Why Are Zebras Stripy?

576 Why Are Zebras Stripy?

Sometimes a problem simply has too many solutions, such as the centuries-old puzzle of why the zebra got its stripes. Various studies have suggested that these vivid black-and-white markings serve many (not all mutually exclusive) purposes—ranging from social cohesion to thermoregulation to confusing predators and parasites using an optical illusion. Now, researchers who’ve developed a computer model to predict zebra striping based on environmental variables have found that temperature was the best predictor of zebra stripe patterns: Zebras living in warmer places tend to have more stripes. The findings, published in Royal Society Open Science this week, suggests how the driving forces behind zebra striping are multifarious and complex.

The patterns found on plains zebras (Equus quagga) vary regionally, from heavy black-and-white striping over the entire body to thinner, lighter stripes covering less of the body. Many animals, including people, have similar gradations in pigmentation because they’re adapted to their local environments, Science explains: People living where there’s a lot of UV light have darker skin, and fruit flies found at high altitude have darker exoskeletons. 


To see how variation in striping is associated with environmental variation, a team led by Brenda Larison from the University of California, Los Angeles, developed computer models that use environmental variables to predict stripes. First, they quantified stripe characteristics—thickness, length, and color saturation covering the forelegs, hind legs, torso, and belly—at 16 sites from Ethiopia through South Africa. Then they measured 29 environmental variables in those areas related to factors like temperature, precipitation, and concentration of green leaf vegetation, as well as the distribution of lions and prevalence of disease-carrying flies. Then they plugged these all into the computer models.

Temperature, they found, can successfully predict stripe patterns. In particular, the most important variables were how consistent the temperature is (or isothermality) and the average temperature during the coldest season. In areas with the lowest seasonal temperatures, for example, zebras have fewer and fainter stripes. 

By contrast, tsetse fly and lion distributions consistently failed to predict stripe pattern variation, they write. But that doesn’t mean pesky flies have nothing to do with striping, especially because the researchers haven’t figured out what causes the correlation between temperature and striping. Last spring, a UC Davis team led by Tim Caro found that zebra stripes provide effective repellent against biting flies, which like it hot. "Diseases carried by horseflies are really nasty," he tells National Geographic. "They can hold a lot of diseases like equine influenza, and it's possible that those diseases are going to be more of a problem under warmer, wetter conditions."

Images: (top), B. Larison et al, Royal Society (middle)


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