Despite what their name may insinuate, American black bears can wear a range of coloured coats: brown, blond, grey, white and even cinnamon.

For the last 9,000 years, the colour of some American black bears has slowly started changing to a red hue, according to researchers. The genetic and evolutionary factors causing this have been mostly unknown – until now.

In a published in the peer-reviewed journal Current Biology, researchers from the United States and Canada say they have identified the genetic mutation – similar to one that causes albinism in humans – that is likely causing American black bears to turn red, or cinnamon-coloured.

By sampling the DNA of 151 American black bears (Ursus americanus) across the United States and Canada, the researchers identified a mutation known as R153C in a gene called tyrosinase-related protein 1 (TYRP1) that appears to have been driving the change in fur colour of some bears, making it a hue of red.

According to the study, it was 9,360 years ago when the cinnamon variant arose, which provides an adaptive advantage by helping bears to thermoregulate in hotter and drier climates or to camouflage as brown bears (Ursus arctos) where the two species are located in the same geographic area.

While the variant is similar to that of oculocutaneous albinism type 3 (OCA3), which is a variant known to cause lighter-coloured hair and skin in humans, and in some cases poor eyesight, the bears in the study did not show any signs of vision issues.

The researchers found that the black bears living in western regions like Nevada, Arizona and Idaho were more likely to be sporting this red-coloured coat than black, while there is a low number of red American Black bears in the east of North America.

Questions of whether thermoregulation, a mechanism that helps mammals regulate their body temperatures, was a factor in the development of this colour-changing gene were examined.

Researchers also looked at whether the change had to do with competition with other cinnamon-coloured bears such as brown grizzly bears.

While the researchers for both these factors, neither were strongly supported as drivers of the change.