Individual Differences in the Phenotypic Flexibility of Basal Metabolic Rate in Siberian Hamsters Are Consistent on Short- and Long-Term Timescales

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 phenotypic flexibility are evolutionarily important, we hypothesized that they are consistently different among individuals. We predicted 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 10 degrees and 28 degrees C for 14 d. The difference in BMR measured after each exposure defined an individual's phenotypic flexibility (BMR). 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 BMR was significant in winter ( = 0.48, P = 0.01) and in summer ( = 0.55, 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 ( = 0.31, P = 0.008). Our results indicate the evolutionary importance of the phenotypic flexibility of energy metabolism and suggest that it may be subject to selection.