Over the past 9,000 years, American black bears have gradually shifted to a reddish-brown color due to a genetic variant.
Some American black bears in the western United States have evolved to have reddish-brown fur. New research suggests that this new coloration may be due to a genetic variant similar to the one that causes albinism in humans.
Researchers from the United States and Japan analyzed DNA samples from 151 American black bears (Ursus americanus) across the United States and Canada. They found that those residing in western states such as Nevada, Arizona, and Idaho were more likely to have reddish-brown fur than their counterparts elsewhere.
The weight of black bears tends to change with age, sex, health, and season. Seasonal weight variation is quite pronounced: in the fall, their pre-hibernation weight tends to be 30% higher than in spring when black bears emerge from their dens. Black bears along the eastern coast are typically heavier on average than those on the western coast, and bears in the northwest are usually slightly heavier than those in the southeast.
Researchers identified a mutation called R153C in a gene known as tyrosinase-related protein 1 (TYRP1), which causes changes in fur pigmentation.
“TYRP1 is a known pigmentation gene involved in the pathway of precursor molecules that ultimately produce eumelanin (black or brown pigment) or pheomelanin (red or yellow pigment),” said Emily Puckett, the lead author of the study and an assistant professor of biology at the University of Memphis in Tennessee, in an interview with Live Science. “What it is doing is changing the amino acid sequence of that gene.”
According to the research, the “reddish-brown variant” is a form of “young mutation”, appearing approximately 9,360 years ago and gradually spreading within the population of American black bears.
Puckett noted that black bears in other parts of the United States, including along the Great Lakes and in the Northeast, are less likely to exhibit reddish-brown fur because this mutation “has not had enough time to migrate naturally.”
“Our demographic model indicates that the most likely occurrence of the mutation is in the western region, likely in the Southwest. From there, it spreads through gene flow across the population.”
However, even so, this is a slow process, with most black bears on the East Coast still exhibiting solid black fur.
The researchers also examined whether the development of this gene in western black bears was related to thermoregulation, a mechanism that helps mammals regulate their body temperature, or competition with another species of brown bear: brown bears (Ursus arctos), also known as North American grizzly bears.
Brown bears (Ursus arctos) are a large bear species found throughout Eurasia and North America. In North America, the population of brown bears is referred to as North American grizzlies, while the subspecies living in the Kodiak Archipelago of Alaska is called Kodiak bears. It is one of the largest terrestrial members of the Carnivora order, second only to its closest relative, the polar bear (Ursus maritimus).
“Our model suggests that this mutation may help them adapt in some way, but we are not 100% sure what it adapts to,” Puckett said. “We tested both thermoregulation and competition with brown bears; however, neither provided compelling evidence. Our new hypothesis is that it is a mechanism for selective advantage.”
Interestingly, this variant is similar to the variant in humans known as type 3 oculocutaneous albinism (OCA3), which causes lighter skin and hair color, two characteristic signs of albinism. In some cases, it can also lead to poor vision.
“Interestingly, bears with this mutation show no signs of visual problems,” Puckett said.
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American black bears are a medium-sized bear species native to North America. They are the smallest and most common bear species on the continent. They typically inhabit forested areas but venture outside the woods in search of food. Occasionally, they invade residential communities, rummaging through trash for leftover food. Despite living in North America, American black bears are not closely related to brown bears and polar bears; genetic studies indicate that they diverged from a common ancestor around 5.05 million years ago. The American black bear and the Asian black bear (moon bear) are considered to be more closely related to each other than to other bear species. The primitive small bear species Ursus abstrusus is the oldest known fossil of the genus Ursus in North America. This suggests that Ursus abstrusus may be a direct ancestor of the American black bear. |
