When it comes to natural selection, variation is the key. One characteristic shared by all organisms that maintain a stable body temperature is basal metabolic rate, `and since all aspects of life use some form of energy' says Claus Bech from the Norwegian University of Science and Technology, `energy has been termed the currency of evolution'. Bech explains that basal metabolic rates seem to vary enormously between species, and this was thought to be due to adaptations to different environments; but this assumption has only been tested rarely. Bech explains that for evolution to act on a characteristic `three fundamental prerequisites have to be met'. First the characteristic must show a consistent variation across the species;second, it must be possible to pass it on genetically; and finally, the characteristic must benefit the organism and make it better suited to its environment. Bech decided to investigate whether or not the variations seen in basal metabolic rate across species could be due to adaptive evolution, but first he needed to test the first prerequisite; were the variations in basal metabolic rate across a flock of birds consistent over a long period of time(p. 4663)?

Bech and his colleagues, Bernt Rønning and Børge Moe, decided to look at basal metabolic rates across a group of zebra finches. Bech explains that zebra finches reproduce rapidly, giving his team the opportunity to investigate the birds' inheritance patterns further down the line. But first, Rønning began working with a smaller group of birds, 18 pairs,measuring each individual's basal metabolic rate over a period of 45 days. Fortunately, Rønning could take metabolic measurements on four birds simultaneously, having constructed four tiny respirometry chambers from paint boxes. Then he and Moe calculated the `repeatability', which gives an indication of the consistency of the ranking of each individual's basal metabolic rate across the group of birds. The value came out quite high; 0.6. The variation in basal metabolic rate across the birds seemed to be largely due to differences between the individuals.

But how would the birds' repeatability fair if their basal metabolic rates were measured again 2 years later? Rønning returned to the aviary,remeasuring the birds' basal metabolic rates and recalculated the repeatability value. The team were astonished when they realised that the value was essentially the same as it had been 2 years before. The bird's repeatability hadn't changed at all; birds with low metabolic rates were still low, while those with higher values were still high. Bech and Rønning admit that this was a surprise, especially given that other measurements of basal metabolic rates in other creatures suggested that repeatability decreased with time.

The Norwegian team's results suggest that there are consistent differences between basal metabolic rate between individuals, so they have satisfied the first criterion for basal metabolic rate to be prone to natural selection. Bech and his team are now focusing on the next criterion that must be met,genetic inheritance. However, he adds that he can only test the genetic effects of evolution in a much larger group of birds where he knows their family tree, and that will take a long time to reconstruct.

Rønning, B., Moe, B. and Bech, C.(
). Long-term repeatability makes basal metabolic rate a likely heritable trait in the zebra finch Taeniopygia guttata.
J. Exp. Biol.