Why the zebra got its stripes

Why the zebra got its stripes

One of the best places in the world to see zebras is the Serengeti in Tanzania, where herds of them stand out against the dusty savannah with their distinctive monochrome marking. A recent study has found these stripes provide a deterrent against another common resident here – flies. If only they were as effective against lions.

When it comes to discerning the reasons for zebras’ distinctive marking, the answers have been far from black and white – until now. A recent study has given the lie to theories such as that the stripes help zebras evade capture by predators, that the markings are there for social reasons and that the animals are striped to keep cool. However, it has found plenty of evidence that the stripes appear to confuse flies, deterring them from landing for a quick bite.

A team at the University of Bristol, in the UK, conducted the research, which involved 16 hours standing in fields and observing how horseflies interacted with three zebras and nine horses, including some draped in black and white sheets to resemble zebras.

Stripes and flies don't mix

The results, published in the open access scientific journal ‘Plos One’, not only back the idea that a zebra’s stripes ward off flies, but reveal this is achieved by disorientating the fly’s vision, causing it to turn away or even collide with the zebra when attempting its bloodthirsty landing.

The study found there was no difference between the amount and behaviour of the flies as they hovered over the horses and zebras – including one rather less than descriptively named ‘Spot’ – but the patterns of the zebras and the horse in zebra disguise had an effect when the flies got close.

Every zebra has a unique coat pattern, much like a fingerprint, yet each zebra in the study had the same effect on the flies as did the striped coats on the horses. The coats were used to ensure it was not the different scent of the zebras that was dissuading the flies from landing.

relatively naturally around both [zebras and horses] until it comes to landing. We saw that these horseflies were coming in quite fast and almost turning away or sometimes even colliding with the zebra, rather than doing a nice, controlled flight.”

Close examination of video recordings by the team of the flies’ activity around the animals suggested the stripe patterns were playing havoc with the flies’ low-resolution vision. Rather than slowing for landing, the insects would often crash into the zebras at full speed and bounce off.

These results bolstered the team’s theories that stripes and flies don’t mix and a zebra’s patterning is the result of evolution, passing on to future generations a protection against biting insects.

Disease carriers

Horseflies are about as bad as it gets for a zebra in a field in Somerset, England, where the study was carried out; but in the zebras’ native environment in southern and central Africa, biting flies carry diseases such as trypanosomiasis, African horse sickness and equine influenza, which can be fatal to equids. Zebras are especially vulnerable to fly bites because of their short-cropped coats, so it’s understandable why a fur pattern that helped evade flies and their deadly diseases would evolve.

And the fly-foxing benefits of stripes are not limited to zebras. Those who enjoy the outdoors in Tanzania could take inspiration and wear zebra-patterned clothing to avoid the attention of biting flies.

Spotting the stripes myths

The UK's University of Bristol study also debunks a few beliefs about the reasons for a zebra’s unique coat.

Stripes offer camouflage

Unless a coachload of Newcastle United supporters turn up in matching home kit, there is very little in the rolling savannah of the Serengeti that will afford much cover for the zebra’s monochrome markings. Field experiments show that zebras stand out to the human eye when they’re among trees or in grassland. However, as a counter argument, there is a story that when the World War One broke out, British mounted troops in Kenya near the border with German-occupied Tanganyika (now Tanzania) would paint zebra stripes with silver nitrate solution on their easily visible grey horses to camouflage the horses from snipers.

Stripes help zebras interact socially

Every zebra has a unique pattern of stripes, which may assist individual recognition. As the University of Bristol team points out, however, solid-coloured horses can recognise each other by sight and sound; and on the rare occasion a zebra is born without stripes, group members do not shun it.

Stripes keep zebras cool in the sun

Given that black stripes might be expected to absorb radiation and white stripes reflect it, one idea proposed is that stripes set up convection currents along the zebra’s back, thus cooling it. The team conducted field experiments in which large water barrels were draped in striped or solid-coloured pelts, or were painted striped or unstriped, but no differences in internal water temperature were found. Furthermore, themographic measurements of zebra, impala, buffalo and giraffe in the wild show zebras are no cooler than their cohabitants.