic
keywords: Basal metabolic rate; Daily energy expenditure; Endotherms; Energetics; Evolutionary physiology; Selection