Long-term repeatability of body mass and body temperature (but not basal metabolism) in the free-ranging leaf-eared mouse

Question: The value of a trait must be repeatable (i.e. consistent in each individual over time) for natural selection to have the power to mould it. How repeatable are physiological (energetic) traits in the field during a prolonged period? Methods: During each of 17 months, I measured body mass, body temperature, and basal metabolic rate in adult males and in non-reproductive females of the leaf-eared mouse, Phyllotis darwini (Muridae) in the Mediterranean habitats of central Chile. I analysed the trait values obtained in the earliest and latest appearances of each individual in the data set. Results: The absolute value of the difference between the two measurements (earliest and latest) of a trait did not depend on the time interval between them. Thus, one could assess repeatability by comparing the measurements of all individuals regardless of the size of that time interval. Both body mass and body temperature were repeatable. However, neither whole basal metabolic rate nor mass-independent basal metabolism was repeatable in this mouse. Conclusions: Both body mass and body temperature may be able to respond to current selective forces, but basal metabolism may not be. That is, most species of endotherms do show repeatability in basal metabolism, but it is not safe to assume that all do.