DNA can mutate and change imperceptibly every time a cell divides and makes a copy of itself.

But when one of these mutations causes a change that is advantageous for the animal - for example, rendering it resistant to a particular disease - it is often "selected for", or passed down to the next few generations of that same species.

Such changes, which create differences within a population but do not give rise to new species, are known as "microevolution".

The idea that microevolution happens faster in warmer environments is not new. But this is the first time the effect has been shown in mammals, which regulate their own body temperature.

"The result was unexpected," said Len Gillman from Auckland University of Technology, who led the study.

"We have previously found a similar result for plant species and other groups have seen it in marine animals. But since these are 'ectotherms' - their body temperature is controlled directly by the environment - everyone assumed that the effect was caused by climate altering their metabolic rate."

Scientists believe that this link between temperature and metabolic rate means that, in warmer climates, the germ cells that eventually develop into sperm and eggs divide more frequently.

"An increase in cell division provides more opportunities for mutations in the population over a given time," explained Dr Gillman.

"This increases the probability of advantageous mutations that are selected for within the species."

'Sister species'

"We suspected the same effect might be happening in mammals, because seasonal changes affect the animals' activity," Dr Gillman told BBC News.

He and his team compared the DNA of 130 pairs of mammals, looking at genetically similar "sister species" - where each of the pair lived at a different latitude or elevation.

They tracked changes in one gene that codes for a protein known as cytochrome b, comparing the same gene in each of the pair of mammals to a "reference" gene in a common ancestor.

By looking for mutations in the DNA code for this gene - each point where one letter in the code was substituted for another - the researchers were able to see which of the two mammals had "microevolved" faster.

Animals living in environments where the climate was warmer, had about 1.5 times more of these substitutions than the animals living in cooler environments.

Dr Gillman explained that, at higher latitudes where environments are colder and less productive, animals often conserve their energy - hibernating or resting to reduce their metabolic activity.

"In warmer climates annual metabolic activity is likely to be greater, so this will lead to more total cell divisions per year in the germline."

These results support the idea that high tropical biodiversity is caused by faster rates of evolution in warmer climates.