Individual Differences in the Phenotypic Flexibility of Basal Metabolic Rate in Siberian Hamsters Are Consistent on Short- and Long-Term Timescales* Jan S. Boratyński 1,† Małgorzata Jefimow 1 Michał S. Wojciechowski 2 1 Department of Animal Physiology, Nicolaus Copernicus University, Toruń, Poland; 2 Department of Vertebrate Zoology, Nicolaus Copernicus University, Toruń, Poland Accepted 10/12/2016; Electronically Published 11/23/2016 Online enhancements: appendix figure. ABSTRACT Basal metabolic rate (BMR) correlates with the cost of life in endothermic animals. It usually differs consistently among individuals in a population, but it may be adjusted in response to predictable or unpredictable changes in the environment. The phenotypic flexibility of BMR is considered an adaptation to living in a stochastic environment; however, whether it is also repeatable it is still unexplored. Assuming that variations in phe- notypic flexibility are evolutionarily important, we hypothesized that they are consistently different among individuals. We pre- dicted that not only BMR but also its flexibility in response to changes in ambient temperature (T a ) are repeatable on short- and long-term timescales. To examine this, we acclimated Siberian hamsters (Phodopus sungorus) for 100 d to winterlike and then to summerlike conditions, and after each acclimation we exposed them interchangeably to 107 and 287C for 14 d. The difference in BMR measured after each exposure defined an individual’s phe- notypic flexibility (DBMR). BMR was repeatable within and among seasons. It was also flexible in both seasons, but in winter this flexibility was lower in individuals responding to seasonal changes than in nonresponding ones. When we accounted for individual responsiveness, the repeatability of DBMR was sig- nificant in winter (t p 0.48, P p 0.01) and in summer (t p 0.55, P p 0.005). Finally, the flexibility of BMR in response to changes in T a was also repeatable on a long-term timescale, that is, among seasons (t p 0.31, P p 0.008). Our results indicate the evolu- tionary importance of the phenotypic flexibility of energy me- tabolism and suggest that it may be subject to selection. Keywords: phenotypic flexibility, repeatability, acclimation, acclimatization, thermoregulation, energy metabolism, photo- responsiveness, season. Introduction Endothermic animals evolved mechanisms of heat production and conservation that enable them to maintain stable and high body temperatures (T b ) over a wide range of ambient tempera- tures (T a ; Hayes and Garland 1995). Since basal metabolic rate (BMR) is related to the size and maintenance costs of the meta- bolic machinery (Kleiber 1961; Daan et al. 1990) and because it correlates with an animal’s total energy expenditure (Ricklefs et al. 1996; but for the most recent discussion of the problem, see Portugal et al. 2016), it reflects the cost of life in endothermic animals (reviewed in Hulbert and Else 2000). Interspecific stud- ies (Nespolo and Franco 2007; Bushuev et al. 2011; Auer et al. 2016) show that in most species whole-animal metabolic rate (MR) is a repeatable trait. Repeatability describes the consistency of among-individual differences in a trait over time and pro- vides information on whether it may be selected for (Lessells and Boag 1987). It may also define the limit of heritability of a trait (Lynch and Walsh 1998; Dohm 2002). BMR and resting meta- bolic rate have been found to be heritable in several natural and laboratory populations (e.g., Konarzewski et al. 2005; Rønning et al. 2007; Sadowska et al. 2005, 2009; Zub et al. 2012), which indicates that consistent differences among individuals in their MR may be evolutionarily important (Rønning et al. 2005; Sza- frańska et al. 2007). Depending on the time of day or year and different envi- ronmental conditions, high or low BMR may be beneficial or disadvantageous. Many studies have shown that animal MR is more variable than fixed and that it changes with changing environmental conditions (for reviews, see Piersma and Drent 2003; Lovegrove 2005; McKechnie and Swanson 2010). Changes in the phenotype, which occur during individual life in response to changing environmental conditions, may be irreversible (i.e., plastic) or reversible (i.e., flexible; Piersma and Drent 2003; Pig- *This paper is based on a talk given at the ICCPB 2015 symposium, “Pheno- typic Flexibility of Energetics in a Seasonal World,” which was sponsored by the Division of Comparative Physiology and Biochemistry, SICB. Marek Konar- zewski, David Swanson, and Michał S. Wojciechowski were guest editors for this collection. †Corresponding author; e-mail: jan.boratyński@gmail.com. Physiological and Biochemical Zoology 90(2):000–000. 2017. q 2016 by The University of Chicago. All rights reserved. 1522-2152/2017/9002-6021$15.00. DOI: 10.1086/689870 000 This content downloaded from 158.075.054.053 on November 29, 2016 06:10:58 AM All use subject to University of Chicago Press Terms and Conditions (http://www.journals.uchicago.edu/t-and-c).