Why are some black bears actually brown?
Sometimes the common names for animals can end up being rather confusing. For example, guinea pigs are not related to pigs and neither are they native to Guinea. In the case of American black bears, not all of them are black. In fact, they come in a range of colors, including brown (also known as cinnamon), blond, or bluish-grey, and some even have coats with mixed colors.
Scientists have wondered why there is this diversity in coat color among black bears. Some have hypothesized that the cinnamon coat is a way of mimicking the generally larger grizzly bear (Ursus arctos), which is brown (but can also be black!) This may provide some form of camouflage or defense for the American black bears.
In new research from scientists at HudsonAlpha, the University of Memphis, and the University of Pennsylvania, the answer to this question has been discovered in the genomes of the black bears, and it involves a mutation in a pigment gene that happened a very long time ago. The research paper is published in the journal Current Biology.
Dr. Emily Puckett is an assistant professor of Biological Sciences at the University of Memphis. She has devoted her career to learning more about the evolution and genetics of bears. With help from partners in state, provincial, and federal wildlife agencies, she collected hundreds of DNA and hair samples from North American bears. Dr. Puckett teamed up with Dr. Greg Barsh, an animal pigmentation expert at HudsonAlpha, to figure out why black bears aren’t always black.
Melanin is the pigment that gives mammals their coat colors. It is produced in the skin by cells known as melanocytes. There are two types of melanin: eumelanin is black or brown, and pheomelanin is red or yellow. It is widely accepted that genetic variation in melanin biosynthesis is what gives rise to differences in hair, eye, and skin color.
The researchers studied photos of bears and analyzed their hair samples. They found that the cinnamon-colored black bears produce reduced amounts of eumelanin, just like grizzly bears. They then sequenced the genomes of nearly 200 bears and identified several different missense mutations in the gene called TYRP1 that codes for making the enzyme tyrosinase-related protein 1.
Cinnamon-colored black bears have a mutation called TYRP1R153C, while most (but not all) grizzly bears have a mutation called TYRP1R114C. The TYRP1 gene produces the enzyme within the melanocytes, and this enzyme helps produce eumelanin, which explains why the cinnamon and grizzly bears have less of this specific pigment.
In addition, studies carried out in the lab of Dr. Mickey Marks at the University of Pennsylvania determined that the TYRP1R153C and TYRP1R114C mutations interfere with melanin synthesis and distribution.
“When we looked at other species, we were surprised to find the TYRP1R153C variant responsible for cinnamon U. americanus is identical to one previously described as a cause of oculocutaneous albinism (OCA3) in humans,” says Dr. Barsh. OCA3 is characterized by reddish skin and hair and frequent visual abnormalities and is most common in people of African or Puerto Rican ancestry. But according to Puckett, bears with TYRP1 mutations have normal skin and can see just fine.
The TYRP1R153C variant was primarily found in bears from the southwest United States, and at lower frequencies moving northward to Southeast Alaska and the Yukon Territory. The TYRP1R153C mutation was associated with the cinnamon color in black bears, as well as with the chocolate and light brown colors, meaning it accounts for almost all of the color diversity among U. americanus.
The researchers used their data to learn more about the TYRP1R153C mutation. One hypothesis is that it may first have occurred in grizzly bears and then been transferred to black bears, but demographic analysis indicated that this was not what happened. Instead, the TYRP1R153C mutation arose spontaneously about 9,360 years ago in black bears living in the western United States, then spread as the bears moved and became distributed across their current geographic range.
“Based on its wide range today, the TYRP1R153C mutation that arose in black bears over 9,000 years ago probably gave an advantage to the cinnamon-colored bears,” says Dr. Puckett. “We used genetic modeling and simulations to predict the selective forces acting on the cinnamon morph. But our predictions ruled out the grizzly mimicry hypothesis as well as another hypothesis having to do with thermoregulation.”
The research team proposed a new explanation for the persistence of the coat color variant: crypsis. This refers to the ability of an animal to conceal itself by blending into the surrounding environment, usually by having similar coloration or patterning as the background. The researchers suggest that this is not only important for prey species or ambush predators, but also for large-bodied species that need to blend into their habitat.
“These results illustrate how genetic variation in melanin biosynthesis can underlie iconic phenotypes and inform our understanding of color variation and recent evolution in large carnivores,” says Dr. Barsh.
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By Alison Bosman, Earth.com Staff Writer
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