The influence of climate on the basal metabolic rate of small mammals: a slow-fast metabolic continuum

The influence of climate (mean annual rainfall, rainfall variability, ambient temperature, T(a)) on the basal metabolic rate (BMR) of 267 small mammals (<1 kg) from six zoogeographical zones was investigated using conventional and phylogenetically independent data (linear contrasts). All climate variables varied between zones, as did BMR and body temperature ( T(b)), but not thermal conductance. Holarctic zones were more seasonal and colder, but rainfall was less variable, than non-Holarctic zones. In general, the BMR was most strongly influenced by body mass, followed by T(a) and the rainfall variables. However, there was significant variation in the strength of these relationships between zones. BMR and T(b) increased with latitude, and mass-independent BMR and T(b) were positively correlated. The latter relationship offers evidence of a slow-fast metabolic continuum in small mammals. The fast end of the continuum (high BMR) is associated with the highest latitudes where BMR is most strongly influenced by T(a) and mean annual rainfall (i.e. mean productivity). The slow end of the continuum (low BMR) is associated with the semi-tropics, low productivity zones, and climatically unpredictable zones, such as deserts. Here rainfall variability has the strongest influence on BMR after body size. The implications of a slow-fast metabolic continuum are discussed in terms of various models associated with the evolution of BMR, such as the aerobic capacity models and the "energetic definition of fitness" models.