As hunters, we’re well aware that black bears, despite their name, come in many color schemes. Spend much time in the woods, and at some point, you’re almost guaranteed to sit, scratching your head, as you watch a strawberry-blonde sow tote around a gaggle of chocolate-colored cubs. And if you’ve been confused by this phenomenon, you’re not alone. It’s perplexed scientists as well, who for decades have pondered the basis for these dramatic differences in pigmentation.
Now, newly-published research from the University of Memphis, indicates that the answer lies in the genetic material of these animals. In a scientific paper published in mid-December, Dr. Emily Puckett and her team dug into the genome, or the entire genetic code, of black bears from across the United States to determine what genes affect coat color.
The researchers used a technique called a genome wide association study, in which they scanned the entire set of DNA of 151 black bears, and identified locations in their genetic structure where there were small alterations, known as mutations, that change the genetic code. And it doesn’t take much to make a big difference. Every gene is composed of up to several-million nucleotide bases—the quintessential “building blocks” of DNA. The precise sequence of building blocks determines what proteins are produced by the body, and it only requires the mutation or deletion of a few of these blocks to change the shape, structure, and subsequently function of these proteins.
After scanning the genomes of the bears in the study, Puckett and her colleagues keyed in on a single gene called tyrosinase-related protein 1, or TYRP1, that they believed controlled the production of an important pigmentation protein. Then, by comparing the genetic code to the coat color of the bears in the study, the researchers determined that bruins with a mutation to the TYRP1 gene had lighter-colored fur—ranging from brown, to red, to cinnamon—than their un-mutated counterparts.
To then test their theory in the lab, the team created transgenic cells by introducing their mutation-of-interest to pigment-producing cells. The researchers discovered that cells with the mutation produced significantly less pigment that those without, corroborating their initial results.
The mutation they discovered, coincidently, is similar to the modification that causes albinism, or light-colored skin, in people and several other animal species as well. But the mechanism is a little more complicated than what meets the eye. People with the mutation actually produce more pheomelanin, or red and yellow pigment, as opposed to the typical eumelanin, or black and brown pigment. People with albinism also tend to have sub-par vision, but surprisingly, the researchers suspect that bears with the mutation appear to have no vision-related problems.
After confirming their results, Puckett and her colleagues then broadened their findings to the greater North American population of black bears to predict when the mutation first occurred. Based on population and genetic modeling, they estimate that it arose in a southwest population of bears just over 9,000 years ago. The exact reason why it first popped up is unclear, but the researchers suggest two theories. The first is that in the hotter, drier climates of the southwest, having a lighter coat could aid in thermoregulation, or the ability to control body temperature. Similar to why wearing a black t-shirt on a hot day is a recipe for a sweat bath, having a black fur coat can absorb more solar radiation, making it more challenging to dissipate heat. The second theory is that lighter coats evolved in the population as a form of mimicry in areas where black and grizzly bears overlapped, in order to avoid predation.
Regardless of which theory is correct, the mechanism is the same, and it’s an elegant example of a basic evolutionary concept: natural selection. Bears that contain the mutation for light fur have an advantage in either regulating body heat or evading predation and, however minuscule the difference might be, have reproduced at a higher frequency than those without the mutation. As such, light-colored bears are slowly increasing in proportion in the North American population. Or, in scientific jargon, the TYRP1 gene appears to be “under weak positive selection.”
So, next time you’re in the woods, bear in mind that those light-colored animals might just have a leg-up. And if you’ve always used coat color to identify grizzly versus black bears, it might be time to do a little more research. Most western states teach hunters to use ear shape, snout length, and the telltale-humped shoulders to sleuth out discrepancies between the two. And being able to tell them apart could be the difference between ending a hunt with a new trophy, or thousands of dollars in fines and a court summons.