Zebra stripes are not black and white

http://www.theguardian.com/science/animal-magic/2015/jan/15/zebra-stripes-function

Version 0 of 1.

Do zebra stripes have some kind of adaptive function? Intuitively, the answer has to be yes. But what is it? As yet another study claims to be homing in on the answer, I thought it might be helpful to sum up the most likely explanations.

1. Camouflage

Hypothesis. Stripy is all about befuddling predators. During daylight hours, the stripes are striking, but illusory. Lions, one of the main predators of zebra, can’t figure out the size, speed and trajectory of their black-and-white quarry. At dusk, stripes help break up a zebra’s outline. This is where Alfred Russel Wallace put his money, though Charles Darwin thought the camouflague idea far-fetched.Prediction. Where there are lots of lions the zebras should have more and bolder stripes.Evidence. Some. Contrasting stripes do create illusions in the mammalian mind: think barbers poles or bicycle spokes. See here for a recent study of the effects created by stripes in motion.

2. Social function

Hypothesis. Stripes are pretty, so perhaps they help zebras make sensible choices about whom to mate with. If peahens can identify the sexiest males by looking at peacock eyespots (and they can), perhaps zebras do the same. “I’m too sexy for my stripes.” That sort of thing. Or perhaps the stripes act like a barcode, with zebras using them to identify friends and relatives.Prediction. Males should have more elaborate stripes than females. There might also be a correlation between some aspect of zebra existence (such as group size) and stripiness.Evidence. Not a lot.

3. Avoidance of biting flies

Hypothesis. Rather like the first hypothesis, the idea here is that the primary function of zebra stripes is to dazzle nasty parasitic insects.Prediction. Flies should avoid landing on stripy surfaces. Where biting flies are a particular nuisance, stripiness should be more common.Evidence. More than you’d expect. A study on horseflies showed that they prefer to land on uniformly coloured surfaces rather than stripy ones. More recently, researchers mapped stripiness to the distribution of the tsetse fly (see this post I wrote last year). In places most suited to biting insects, stripiness is a more common trait among equids.

4. Thermoregulation

Hypothesis. Black stripes get hotter than white stripes, causing air to flow over the zebra’s skin.Prediction. In places that are hottest, zebras should have bolder and blacker stripes.Evidence. Some. This week, researchers published an analysis of factors that might explain variation in stripes within a single species, the plains zebra (Equus quagga). Temperature, they report in Royal Society Open Science, “was a significant predictor of zebra stripe patterns across their entire range in Africa.”

In spite of this latest finding, we are still some way from any kind of definitive answer. “Much additional work is needed to elucidate the true functionality of striping in zebra,” the authors conclude. Zebra stripes then (OK, at least their function) are anything but black and white.

Let’s reconvene when the next study comes out.