Thursday, January 19, 2017

The Horse in Striped Pajamas

The Man in the Zebra Suit Knows the Secret of the Stripes

Wired  Nick Stockton   01.17.17 

The University of Chicago Press

At four in the morning, Tim Caro roused his colleagues. Bleary-eyed and grumbling, they followed him to the edge of the village, where the beasts were hiding. He sat them down in chairs, and after letting their eyes adjust for a minute, he asked them if they saw anything. And if so, would they please point where?

Not real beasts. Despite being camped in Tanzania’s Katavi National Park, Caro was asking his colleagues to identify pelts—from a wildebeest, an impala, and a zebra—that he had draped over chairs or clotheslines. Caro wanted to know if the zebra’s stripes gave it any sort of camouflage in the pre-dawn, when many predators hunt, and he needed the sort of replicability he could not count on from the animals roaming the savannah. “I lost a lot of social capital on that experiment,” says Caro. “If you’re going to be woken up at all, it’s important to be woken up for something exciting or unpredictable, and this was neither.”

The University of Chicago Press 

The experiment was one of hundreds Caro performed over a twenty year scientific odyssey to discover why zebras have stripes—a question that nearly every major biologist since Alfred Russel Wallace has tried to answer. “It became sort of a challenge to me to try and investigate all the existing hypotheses so I could not only identify the right one,” he says, “but just as importantly kill all those remaining.” His new book, Zebra Stripes, chronicles every detail.

This is not a book for casual pop science readers. It is a book about doing science, full of every detail you’d need to reproduce any of the experiments done in the book: distances for viewing pelts; reflectance values for zebra hair; thermal camera settings for taking infrared pictures; speaker settings for playing predator noises; histograms, leaf and tree diagrams, scatter plots; page after page of references. This book is for scientists, or those who wish they’d become scientists. And as treats for the latter, there the anecdotes of Caro’s scientific antics: Tales of how he systematically worked through each hypothesis until he figured out the secret of the stripes.

The University of Chicago Press 

Hypothesis: To hide from predators

The most popular theories about zebra stripes coalesce around the idea that the markings evolved to help the animals from getting eaten by predators. Zebra stripes could be camouflage. As mentioned earlier, Caro tested this hypothesis by annoying people at dawn. He also annoyed people at dusk. For a week, as the sun went down, he asked his colleagues (every five minutes) if they could see zebra, and other animal, cutouts in the dimming light. With a notepad, he’d record their answers, the light conditions, etc. Then he’d ask again, every five minutes, until dark.

He took this data and applied it to anatomical information about the zebra’s predators—a lion or hyena’s eye shape, its number of cones and rods, and the animal’s spectral sensitivity to light. “But we should have known in advance that most predators, with their bichromatic vision, are very bad at picking out stripes,” says Caro. “Certainly at dusk and dawn, the zebra is a grey blur and its stripes have no effect on breaking up its outline.” 


Hypothesis: To warn predators

Camouflage is only one category in the safe-from-predators theory of striping. Another is the idea that the stripes warn predators that zebras are dangerous, a strategy called aposematism. “Aposematic animals are warningly colored, like a skunk or porcupine,” says Caro. “In addition to their colors, they tend to be noisy or sluggish or smell a lot to advertise their danger.”

Zebras aren’t slow, but they are noisy. But are they noisier than other, more blandly-coated savannah herbivores? Caro tested this by parking his Land Rover near herds of impala, zebras, and topi, and recording every snort, grunt, and whinny for half hour increments. “But it’s really the impalas that are the noisiest species,” he says. Being up close, he also noticed a lot of wounds on the zebra. “Not what you’d expect from animals warning predators not to attack them.” 


The University of Chicago Press

Hypothesis: To confuse predators

Another popular predatory theory is that zebra stripes confuse hunters, by creating an optical illusion. Two or more zebras could look like one animal by fleeing in a tight group. “One way to test that would be to sit at a national park for months waiting for a predation attempt,” says Caro. “But I didn’t have time or patience to do that.”

So he made himself the predator, by walking briskly towards large herds of zebra. Then, the moment they started to run, he would take out his stopwatch, pencil and paper, and start writing down details. “There are many other forms of the confusion hypothesis,” says Caro, all of which he tested.

They all failed. In fact, so did every tack he took to testing whether the stripes deter large predators. If you look at the data, lions are really good at killing and eating zebras.


Hypothesis: Social recognition

A less popular hypothesis is that zebras use their stripes as a form of individual recognition. The fact that zebras could recognize each other isn’t controversial—horses do this. But horses have fairly big brains. So, the theory is zebras would have smaller brains, and would need the stripes as a memory aid. But when they compared zebra brains to other equids, they found no meaningful size difference. “Why should zebras need this extra stripe recognition capability to do what horses can do already?”


Hypothesis: Temperature regulation

In the early 1990s, great naturalist Desmond Morris suggested that, because black stripes absorbed heat, and white reflected it, the temperature difference between the two in the midday sun would create a convection current—a cooling breeze across the zebra’s back. To test this, Caro took an infrared camera into the bush. “I took pictures of various species, and found that zebras are not any cooler,” says Caro. Not to mention that the physics don’t quite work out. Once a zebra starts to move, any air currents flowing across the animal’s back would break up.


Hypothesis: Flies

Looking back on how he wound up walking down a dusty Tanzanian road in the midday sun draped in a zebra pelt, Caro admits he should have consulted an insect expert. “I knew from the literature that certain kinds of biting flies didn’t like landing on black and white surfaces,” he says. He also knew that the insects were attracted to movement. So, he would put on the pelt, trudge for an hour, and have his assistant count the number of tsetse flies that had landed on him. For science, he did the walk again, draped in a wildebeest hide.

And? “I really started to see results at this point,” he says. The flies did not like the stripes! “It was an elevating experience, at last after ten years working on this project I started to see a positive effect on one of these hypotheses.” He did more experiments, including setting up striped fly traps (no more walking down dusty roads). With each new experiment, the evidence lined up to support the anti-insect hypothesis. Eventually, Caro and his colleagues did a map analysis, overlaying the ranges of various biting flies and insects with the places where zebras, and their non-striped cousins like the Asiatic wild ass, ranged. “It’s a slam dunk, if you like,” he says. “You find striping where you have high biting fly abundance.”

Caro has no lingering doubts about the connection between flies and stripes. Now, he wants to find out exactly how the flies forced the stripes’ evolution. One question is about the flies—why are they repulsed by black and white? Another is whether the zebras adapted this anti-fly defense because they are particularly susceptible to blood loss, or to diseases the flies carry. Not so annoying anymore.

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