Basal Metabolic Rate (BMR) is the lowest amount of energy used by a resting, fasting individual who is not consuming any energy to warm up or cool down, and is one of the most studied aspects of animal physiology. However,scientists haven't yet answered all the questions associated with BMR, as a recent publication by Greg Russell and Mark Chappell from the University of California, Riverside, tells us. They tested a common assumption that BMR is consistent over an animal's lifetime in the deer mouse Peromyscus maniculatus, and found some unexpected results.

For decades, physiologists have extensively measured the BMR of many warm-blooded animals and linked their findings to an animal's fitness. By doing this, scientists are assuming that offspring inherit their BMR from their parents, meaning that BMR should be consistent over an animal's lifetime, but will change between generations in response to selection pressure. A few previous studies have measured consistent BMR over an animal's lifetime, supporting this assumption. However, other studies have shown the opposite: that environmental factors such as altitude and temperature influence BMR. With this in mind, Russell and Chappell wanted to confirm if the assumption that BMR is consistent and responds to selection pressure was accurate in deer mice, and if BMR responds to changes in altitude and temperature.

First, the team investigated whether altitude and temperature influence BMR. They bred two groups of experimental captive mice: one group were born and bred at 340 m, and the other group in the mountains, about 4000 m above sea level. Measuring the animals' BMR, they found no difference between low-and high-altitude mice, suggesting that altitude alone doesn't affect BMR. To find out how temperature and altitude together affect BMR, the team divided both the high-altitude and the low-altitude mice into two groups, keeping one group at a warm 21°C and the other at a chilly 5°C. They found that the combination of high-altitude and 5°C temperatures raised BMR, but the other groups were unaffected. This finding agrees with previous studies which showed that cold temperatures and high-altitude influence BMR during an animal's lifetime.

To find out if BMR remained consistent in the mice, supporting the assumption that it responds to selection pressure, the team measured BMR in the mice again after 1–2 months of adapting to their new temperatures. Expecting to find the same BMR in both measurements, the research duo were astonished to find out that BMR in deer mice was not consistent over time. This was the team's most surprising result and heavily contradicts previous studies that show BMR consistency.

The authors conclude that – at least in deer mice – researchers can't make the assumption that BMR responds to selection pressure because it is not consistent over an animal's lifetime. Instead, the researchers propose that selection could affect BMR indirectly, for example by acting on physiological traits that experience selection, such as maximal aerobic capacity, which in turn influence BMR by increasing or decreasing metabolic demands. Russell and Chappell's findings show that researchers should be cautious when assuming that BMR changes in response to selection pressure in all mammalian species, and will probably start a lively discussion on the evolutionary basis of BMR and the assumptions that researchers make in experiments.

Russell, G. A. and Chappell, M. A. (
). Is BMR repeatable in deer mice? Organ mass correlates and the effects of cold acclimation and natal altitude.
J. Comp. Physiol. B