A Coat Of Many Colors
By Dr. Jenn Ballard
Few things define a species so distinctly as their outward appearance—bright feathers, sharp spines, sleek scales—but among mammals, the American black bear stands out for the wide range of colors, or more technically “colourmorphs,” it displays across its range. The quintessential black morph is the most common overall, characterized by inky black fur with tan accents across the muzzle. Then there is the spectrum of lighter shades referred to by descriptive names like chocolate, cinnamon, or blonde. Finally, there are the legends, the gray glacier and white Kermode morphs of coastal Alaska and British Columbia. But what creates this remarkable diversity and what does it mean for bear management?
In 1987, a researcher from Manitoba named Dr. Richard Rounds published a seminal paper comparing decades of regional reports about black bear color distribution with contemporary surveys from biologists and outfitters. In total, they examined over 40,000 records of harvested or tagged bears and documented the percentage of black and non-black morphs, the latter being a collective term for all brown, gray, and blonde variations. According to that analysis, non-black morphs were virtually absent from eastern populations at the time. They gradually became more common on a gradient from east to west across the Great Lakes region and into the Rocky Mountains. Throughout the west, small pockets were identified where non-black morphs comprised the majority of local bears and prevalence was mapped out in a vaguely topographic fashion. The study explored several hypotheses for why brown morphs occur in certain areas, but these have remained a topic of debate. Large observational studies like this were groundbreaking in their time, but new technologies allow researchers to validate and expand our understanding of these age-old questions.
One research team, led by Dr. Emily Puckett of the University of Memphis, is examining the question of black bear coloration through the lens of modern genomic science. In a study published earlier this year, Dr. Puckett’s team reported finding a genetic mutation that explains the vast majority of brown and blonde colourmorphs. The researchers examined hair samples from across the black bear range, measuring their chemical composition and ability to reflect light. They expected to find measurable differences between hairs plucked from black and brown morphs, which they did. Hair from brown morph bears contained less pigment and reflected more light than hair from black morphs. The team went on to compare these differences with genetic sequences from the donor bears and identified a single, shared mutation in nearly all lighter morphs.
The mutation works like this: under a microscope, a bear’s skin looks like a rock wall with an overlapping arrangement of flat cells. At the base of the wall are specialized cells that produce a pigment called melanin; this pigment diffuses through the layers like a dye giving the skin its color. Hair follicles are lined by these same layers and pigment-producing cells. As hairs grow, melanin is incorporated into their structure, the shade and concentration of the pigment determining the color of the individual hairs and, ultimately, the entire hide. To make melanin, these special cells use a series of chemical reactions nudged along by enzymes. The mutation Puckett’s team discovered changes just one of those enzymes, affecting the way it works and moves in the cell. That small change is enough to interrupt the entire assembly line, reduce the amount of melanin made, and produce bears with permanently light-colored coats.
After identifying the underlying cause of these lighter morphs, Puckett’s team went on to map the prevalence of the mutation across bear populations as well as using mathematical models to estimate the time and location where the change likely originated. Imagine their excitement when the genetic maps turned out virtually identical to the color distribution maps developed by Rounds nearly 40 years prior. Their models also suggest that the mutation arose in a western bear population around 9,000 years ago, spreading outward as bears dispersed across the landscape. When I spoke to Puckett about their work, she felt that these were the most surprising findings of the project. The near perfect alignment of the two studies despite drastic differences in time and technique lends tremendous confidence to the results, and she emphasized how shockingly new the genetic change is from an evolutionary standpoint. It’s hard to consider 9,000 years ‘new,’ but black bears emerged as a species between 1 and 2 million years ago. That means brown color morphs have been present for less than 1% of the total time black bears have wandered North America.
Even though mutations arise by random chance, they don’t become common in wildlife populations unless they prove beneficial to their hosts. Puckett’s research suggests that brown colourmorphs are an advantage to black bears, at least in certain ecosystems. Theories abound as to why, but mimicking the formidable grizzly bear, improved camouflage, and reduced heat absorption in open terrain have all been proposed. Neither Rounds nor Puckett were able to identify a singular cause and, ultimately, the selection may be multifactorial or too complex to tease out.
Puckett readily points out that their findings don’t apply to the two rarest colourmorphs. While the mechanism responsible for the smoky, blue-gray appearance of glacier bears remains a mystery, a study from 2001 documented a remarkably similar mutation affecting the pigment-producing cells of Kermode morph bears. The mutation in these bears occurs in a hormone receptor on the outside of the cells meant to regulate melanin production. Instead of interrupting the assembly line, this change may prevent the process from starting altogether and lends to the characteristic white coat of the revered “Spirit Bears.”
Puckett concluded our conversation by graciously acknowledging the contributions of her expansive research team, with over 40 coauthors on the most recent paper; she also expressed gratitude for the support of hunters and conservation agencies, without whom such intensive sample collections would not be possible.
The conservation of wildlife species in a modern world requires a variety of tools, and the ready availability of cutting-edge genetic technologies has been a game changer for understanding the health, diversity, and connectivity of populations. Genetic diversity is fundamental to species adaptability. Where animals move, they breed, leaving behind genes like a trail of breadcrumbs and infusing diversity. Where connectivity is lost, gene pools stagnate with sometimes dire consequences. As human development continues to expand, the isolation of wildlife populations is certain to increase. Decisions about when and how to protect unique genetics versus ensuring sufficient overall diversity are surprisingly complicated, but genetic research can inform these difficult questions by quantifying diversity, defining biologically relevant populations, and detecting early signs of stagnation. Puckett’s study is a great example. It doesn’t just answer a question about bear color, it is a snapshot of the ongoing evolution of a species and a roadmap of bear movement over millennia.
Puckett EE, Davis IS, Harper DC, Wakamatsu K, Battu G, et al. (2023) Genetic architecture and evolution of color variation in American black bears. Current Biology 33(1):86-97.
Ritland K, Newton C, Marshall HD (2001) Inheritance and population structure of the white-phased “Kermode” black bear. Current Biology 11:1468-1472.
Rounds RC (1987) Distribution and Analysis of Colourmorphs of the Black Bear (Ursus americanus). Journal of Biogeography 14(6):521-538.