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Thursday, April 17, 2014

Why do Zebras have stripes?

Why do zebras have those flashy stripes? In a world where predators could be lurking anywhere, being so boldly conspicuous seems like a huge handicap. There must be some kind of evolutionary advantage to that distinctive coloring and biologists have come up with several ideas. The stripes might be courtship or other social cues, helping the animals mate or bond in groups. They might allow the animals to identify each other. They might confuse predators by making the zebras blend together. Perhaps they offer some type of temperature control. Nope. Tim Caro of the University of California, Davis and his colleagues ruled out those possibilities. It turns out that it’s all about the bugs.

That’s right, bugs. More specifically, biting insects like tsetse flies, stomoxys stable flies and tabanid biting flies. The researchers found that the ranges of animals with body stripes (zebras) exactly matched the ranges of these bloodsucking insects.

In the diagram below, you can see the different species of wild equine. Some have full body stripes, some leg stripes, some neck stripes and some are not striped at all. The blue dots show species not plagued by tabanid flies. Notice that those are the animals with no stripes. There was no such correlation with other factors like the presence of certain predators or temperature.

Striping and tabanid activity.
Phylogenetic tree of equid subspecies showing leg stripe intensity (inside circles) and proportion of geographic range overlap with 7 consecutive months of temperature lying between 15 and 30 C and humidity between 30 and 85% (outside circles).
Drawings by Rickesh Patel.
DOI: 10.1038/ncomms4535,

But aren't there plenty of other animals living in those same areas that are not striped? Yes, but zebras are covered with short hairs that biting insects can penetrate with their mouth parts. Non-striped mammals living in the same areas typically have longer, thicker fur that the flies can't get through.

How do stripes help animals avoid biting insects? Apparently, many insects, including tabanids and tsetses, don’t like to land on striped surfaces. And indeed, solid colored feral horses suffer much more harassment from flies than do their striped cousins.

The idea that insect parasites drove the evolution of stripes isn’t settled science yet, but it’s a fascinating hypothesis. I can’t wait to see if further studies confirm it.

Caro, T., Izzo, A., Reiner, R., Walker, H., & Stankowich, T. (2014). The function of zebra stripes Nature Communications, 5 DOI: 10.1038/ncomms4535,

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