A festivity of stripes

Then the Ethiopian put his five fingers close together (there was plenty of black left on his new skin still) and pressed them all over the Leopard, and wherever the five fingers touched they left five little black marks, all close together. You can see them on any Leopard's skin...

‘How the Leopard got his spots’ from The Just So Stories.

Rudyard Kipling, 1902.

I’ve always disliked the Just So Stories.
As an animal-obsessed child I encountered them often but they inevitably left me feeling cheated and irritated. I really wanted to know why the leopard sported spots or how the elephant acquired its trunk and these silly tales could not enlighten me. (Maybe zoologists are born and not made??)
Fortunately neither Kipling, nor Aesop, tackled the tricky question of How the Zebra got its Stripes. A San (Bushman) legend, however, attributes the charcoal stripes to singe-marks...

But why am I pondering equine apparel?
Well, firstly it’s the festive season and what could be more festive than a zebra?
Romping, head-tossing and snorting, these roly-poly creatures are guaranteed to lift one’s spirit. They’re a dazzling chimera: part mythical beast, part fairground runaway, part my-first-pony.

Who can resist a zebra?

My second reason is that the local zebra mares are celebrating the season by dropping their foals. This has transformed my drive to work into a wonderful, ooh-ah experience. The new mums are cautious and they gallop off as I approach, their gangly-legged foals racing and weaving out ahead of them. The herd’s stallion trots a few paces before swinging around to confront me. Standing tall with nose lifted, ears pricked and nostrils flared, he paces forward a stride or two, doing his best to look fierce and intimidating. Once I’ve actually passed by, he wheels around and thunders off after his disappearing family.

Now before we start let’s get one thing straight: zebra is not a proper taxonomic term. In fact it’s a brazen act of colour discrimination.
Zebra simply means ‘striped horse’ and it’s like using one collective name for all spotted cats or all blue birds.

The great grandmamma of today’s six horse species galloped into existence on the North American plains about two million years ago, and then clip-clopped off to world domination. This equine granny is believed to have worn stripes and three of her great grand kids maintain traditional dress. However, these purists are no more related to one another than they are to the more individualistically attired horses and donkeys.
While the Grevy’s zebra (trotting about northern Kenya) is clearly an ass (no offense meant), the mountain zebra (counting down its days in the pointy bits of southern Africa) and the savannah-loving Burchell’s zebra are more closely allied to horses, enjoying the same free-wheeling lifestyle as mustangs and brumbies (i.e. itinerant harems).

The Grevy’s zebra (Equus grevyi) voices its dismay over its endangerment with a classic donkey’s ‘hee-haw’. As with the wild asses, stallions hold down real estate and then have their wicked way with mares that wander by. Photo by Steve Garvie.

The mountain zebra (Equus zebra) may look similar to the Burchell’s (give or take a dewlap and a bare tummy) but their differences run deep: the Burchell’s zebra boasts 44 chromosomes while the mountain zebra makes do with 32. Photo by Vince O’Sullivan.

The critically endangered Somali wild ass (Equus africanus) sired every donkey and mule on the planet; obviously a smart decision to hang on to those lewd stockings. Photo by Lisa Brown.

So why are some horses stripy?
I mean virtually every other grass-munching mammal favours demur browns or greys. What do zebra gain from sporting so much razzmatazz?
Well, sit back and enjoy some scientific just-so-stories.

Black and white stripes may dazzle and confuse leaping predators, or rouse in them a sense of danger (if skunks can do it...). Those stripes may deter blood-sucking flies, act as camouflage (at a distance) or break up a zebra’s outline. They may generate cooling air currents, enhance the wearer’s sex appeal or machismo, or allow individuals to recognise their nearest and dearest (every zebra is fingerprint-unique).

Of course ‘stories’ like these are the raw material of science. Whittled away by scientific endeavour, they’ll eventually yield a shabby effigy of reality. So I’m delighted that - after centuries of speculation - researchers are finally starting to test them.

The Burchell’s (or plains) zebra (Equus burchelli) looks like it’s been gift-wrapped by, er... someone like me. The stripes are aligned horizontally on the back half but vertically on the front.

The old razzle-dazzle, predator-befuddling fable has been given scientific credence by Martin How and Johannes Zanker. Using piccies of Burchell’s zebras, they produced computer models which show that the zebra’s stripes create an optical illusion. Most animals (including you and me) have neural circuits in their brains that detect the direction an object is moving by monitoring how its contours appear. But when a zebra moves, its diagonal flank stripes, juxtaposed with the narrower vertical stripes on its neck and shoulders, produce confusing signals that flummox these motion detectors. Just like the classic barbers’ pole (whose spiral stripe appears to move upward when the pole spins), the zebra’s stripes make it appear as if the creature is moving in the wrong direction. In the pell-mell of a hunt, with multiple zebras leaping and jostling, these fickle signals may throw a lion off its stride.

Although it’s great to have this fable confirmed, it doesn’t fully explain why zebras are stripy. The problem is, not all species have the Burchell’s stripe pattern: Grevy’s zebras don’t have diagonal flank stripes, and the ill-starred quagga had no stripes at all on its back half (the bit most frequently viewed by blood-thirsty zebra-consumers).

Have I got them at the back too?



The brown quagga (a South African subspecies of the Burchell’s zebra) rashly lost many of its stripes. This sacrifice, however, failed to save it from being hunted to extinction for its skin during the 1800s. This scary spectre haunts a museum in Berlin (photo from Wikipedia).

The bug-proofing fable has also earned itself the scientific stamp of approval. In a horse paddock in distant Hungary, Adam Egric and a whole troop of researchers have been testing how colour influences the avarice of horseflies (tabanid flies). By leaving out coloured boards coated in sticky bug-glue, they’ve shown that dark colours (which reflect horizontally polarised light) are much more attractive to the little winged vampires than is white (which scatters light in all directions). Yet when the researchers put out a board with black and white stripes (which does both) they were startled to find that it lured fewer flies than just plain white. After trying various widths of stripe, they found that typical ‘zebra-width’ (i.e. Burchell’s) attracted the fewest flies of all. As a final confirmation, they created plastic horse models, smeared them with glue and left them outside for a two-week period. They then counted the number of ensnared horseflies and found that the black model harboured 562, the brown model 334, the white 22 and the zebra-striped model 8.



Sticky equids. Photo by Gabor Horvath (I’m unsure of the legality of using this image but I couldn’t resist it).

 
I must admit I’d be more convinced by this study if the researchers had done it with African bugs. Do the local tsetse flies show the same proclivities? Blood-thirsty insects can certainly pose a serious risk to their hoofed victims (because they transmit noxious illnesses) but if stripes are such an effective fly-repellent why don’t other succulent herbivores flaunt them too? And why is the Somali wild ass - who lives only a teeny bit further north than the very stripy Grevy’s zebra – unashamedly stripe-free?

Of course, the reason that ancestral horses opted for stripes in the first place may be different from the reason that stripes are a fashion item today. And different species may hang on to their stripes for different reasons... (oh, the trials of an evolutionary biologist).

To be honest, I’d put my money on a social function being the primary one (but maybe my gregarious little mongooses have biased my thinking?). Although the stripe pattern on the rear half of zebras varies greatly (both between and within species), everyone looks remarkably similar up front (i.e. the bit spied by other zebras during social encounters). And we all know that transverse stripes make one look fat. Since prancing about on tippy-toe with an arched neck is standard protocol for equids out to impress, it seems very likely that stripes function to make the wearer look bigger and heftier.
But of course this is just one more just-so-story until some dedicated soul heads out there with a little pot of hair dye...