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In my last blog, I wrote about a study that found the genes for spotting in horse bones that were 25,000 years old—just like the fabulous prehistoric paintings of horses the Pêch-Merle cave in France. I love the research because I have seen the painted caves & they are marvelous.
But there's more, much more, to this story.
The 2011project was actually an extension of an earlier study, led by Arne Ludwig of the Leibnitz Institute for Zoo and Wildlife Research in Germany. In 2009, DNA from ancient horses showed only bay genes in the oldest bones roughly 25,000 years ago, but an explosion in color in horses shortly after domestication (about 5,5000 years ago). Apparently the people who first domesticated horses liked a variety of colors and patterns, perhaps to make their "special" horses easier to recognize.
Why did this new work reveal ancient genes for spotting? In 2009, the team simply hadn't asked the question, are the genes for spotted coats present in these horses? Perhaps they expected a variation in coat color to be a side-effect of domestication because it appears to be an unintended consequence of domestication in other species. Having found genes for spotting in ancient horses, the geneticists suggested that perhaps spotted horses had a camouflage advantage.
Thiss image shows both crypsis (background matching) and disruption (breaking up the animal's outline).
Because of the extreme kindness of a highly talented artist, Bev Doolittle, I can demonstrate with a few illustrations just how effectively spotted horses can be camouflaged on a snowy landscape. (The three images on this page are details from Hide and Seek ©Bev Doolittle, courtesy of The Greenwich Workship, Inc. I should add that prints of originals of Bev Doolittle's gorgeous paintings are available commercially).
This spotted horse is beautifully camouflaged against the snowy landscape.
Clearly being a largely white horse with dark spots could have had a significant advantage as camouflage in Pleistocene Europe, when wolves, cave lions, cave hyenas and deadly human hunters roamed the landscape seeking prey.
This type of camouflage—in which an animal matches its background—is called crypsis. Crypsis is fairly rare among mammals, which are unable to change their skin and fur color in most cases, although some species, like Arctic foxes, have a winter coat of one color and a summer coat of another. (No mammals can execute the trick played by chameleons and other lizards, squids, and many fish, where color and pattern can change in moments.) As Bev Doolittle's paintings show, spotting in horses may function as disruptive camouflage as well as crypsis.
Did camouflage through spots work? It is difficult to know. Wild horses survived, despite the presence of both Neandertals and modern humans in Ice Age Europe. In fact, in southwestern France, bony remains show that horses were one of the four most commonly hunted Pleistocene animals. Would they have been hunted to extinction without the spotted coats? Perhaps, or perhaps not.
If spotting is a form of camouflage in horses, what about zebras? I spent many years working in Africa and found zebras gorgeous, but I never thought they were hard to see.
Marcel Oosterwijk took this photo of zebras in Serengeti National Park, Tanzania. Their stripes don't camouflage them well!
So what is the point of all those glorious stripes? Disruption is a second type of camouflage, in which patchy coloring helps break up the animal's outline and makes it hard to pinpoint the location of a single individual in a herd.
In 2008, ecologist Tim Caro took a hard look at roughly 5000 species of mammal that have black-and-white coloring and what its adaptive value might be. After discussing various primates, carnivores, antelopes and so on, Caro remarks, "Zebras are more problematic."
He discounts the idea that background matching is the function of a zebra's stripes, because the species spends so much time in open habitats. He also is skeptical that the bold coloring functions as a warning color, like the startling blues or reds of poison frogs who want potential predators to recognize that they are dangerous to eat. Horses are big and strong, but their only two defenses are running away and kicking: just not in the same league as being poisonous.
Then again, animal trainers who work with zebras have found that they are remarkably difficult, especially when compared with domesticated horses. People have tried numerous times to domesticate zebras and—to put it simply—zebras have said, "Heck no! Not doing it."
A zebra's stripes do disrupt the outline of its body, particularly when they are clustered in herds.
Where does one zebra stop and another start?
This feature probably makes it harder to single out a specific zebra to hunt. Whether you are a wolf or a human. And some ecologists have speculated that the variation in precise striping pattern is a bit like fingerprints, enabling individual zebras to recognize each other and form alliances.
I guess being a horse of a different color pays off in its own way.
