The truth behind why zebras have stripes

Flies that bite and suck blood are a common menace to animals in Africa. Horseflies and tsetse flies also transmit diseases like sleeping sickness, African horse sickness, and the potentially fatal equine influenza. The thin hair of a zebra would pose little barrier to biting flies. But analyses of tsetse flies’ diets found no trace of zebra blood.

For almost a century now, anecdotal evidence and experiments with inanimate models have repeatedly shown that flies tend to not land on striped surfaces. Large-scale evidence came in a 2014 study by Caro and his colleagues. They collected weather, lion presence and zebra herd size data, and compared these factors to the stripes of zebras living in the area. Striping was more pronounced in environments that favour horseflies, according to Caro.

“[The 2014 study] really showed something really remarkable to us,” says Caro. “We also found absolutely no support for the other hypotheses.”

The research at Hill Livery earlier this year, shed new light for Caro’s team. They observed horseflies around zebras and horses; some horses had black, white or striped coats placed on them. The horseflies hovered around zebras and horses in similar amounts, but far fewer flies landed on zebras – or horses with striped coats.

The flies would try to land on the stripes, but then fail to decelerate as they normally would approaching a non-striped surface, and bounce off. “It looks as if they cannot recognise that black and white surface as a good landing spot,” says Caro.

Caro says his team is working with “lots of unpublished data” from videos of flies approaching different patterns to learn how the stripes mess up a fly’s landing. And in Princeton University, evolutionary biologist Daniel Rubenstein and his collaborators are tackling the question using “fly vision in virtual reality”.

Cooling down

However, other zebra researchers, like retired animal lab technician Alison Cobb and zoologist Stephen Cobb of Oxford in the UK, aren’t convinced by the parasite-deterring explanation. They believe that zebra stripes aid primarily in thermoregulation. While Alison agrees with Caro’s findings, she thinks biting flies “seems too unimportant an effect” to have driven the evolution of zebra stripes. “Every zebra must avoid getting hot, and biting flies will come at certain places, and certain times of the year, but they are by no means as definite or frequent a threat as overheating,” says Cobb.

Thermoregulation has long been suggested by scientists as the function of zebra stripes. The basic idea is that black stripes would absorb heat in the morning and warm up zebras, whereas white stripes reflect light more and could thus help cool zebras as they graze for hours in the blazing sun. This seemingly straightforward logic has, however, received mixed support.

Caro and his team found only a weak spatial overlap between striping patterns and maximum temperatures. A year later, a spatial modelling study of plains zebras – the most numerous species that range from eastern to southern African – led by Brenda Larison of University of California, Los Angeles, found stronger striping patterns in areas that are warmer or receive more intense sunlight.