Are zebras black and white, or white and black?
I was recently part of a discussion among other artists about zebras. The age-old question arose about whether they are white animals with black stripes or black animals with white stripes. I thought it might be fun to look at zebra coloration in light of recent discoveries in horse color. It turns out that genetics has a different answer: zebras are dun.
Black on white, or white on black?
First, let’s look at the question of white-on-black or black-on-white. Most scientists will tell you that zebras are black with white stripes. That might seem counterintuitive when white hair covers a larger portion of the body than black hair. The average person also puts a lot of weight on the fact that the underside of the zebra is predominantly white.
However, the area that tells the story is the face—particularly in the thin-coated regions around the nose and eyes. Those are black.
It’s not just the nose and the area around the eyes. Zebras have black skin—even under their white stripes. Their dark skin says that their default coloration is black.
So if zebras are black with white stripes, how can they possibly be dun?
Dun and wild equids
One of the more interesting discoveries from the initial study that identified Dun (D) was that all existing wild equids were homozygous for Dun. Included in the testing were Plains zebras, Mountain zebras, Grévy’s zebras, and a museum sample from a Quagga. Several wild asses were tested and were also genetically dun.
This probably seems strange. In the visual classification of horse colors, dun is considered a dilute color. How can a dilution gene give a black-and-white pattern? This isn’t as improbable as it might seem. That’s because the other interesting finding from that study was that Dun isn’t a dilution at all.
Dilutions vs. White Spotting
Dilute colors occur when pigment cells are present but don’t function effectively. That is why the face of a cremello still has some color (hair and skin), but the bald face of a pinto does not. A cremello has dramatically impaired pigment production, but the bald face of a pinto doesn’t have pigment cells at all. It’s not that the cells don’t work correctly; they are missing. It is what white spotting is: the absence of pigment cells.
Although duns look like they have reduced pigment production—like they are diluted—the study found that the hairs in those areas had fewer pigment cells. The author, Freya Imsland, referred to the phenomenon as “microscopic white spotting.” Where a pinto has areas of the body missing pigment cells, a dun horse has missing pigment cells in the hair itself. When viewed in cross-section, the paler hair has areas without pigment.
Dun horses have hair on their body that looks like the first cross-section and fully pigmented hairs (last cross-section) on their points and factors.
The mutation for non-dun1 changes this so that the body hairs have more pigment cells, but some are still missing (middle cross-section). In the last variant, non-dun2, the hairs are all fully pigmented. Horses that are homozygous for non-dun2 are factor-colored all over. With no contrast between their body and the factors, they are truly not dun at all.
How does this explain zebras?
Zebra coloring makes sense if you imagine that the same dun pattern of factoring (full-color hair) and microscopic spotting (pale hair) was altered to maximize contrast. For this, two things must change: the dun factors have to become full-body striping, and the dun’s two-tone coloring has to become black and white.
Dun factors become stripes
The first step in transforming dun is increasing the dun factors until striping covers most of the animal. From time to time, dun horses are born with pronounced body striping. The Morgan Coulee Bend Talisman and the Hucul pony Arkan-Goran are good examples. In zebras, intense striping is the change that persists when they are crossed with horses.
The colors become black and white
Once the striping pattern is there, the only thing left is to change the “factors” to black and the non-factored areas to white. In a dun horse, the factors are the fully pigmented base color, and the non-factored areas are the hairs with reduced pigment cells. With a zebra, the factors (stripes) are going to be melanistic—black—and everything else will have no pigment cells in the hair, leaving it white. (Because the lack of cells happens in the hair shaft, the skin under the white hair is still black.)
Comparisons
In this context, zebra patterning can be more easily seen as a modified form of the wild color common to equids. It is essentially dun on steroids.
The dorsal is still there, going right down the center of the tail.
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From this angle, you can also see “zippers” on the backs of the legs.
The stripes on the forehead are similar to the dun factor known as cobwebbing. If you look, you can also see a similar pattern of banding on the ears. Both even have pale ear tips.
Horses and zebras both have modified versions of the standard equid dun color. They just went in a very different direction. In horses, the change minimized the contrast by reducing or eliminating the (microscopic) white spotting. With zebras, the changes were made to achieve maximum contrast between the fully pigmented and white spotted parts. Underneath their very different coloring, it is possible to see their common origin.
The zebra photos for this post all came from Wikimedia Commons. Clicking them will take you to that site, where more information on species and location are available.