Why do zebras have stripes?

kiligirl (66)in #africa • 7 years ago (edited)

Some of you might have seen that last week researchers from the University of Bristol and the University of California, Davis announced they'd cracked the zebra code with research carried out in the English countryside. Serious references are provided at the end of this post, but we're talking zebras here - let's have a bit of fun, shall we?

Years ago, I went to one of the fancier private game reserves in South Africa, the kind where the game rangers have to have degrees and be extremely serious about everything. Our ranger fit the mould perfectly, demonstrating a vast array of knowledge on everything from soil types to grasses to leopard scat. At one of our sundowner breaks, having lulled us into a false sense of security that we could trust his vast ranger expertise, he asked our group whether zebras were white with black stripes, or black with white stripes.

Apparently that's some hilarious game ranger type question (I don't remember the witty answer), but when I shot back, "Why do zebras have stripes?", he said something lame about it confusing predators...which confused me, because it doesn't seem to be that good a strategy to completely not blend in with your environment if you're trying to hide from creatures with big teeth and claws out on the open plains. And it was obvious he truly didn't know the answer. But he wasn't the only one - this question has apparently been nagging scientists for over 150 years, as summarised somewhat tongue-in-cheek here.

It's not to confuse predators

But don't take my word for it. The researchers have thoroughly debunked the "stripes confuse predators" hypothesis thus:

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.

This isn't how it usually works out in practisevideo source

While this could be confusing to predators, isn't it just easier to blend in with the background?gif source

And it's not for socialising either

The authors go on to topple that second hypothesis like a house of cards:

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.

Not for heat regulation, either

Others have hypothesised that stripes were good for zebra socialising or for heat regulation (really? why not just be all-white, then?). Not so, say the authors of the study.

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.

"...when you have eliminated the impossible, whatever remains, however improbable, must be the truth"

With a nod to Sherlock Holmes, perhaps these researchers have indeed found the "zebra truth".

As part of the research, they dressed horses up in striped coats (I kid you not).

Equid lending itself to the research cause - not a dedicated follower of fashionImage source

The study, published in the journal Plos One, involved horses, zebras, and horses dressed as zebras. The team said the research not only supported previous work suggesting stripes might act as an insect deterrent, but helped unpick why, revealing the patterns only produced an effect when the flies got close.

Dr Martin How, co-author of the research from the University of Bristol, said: “The flies seemed to be behaving 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.”

Researchers made their discovery by spending more than 16 hours standing in fields and noting how horseflies interacted with nine horses and three zebras – including one somewhat bemusingly called Spot.

Spot, the zebra - oh, that hurtsgif source

Okay, back to the research, because this is where it gets really interesting.

While horseflies circled or touched the animals at similar rates, landing was a different matter, with a lower rate seen for zebras than horses.

Examples of horsefly trajectories around domestic horses (a-c) and captive plains zebra (d-f)Image source

Perhaps the horses and zebras have different smells, and the horses are more attractive to the flies?

Here's how the researchers controlled for that potential variable.

When we placed black coats or white coats or striped coats on the same horse so as to control for any differences in animal behavior or smell, again flies did not land on the stripes. But there was no difference in landing rates on the horse’s naked head, showing that stripes exert their effect close up but do not impede fly approaches at a distance.

And yes, horseflies are seriously annoying

If you've never had the pleasure, imagine something about three times the size of a housefly, with a tremendously annoying buzz, fast flying and a tendency to take chunks of flesh from your person. One of the most satisfying moments of my life (it may have grown in my memory over the years, I'll admit) was out on the middle of a lake, peacefully canoeing, when one of those terrors decided I looked like a juicy target - and I gave it an almighty thwack with my canoe paddle. I heard the thwack, that's how big that toothy monster was. It dropped lifeless into the depths of Lake Massawippi, and peace reigned once more.

Back to the study - what have they concluded so far?

Their current thinking is that the horseflies get confused close up to the stripes, since the landing rates around the horses' and zebras' heads didn't change. Here's their thinking on the effect of the stripes on the horseflies' perception.

So now that we know that stripes affect horseflies really close up, not at a distance, what is actually going on inches away from the host? One idea is that the stripes set up an optical illusion that disrupts the expected pattern of movement the fly experiences as it approaches the zebra, preventing it from landing properly. Another idea is that flies don’t see the zebra as a solid entity but a series of thin black objects. Only when very close do they realize that they’re going to hit a solid body and instead veer off. We are looking into these possibilities now.