Black bears are black, and grizzly bears are brown, right? Turns out, it’s not so simple. Some black bears are, in fact, brown—and perhaps increasingly so. A new study published in Current Biology sheds light on the cause.
“Sometimes called cinnamon bears, black bears can appear in a variety of shades: a chocolate, a blond, a brown,” says Emily Puckett, an assistant professor of biological sciences at the University of Memphis.
Puckett wanted to know whether black bears are appearing brown due to interbreeding between grizzlies and black bears. “Because they come out similarly coloured we had a reasonable hypothesis that it was an interspecies movement of alleles [or genes].” But the answer to that, they discovered, is no. “It’s a unique mutation and it’s unique to black bears,” she says.
One of the most fun aspects to the work, and what gave them a hint as to the actual mechanism for the change in bear colour, according to Puckett, was looking back at a study that was done in the1980s that drew on the diligent work of natural historians and wildlife communities. “I was astounded at this paper. They surveyed managers across the US and Canada, and came up with a series of maps of the percentage of black animals on the landscape across the geography.” That study looked at about 40,000 bears, and there was an older paper from the ‘70s that tracked the colour of female bears and their cubs. From those frequencies, which were remarkably consistent, Puckett and her team could guess that the mechanism probably arose from a dominant mutation.
Wild profile: meet the black bear
To find that out Emily Puckett and her team used tissue samples from bears that were hunted, killed by vehicles, or animals captured for other research studies or management purposes. They did genetic analysis and hair colour analysis on samples from hundreds of black bears (Ursus americanus) and a small number of grizzly bears (Ursus arctos). In the end, they concluded that the “cinnamon morph” is caused by a mutation in the TYRP1 gene. Puckett effuses about being able to build on the work of past citizen scientists and wildlife biologists. “These natural histories from wildlife communities were spot on,” she says, “then I get to come in with the latest technology and create 200 bear genomes.”
Puckett was amazed to discover that her allele frequency data (what the genetic analysis shows) “matches up very very closely to the data measuring phenotype frequency” (or what the animals actually look like). “Which actually makes sense,” she says, “because it’s a dominant mutation.”
Not only did they discover the gene mutation that causes the colour morph, they also found out when, historically, the mutation took place. “We used a very fancy population genomics coalescent model that estimates when in time the mutation arose on the chromosome,” she says. “And we ran that for the specific point mutation that we identified that caused the brown colour and estimated that it was 9,360 years—or 1,440 generations—old.”
So, where could you expect to see a so-called cinnamon bear? In the US, in the Southwest, the Sierras, and California is where the researchers saw the brown version of the gene showing up in the highest frequency, decreasing as you move north up into the Rocky mountains. “The Rockies are of course this massive barrier even for a large, strong animal like a bear to move through,” says Puckett. “So once you get east of the Rockies, or north of the Rockies, into the Yukon, you see that gene showing up, but in lower frequencies.”
Puckett explains that genes spread as one animal moves from its natal area to a second area and breeds, moving alleles, or genes, from population one to two. “This is such a fun part of the paper—it’s basically the same piece of DNA being copied over and replicated—from bear parent to kid—found in the southwest, found in Alaska, found around the Great Lakes, Manitoba, Ontario, and then in Connecticut—that’s as far east as we’ve found it.”
The last piece of the puzzle was what adaptive advantage did it serve to have brown fur instead of black? The researchers tested two proposals. First, could being a lighter colour give an advantage for thermoregulation, since the trait arose in a hotter, drier environment? Second, could it have allowed the American black bears to ride on the coattails, or reputations, of grizzlies where their ranges overlapped, allowing them to better compete? They tested the different factors and didn’t get support for either hypothesis. “So we don’t know why it happens,” says Puckett. But they now wonder whether some type of selective advantage, such as, perhaps light coloured fur might be hard to spot in different environments. “In forest habitats, they might blend in more if they are black. In more edge habitat, where the forest cover is more open, maybe those are places where brown allele might be more favoured.”
In the meantime, we can say that for some bears at least, cinnamon is the new black.
Wild profile: meet the grizzly bear
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