The zebra’s stripes are one of the most stunning and extraordinary patterns in nature, a pattern that has been pondered over by humans since they first clapped eyes on the zebra.
Despite its beauty, researchers have long struggled to explain the purpose of the zebra’s unique black-and-white coat. 150 years ago great Victorian biologists like Charles Darwin and Alfred Russel Wallace waded into the debate of why zebras have stripes, and scientists have come up with various theories since then, but there has been no widespread scientific consensus.
In this article, we look at the pros and cons of the various theories as to why zebras have their distinctive stripes.
A quick intro to zebras… and their stripes
The zebra is one of the oldest members of the horse family, the ancestor of today’s modern horses, closely resembles the earliest equine ancestors. Together with horses and asses, zebras are members of the Equus genus, and the only striped member of the family.
There are three living species of zebras that roam eastern and southern Africa. Whilst they’re all similar in appearance, with characteristic black and white striped coats, there are some distinct differences in striping patterns and intensity between the species:
Burchell’s/Plains/Common zebra (Equus burchelli):
The most populous zebra species with a fragmented range covering much of southern and eastern Africa south of the Sahara. They have broad stripes that fade to grey as they move down the body – called shadow striping – and predominantly white legs.
Grevy’s zebras (Equus greyvi):
The largest (and most threatened) of the zebra species, found in Kenya and Ethiopia. They have narrower stripes with clear black stripes down their necks and the middle of their backs and white undersides.
Mountain zebra (Equus zebra):
Native to southern Angola, Namibia, and South Africa, mountain zebras are the least common of the three species. They have wide dark stripes against a cream-colored background and a white belly. A dewlap (flap of skin hanging from their throats) also distinguishes these zebras from the other two species.
Bonus zebra: Quaqqa (Equus quagga) The quagga was a plains zebra that lived in South Africa until extinction in the late 19th century. Its unique colouring of a striped head and neck fading into a solid coat towards its rump made it look something like a cross between a zebra and a horse.
Genetics causing selective pigmentation determines the variety of striping in any given animal. The resulting pattern is completely unique to each zebra, much like a human fingerprint.
One common question about zebras is ‘Are zebra’s white with black stripes, or black with white stripes?’ For two reasons, most experts consider zebra to be black with white stripes:
1. Zebras have dark skin beneath their fur
2. Each zebra’ss unique striped pattern is a result of pigment activation (the black) and inhibition (the white). Meaning a zebra’s fur is actually black, and the white areas are fur that lacks pigmentation.
So, why do zebras have stripes? Let’s look at the theories:
Why zebras evolved to have such unique black and white stripes is a question that scientists – and the average safari-goer – have been asking for well over a century.
Six theories have been put forward over the years, and recently there seems to have been a renewed interest, with scientists such as Tim Caro testing and investigating many of the theories, all of which fall into four categories:
- Zebras are striped for social reasons
- Zebras are striped to improve their chances with predators
- Zebras evolved stripes to help manage their temperature
- Zebras stripes help them avoid fly bites
Let’s explore the six theories in more detail:
1. Stripes camouflage a zebra
Lions are actually color blind. If you can only see black, white and shades of gray a single colour dark animal standing in light-coloured tall grass or under trees would be very obvious. In theory, the zebra’s stripes should help it blend in with its surroundings when in grass or forest, making it harder to see.
Alfred Russel Wallace first put forward the theory in his book Darwinism:
“It is in the evening, or on moonlight nights, when [zebras] go to drink, that they are chiefly exposed to attack… In twilight they are not at all conspicuous, the stripes of white and black so merging together into a grey tint it is difficult to see them at a little distance.”
Even Rudyard Kipling lent his weight to this theory, writing in his ‘Just so’ stories that zebras stripes were down to:
“the slippery-slidy shadows of the trees” falling on its body.
Some scientists are dismissive of this theory, suggesting that as zebras stand out to the human eye whether in trees or in grassland their ‘camouflage’ is very poor, and that anyway they tend to run from predators rather than hide.
Likelihood of being correct 6/10
2. Stripes confuse predators
A variation on the camouflage theme. Since zebras herd together, this theory suggests that the mass of stripes in a zebra herd could confuse and/or dazzle predators by acting as an optical illusion.
To a predator, a group of zebras could look blended into one large striped animal, too big to take on – particularly when the zebras move in a herd. The illusion would have the added benefit to zebras of making it difficult for predators to pick out a single animal to attack.
The counter-argument to this theory is that when fleeing predators, zebras don’t behave in a way to maximise any potential confusion caused by their stripes. Also bad for this idea is that lions and spotted hyenas actually have quite weak eyesight (compared to humans), and can only resolve stripes when they’re very close to a zebra, so stripes are unlikely to be of much use as a defense against predators.
Likelihood of being correct 4/10
3. Stripes allow zebras to recognize each other
As each zebra has their own unique striping pattern, one theory states that their stripes might act as a unique identifier, allowing individuals to recognize one another.
This theory is highly unlikely to be the why zebras have stripes, given that a) uniformly coloured horses are able to recognize other individuals by sight and sound, and b) unusual unstriped zebras have been observed mingling happily with a herd and breeding successfully.
Likelihood of being correct 2/10
4. Stripes impact a zebra’s sexual selection
In The Descent of Man Charles Darwin observed that:
“He who attributes the white and dark vertical stripes on the flanks of various antelopes to sexual selection, will probably extend the same view to the… beautiful zebra.”
His take here is that zebra stripes may in some way help males and females decide who they will mate with.
Although each zebra’s stripes are unique, the overall pattern is similar between zebras of the same species, so it’s hard to see how this can be the case.
Likelihood of being correct 2/10
5. Stripes reduce the number of fly bites
A theory that has been around since the 1930s and has gained traction in recent years is that zebra stripes evolved to help them avoid bites from flies carrying diseases (such as African horse sickness, trypanosomiasis, and the potentially fatal equine influenza). Having a pattern that increased protection from biting flies, such as tsetse flies, could be a strong evolutionary advantage, meaning stripes would be passed on to future generations.
This theory was recently put to the test in a new study by biologists from the University of California Davis. They studied zebras and horses which were dressed in different coats: all black, all white, and a black-and-white zebra pattern. The behaviour of flies around each animal was then recorded and analysed.
The test found that far fewer biting flies landed on zebras than on horses, and, crucially, that horses dressed as zebras attracted fewer flies than horses wearing no coat or an all-white or all-black coat.
The University of California Davis scientists are not sure why the flies exhibit this behaviour, though suggest that zebra stripes may have some sort of dazzling effect on the flies.
Likelihood of being correct 8/10
6. Stripes help zebras cool down
Our final theory for why zebras are striped has gained support in recent years, and states that their alternating colour pattern works to control their body temperature.
Historically the thinking was that black stripes might absorb heat in the morning to warm the zebra up, while white stripes might reflect more light and cool the zebras whilst grazing for hours in the direct sun.
A recent scientific study found that during the warmer hours of a day, zebras black stripes were consistently 12-15 Celsius higher than white stripes. The scientists behind the study have suggested that this steady temperature difference between stripes would drive a ‘mildly turbulent air’ along the animal’s back to help cool it.
The same study also found that the hairs on a zebra’s dark stripes erect during early morning and noon. In theory, this action could trap heat in the cool morning and lead to sweat evaporation during the peak sun hours.
Whilst all savanna grazers need to regulate body temperature, zebras may especially benefit from an additional cooling system because they digest food less efficiently than other African grazers, so need to spend more time eating in the heat of the sun.
Likelihood of being correct 7/10
So which theory is correct?
At the moment we simply don’t know for sure, but zebra stripes probably serve multiple purposes.
Our view is that the true reason is a combination of the two last theories explored – that zebras are striped as a result of evolutionary benefits of both reducing biting flies and regulating their temperature. We’ll keep this post updated as the science progresses, so be sure to check back for the latest developments.
One last (very) alternative theory
Still with us and would like to hear one last theory as to how the zebra got its stripes? In which case, this beautiful Tinga Tales animation is well worth a watch (though the chances of it being correct are minimal :).
And that’s the lot for this exploration of why zebras have stripes. Which do you think is the most likely theory? Or perhaps you have a theory of your own to share? Let us know, we’d love to hear from you in the comments section below!