Why do zebras have stripes?

Many African animals fashion some form of stripe or pattern, but none quite so striking as the zebra. Researchers have spent many years trying to establish the purpose of these iconic markings - but it has proven more complex than was initially thought. 

The black stripes on a white background were traditionally thought to be a form of camouflage. This might seem a little strange when encountering these distinctive looking animals in a zoo setting, but it is a completely plausible idea when considering their native African habitat. Zebras blend into the Savannah background, with their profiles broken up by stripes mimicking sections of vegetation.

A study published in the journal PLOS ONE in February 2019 provides a new perspective on the role of zebra stripes, which relates to protection from flies and parasites. 

Researchers based at the University of California, Los Angeles (UCLA)  and University of Bristol, UK, produced one of the most comprehensive zebra stripe studies to date. They examined how 29 different environmental variables influence the stripe styles of zebras at 16 different sites from south to central Africa.

The study found that zebra’s stripes deter horse flies from landing on them. The researchers compared the behaviour of horse flies as they attempted to prey on zebras and uniformly coloured horses held in similar enclosures. Flies circled and touched horses and zebras at similar rates, but actually landed on zebras around 25% less than horses.

According to researcher Dr Martin How from the University of Bristol: "This reduced ability to land in the zebra's shelter may be due to the fact that the stripes interrupt the visual system of the flies in their last moments of approach.”

In other words, the stripes have the function of somehow ‘dazzling’ flies once they are close enough to see the stripes with their eyes, which ultimately stops them from landing. 

The second part of the research process was observing the behaviour of the horse flies around the horses with different coloured cloth coats: black, white or zebra stripes. This was a key part of the experiment - in an attempt to determine the significance of zebra stripes. 

It was found that when horses wore coats with stripe patterns, flies made fewer landings compared to when they wore single-coloured coats.

The research also looked at zebra and horse behaviour in response to biting flies. Zebras showed preventive behaviour, such as running away and wagging their tails at a much higher rate than horses. As a result, the few flies that eventually landed successfully on zebras spent less time on the zebras compared to those that landed on horses. 

The researchers added: "Zebra stripes are now believed to have evolved to thwart attack by biting flies. We observed and filmed the behaviour of horse flies near captive zebras and horses and found that flies failed to decelerate close to stripes preventing controlled landings. Combined with zebras' anti-parasite behaviour, few flies landed successfully or probed their hosts for blood."

Dangerous and debilitating diseases, such as trypanosomiasis and African horse sickness (AHS), which cause weakness and often death can be spread by flies. As zebras have evolved, it seems logical that their appearance and mannerisms have adapted to help combat this threat.

Flies remain a widespread problem for domestic animals. Fly mitigation techniques inspired by this groundbreaking research, such as the creation of fly-proof clothing designed to look like zebra stripes, could be a vital development for the world of animal health and welfare.

Reference: 'Benefits of zebra stripes: Behaviour of tabanid flies around zebras and horses' by Tim Caro, Martin How et al in PLOS ONE.

Image credit: iStock Photo.

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