Zebra’s stripes are a no fly zone for flies
Zebras are famous for their contrasting black and white stripes – but until very recently no one really knew why they sport their unusual striped pattern. It’s a question that’s been discussed as far back as 150 years ago by great Victorian biologists like Charles Darwin and Alfred Russel Wallace.
Since then many ideas have been put on the table but only in the last few years have there been serious attempts to test them. These ideas fall into four main categories: Zebras are striped to evade capture by predators, zebras are striped for social reasons, zebras are striped to keep cool, or they have stripes to avoid attack by biting flies.
Only the last one stands up to scrutiny. And our latest research helps fill in more of the details on why.
What’s the advantage of zebra stripes?
Could stripes help zebras avoid becoming a predator’s meal? There are many problems with this idea. Field experiments show that zebras stand out to the human eye when they’re among trees or in grassland even when illumination is poor – they appear far from camouflaged. And when fleeing from danger, zebras do not behave in ways to maximize any confusion possibly caused by striping, making hypothetical ideas about dazzling predators untenable.
Worse still for this idea, the eyesight of lions and spotted hyenas is much weaker than ours; these predators can only resolve stripes when zebras are very close up, at a distance when they can likely hear or smell the prey anyway. So stripes are unlikely to be of much use in anti-predator defense.
Most damaging, zebras are a preferred prey item for lions – in study after study across Africa, lions kill them more than might be expected from their numerical abundance. So stripes cannot be a very effective anti-predator defense against this important carnivore. So much for the evading-predators hypothesis.
What about the idea that stripes help zebras engage with members of their own species? Every zebra has a unique pattern of striping. Could it be useful in individual recognition? This possibility seems highly unlikely given that uniformly colored domestic horses can recognize other individuals by sight and sound. Striped members of the horse family do not groom each other – a form of social bonding – more than unstriped equid species either. And very unusual unstriped individual zebras are not shunned by group members, and they breed successfully.
What about some kind of defense against the hot African sun? Given that black stripes might be expected to absorb radiation and white stripes reflect it, one idea proposed that stripes set up convection currents along the animal’s back, thereby cooling it.
Again, this seems improbable: Careful experiments in which large water barrels were draped in striped or uniform colored pelts, or were painted striped or unstriped, showed no differences in internal water temperatures. Moreover thermographic measurements of zebra, impala, buffalo and giraffe in the wild show that zebras are no cooler than these other species with whom they live.
The last idea for striping sounds preposterous at first blush – stripes stop biting insects from obtaining a blood meal – but it has a lot of support.
Early experiments in the 1980s reported that tsetse flies and horseflies avoid landing on striped surfaces and has been confirmed more recently .
Most convincingly, however, are data from across the geographic range of the seven living species of equids. Some of these species are striped (zebras), some are not (Asiatic asses) and some are partially striped (African wild ass). Across species and their subspecies, intensity of striping closely parallels biting fly annoyance in Africa and Asia. That is, wild equids indigenous to areas where annoyance from horseflies is prolonged over the year are those most likely to have marked striping patterns.