Over the past five years I’ve written several posts on the long-standing and vexing question, “Why on earth do zebras have stripes?” (See posts here, here and here.) If you’ve read those posts, you’ll know about the experiments that seemed to settle the issue, or at least that gave a good indication of the evolutionary forces that promoted the evolution of this striking pattern.
One clue is shown below: a figure from a paper that I described in an earlier post (my emphasis). The top part shows the distribution of various striped equids in Africa (the green and orange are unstriped Asian equids) and the bottom shows range maps of two groups of biting flies: tabanids (horseflies) and Glossina, the tsetse fly; both of these carry equine diseases and also promote infections and blood loss. As I wrote at the time (my emphasis):
Here’s the association between the historical (not present!) ranges of equids and of tabanids and tsetse flies; equids at top (zebra ranges striped!) and flies at bottom. Note that tsetse flies (Glossina) aren’t found outside Africa. E. kiang is an unstriped wild ass, E. africanus is the African wild ass, having thin stripes on its legs, E. hemionus is the onager, an unstriped wild ass, and E. ferus przewalskii is Prezewalski’s horse, a rare wild horse thought to be the closest living relative of the domestic horse.
The correspondence is pretty good, although not perfect, since flies live in some areas where zebras don’t. The crucial observation, though, is that biting flies always occurred in areas where zebras lived.
Note, too, that unstriped equids don’t generally coexist with either kind of fly, though the African wild ass, which does have thin striping on its legs, does live in areas with horseflies.
This is a correlation, not a causation, but an earlier experiment (link given below) showed that these flies indeed do avoid landing on striped surfaces. We’re not quite sure why.
Now the entire issue, and all the various hypotheses that have been raised, are discussed in a new book, Zebra Stripes by Tim Caro, which itself is summarized in a new piece in Wired magazine, “The man in the zebra suit knows the secret of the stripes.” The book is shown below, but, as Wired notes, it’s not written for the layperson:
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.
But that’s okay, for if you’re not a scientist, the Wired article is a good description of all the hypotheses raised about the pattern (Alfred Russel Wallace was the first to consider its evolutionary advantage) and how they were tested and refuted, winding up with what seems to be the correct idea. I’ll just list the leading hypotheses and you can read about the tests at Wired.
- Hiding from predators (breaking up the animal’s outline). Nope.
- Warning predators: an “aposematic” pattern indicating that zebras are dangerous to attack. Nope.
- Confusing predators by making it difficult to single out an individual to kill. Nope again
- Social recognition: a way to enable one zebra to recognize a member of the same species. Still nope.
- Temperature regulation: the contrast between heat-absorbing black stripes and heat-reflecting white ones might create a convection current to cool the animal. Not likely.
- Avoiding flies (something I hinted at above). Experiments described in my earlier posts show that flies simply don’t like to land on a striped pattern.
And indeed, the last hypothesis seems to be the right one. As the Wired article notes, Caro did a cool experiment, though he risked incarceration! (There were of course other experiments too.)
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.
I summarized Caro and his colleagues’ 2014 Nature paper on the fly hypothesis (the paper was tersely called “The function of zebra stripes“) in an earlier post, from which the range maps above were taken. (See also a suggestive 2012 paper in the Journal of Experimental Biology by Ádám Egri et al.)
I think Caro’s conclusion is pretty strong, but not quite so strong as to say, “We needn’t do any more experiments.” Still, the Fly Hypothesis is the best one going, and if you take no other lesson from this post, just remember that zebras probably have stripes to ward off biting flies.
Here’s Caro in his zebra suit, looking for all the world like an escaped prisoner. What sacrifices we make for science!
h/t: Greg
Would this work in northern climes where biting flies live? (I’m thinking of northern Minnesota/Wisconsin, southern Canada, and such places.)
Oh, wouldn’t that be nice? 🙂
Must try purchasing stripey tops to test hypothesis, for next time I’m out in high summer in the woods being pursued by hundreds of flies.
I doubt that it would work against the dreaded Scottish Midge.
I suppose not. I’m thinking of the larger deer flies and horse flies we have.
Tartan doesn’t seem particularly effective. Then again, nothing seems particularly effective – though as a non-sufferer, I’ve not looked particularly hard.
Unless there is a very good reason for flies avoiding stripped animals (is there?), wouldn’t there have been strong selection pressure for flies that didn’t have this aversion?
I was just going to say that this conclusion leads me to ask “why do flies tend to avoid stripes?”
I am going to speculate here, but maybe the stripes throw off their visual navigation so they can’t fly in a singular direction when trying to land on a striped surface.
As to why they don’t evolve a countermeasure, I only have another speculation which is that this navigation system, with this weakness, is necessary for other reasons. Also, I expect the flies have other hosts to choose so their selection pressure is not that strong.
Sometimes there’s no genetic variation for that kind of avoidance, and (more likely, to my mind), there may be important selective reasons for flies to retain that behavior. Remember, too, that they bite other species.
I like Mark Sturtevant’s hypothesis. OK – I’m talking about human’s now, but perhaps it can be extended to flies…
[1] Stripes painted on roads across the grain of traffic tends to slow traffic & the traffic gets slower as the stripes get closer together. Perhaps a flies ‘landing computer’ is thrown off by stripes giving a false reading of relative speeds. Not sure if this is a related observation, but how about that strange experience of sitting on a moving train & the world inverts such that it’s the scenery that’s perceived as moving all of a sudden?
[2] If you watch people walking on a floor with no change in levels, but with a distinct change in pattern or colour you will note quite a few people will hesitate at the boundary. Perhaps the fly hesitates a little & ‘stalls out’ from a graceful landing on target
Maybe there is, it’s ongoing, and humans just happen to have invented civilization at this particular moment in the fly-zebra arms race. After all, our window of scientific exploration is perhaps three thousand years wide. That’s not much, all things considered. Probably not enough to see a biological/evolutionary arms race change dramatically, unless we’re talking about bacteria.
sub
Very interesting. Seems like maybe the striped herbivores have a temporary advantage in the arms race.
From Wired quote:
There are a number of striped antelope in sub-Saharan Africa also. Their colors are more typically a light brown and white. So I’d test to see if light stripes on a brown background work before I’d attribute the effect to the specific shades the zebra has.
IIRC zebra hair starts off black and a specific inhibitor has to be activated to get the white. So both the zebras and the antelope may simply be examples of dark-haired animals that evolved white stripes (rather than what I think most people assume, that zebras must have started out some other color and both the black and white coloring evolved together).
ohmigosh you mean they are black with white stripes?!??!
And we always thought that they were white with black stripes. Next you’ll be telling us that the stripes go from bottom to top, not top to bottom.
Thanks for the post! It would be really neat if we could find a way of measuring the fitness advantage of having stripes. If only we could alter the coat color pattern of zebras in enclosures. But I guess there would be ethical concerns that would prevent that from being done.
It shouldn’t be too difficult to find or produce an albino zebra.
Just relocate some of those polar bears?
To the tropics? We’d be dragged to court for cruelty to animals, and this time for good reason.
After dragging the first polar bear from Svalbard to the Serengeti, there probably wouldn’t be much left for you to be sponged into court, let alone dragged.
And the bear wouldn’t have reached Nordkapp. Bjornoya, not Finnmark
IIRC, earlier work along these lines was done by setting out pieces of fabric in various patterns, and counting the numbers of flies per unit area on different patterns. No ethical issues to be majorly concerned over.
Perhaps it would have been good for lions to have been striped like tigers…?
I think god thought stripes made a change from spots .Snigger snigger .
So unrelated, then, to the reason tigers are (and thylacines were) striped?
I suspect they might not be repulsed by black and white, but rather that the stripes are effectively invisible to the flies.
If you have a low-resolution visual processing system, including the analytical part, zebra stripes are going to very closely resemble foliage — a mess of branches, brambles, that sort of thing. There’s nothing in there that’s going to scream, “EAT ME!” to a bloodsucker. Plus, when in motion, it’s not going to seem like a good resting spot, but rather something being madly tossed about by a furious wind.
A resting lion could be mistraken for a rock by such a visual system, but there’s no harm in landing on either a rock or a lion — and, if a lion, there’s a meal to be had. Further, a moving lion is obviously a meal — since rocks typically don’t move.
Remember, even if the optical systems of fly eyes are sophisticated with lots of resolution…their nervous systems simply don’t have enough circuits for complicated analysis. And we see with our minds, not with our eyes. The real question isn’t what sort of image gets projected on the retina-equivalent or sent down the optic nerve, but the 3-D model of the environment that gets created in the brain.
Now that the zebra stripe question has been reasonably satisfactorily answered, as is nearly always the case, the real fun comes from the new questions the answer raises.
Cheers,
b&
This sounds plausible, but one minor quibble.
I am not so sure that is valid. A counter example that comes to mind is jumping spiders. They have the ability to see prey at a relatively large distance, plan a route from their location to the prey and then follow that route successfully even though they can’t see the prey while they are in transit. This was very much a surprise to the people who did the studies because as far as they knew the spiders’ brains did not have enough neurons to do the modeling necessary.
Another example of a slightly different kind is “optic flow” that some birds of prey that fly fast through obstacle dense spaces such as forests utilize for obstacle avoidance. Optic flow is a neat “trick” that hugely reduces the processing power and speed that more conventional brute force methods require.
I’ld like to see research on the fly’s imaging capabilities.
These are all good ideas and counter-ideas, but I rather like Bens’ idea. One must consider how things look to a fly, which will be with a mosaic image, and sensitivity of UV light. A brambly area would have high contrast in light and dark along with contrasty differences in UV brightness. So it seems reasonable to me that the zebras are leveraging on a deep visual cue that the flies use for navigation and acceptance of landing sites.
As for why we don’t see systems like this in other species, well, I can wave my magic wand and say its’ because they are different. Birds of prey have bigger brains and keen vision and they can see the gaps in the stripes (branches) as spaces.
Oh, and welcome, Ben! We have missed you.
Could be a link to flies mortal enemies the many striped wasps? Maybe zebras look like big giant wasps from a distance and hence not worth risking life for?
Does this phenomenon tell us anything about fly vision?
I think it certainly does. Though figuring out what is likely to take a lot more work!
I still feel a bit skeptical of the map analysis, and of the maps themselves. There is some cherry-picking as to how many genera to include in the category of “biting flies that matter to equids”. The tsetse fly hypothesis did not fit the map (slightly striped E africanus live in northern Africa without tsetse flies), but when the group of “flies that matter” was enlarged to include Tabanus, it did. But there are a lot of biting fly genera. There are probably matching fly genera distributions for just about any evolutionary-biogeographical hypothesis one wanted to support.
Also I can’t help wondering if those areas that are blank on the Tabanus map register genuine absences. A quick look at the literature reveals many species of tabanids in western Turkey, including multiple Tabanus; Tabanus kathmanduensis from Nepal, Tabanus and other tabanids in Mongolia, etc. I can’t belive that tabanids, or even just Tabanus, are really absent from the range of the unstriped equiids.
I admit I haven’t read the paper though….
Wondering if this finding of flies avoiding striped patterns is specific to the tsetese flies or if it applies to other flies as well. If it applies to other flies, this technique may make spending time in the Adirondacks or other afflicted areas during black fly season, late spring and early summer, more tolerable. Wonder if it would help for mosquitoes too? Very interesting.
The evidence piles up. Interesting.
I can imagine experiments to determine how flies react to variations on stripes, such as line thickness, angle, etc.
Flies in captivity would be exposed to food with various patterns to see where the flies congregate. Perhaps this information could be correlated with what is known about the flies vision.
Caro looks like an extra from “O Brother, Where Art Thou?”
Lol
Wonder if anyone turns into a frog?
Did you see Madgascar, with Chris Rock “voicing” the zebra? I swear the animator got the zebra to look like Chris, including the front tooth gap. At one point the zebra asks “Am I white with black stripes, or black with white stripes?”
Tell Caro he should sell those pajamas to hte tourists travelling to Africa. They probably work better than insect repellents and are totally harmless. Hahahahaha!!!
I also saw the business opportunity, but I thought of Africans producing and selling striped clothes in situ.
Having got several “ensembles” from West Africa, I’d be surprised if a fly advantage to a particular type of pattern had avoided attention before now. The markets are full of very diverse patterns, mostly in bold and often contrasting colours.
If any of those patterns had a significant fly advantage, I’d expect the locals to know about it, and charge double for it (triple or more for tourists). I deduce that no such significant advantage exists.
Wish I could buy some. Heading to Tanzania next month. All the literature says that Tsetse flies are attracted to darker colors, especially blue. Don’t know if that has been tested empirically, but will leave all my blue clothes at home.
So when it comes to stripes or patterns in predators like tigers, leopards or jaguars, do the stripes/patterns in these cases help with camouflage or social recognition? I have never read any scientific/evolutionary explanations for these markings in predators.
Either way, I admire Caro’s scientific efforts and savvy.
Why do the flies avoid stripes though?
I don’t see an explanation for that, and that’s a major gap in understanding even if the hypothesis is correct.
And certainly, that would open a niche for flies that do not avoid stripes…
Quaggas?
Any historical correlation?
Here’s a thought that sort reverses cause and effect. There is something else about zebras that the flies don’t like and the stripes evolved as an aposematic pattern to warn them off. Let’s say zebra blood is lacking (or even just limited) in some component that flies need to reproduce. For them, biting a zebra is a poor choice compared to other targets. Nevertheless, a bite could still transmit disease. So the zebras evolve a warning pattern so the flies can avoid them before biting.
That pic is Ignobel worthy.
Anecdote : in summer – warm – I’ve seen particular bugs standing on yellow surfaces before – metal park structures and my T-shirt. I’ll have to read the links…
… as in, a lot of bugs.
Thank you for an informative post about a well-written article on a fascinating issue!
A dull question from a mere ex-chemist. If stripes offer such a good deterrent against biting flies, why haven’t even more animals adopted them?
That’s another point in favor of my suggestion above (comment 21): it explains the absence of the striped adaptation in other victims of the flies.
Read something a while back that says the sharks avoid people wearing black and white striped wetsuits. Don’t know if this has been verified.
I wouldn’t want to be the one testing that particular hypothesis in person! 😉
Given that South African fishermen have been routinely persuading great whites to “jump for the kamera!” (I can’t type in a Sif Ifricann accent.) for years, by towing a decoy behind their boat. The decoy is a sheet of foam (surf board material, or wetsuit fabric, black, cut into the silhouette of a seal. Shark sees “seal” from the depths, rushes up to get dinner, breaches and people get photos.
Easy enough to modify to test your hypothesis.
Sorry. Doesn’t count since he didn’t wear a zebra head and a tail. Probably should prance about on all fours as well.
For science.
That, he’s probably saving for his Ig acceptance speech. To forestall the 8-y.o. speech-limiting system.
Hey – isn’t it approaching the Oscer Bafta movie award season? Will the Ig speech-limiting system be deployed this year, I wonder?
And of course there is now a commercial application:
http://www.bucas.com/products/buzz-off-zebra/
Very cool application!
Reblogged this on The Logical Place.
Looking forward to striped hiking and camping gear!
Reblogged this on Foen Peng and commented:
This piece is very interesting! I am very much amused by the last picture.
I’ve always thought the striped pattern confuses predators. I don’t have the patience to look at all of the comments above so I’ll ask an obvious question that may have been commented on already, given that the selection pressure of biting flies on large mammals would be extremely powerful, so powerful that it produces the amazing stripes of zebras, why wouldn’t ALL the African fauna, including predators, in the ranges on the above map simply be covered with stripes? The flies don’t just bite zebras. Maybe Tim Caro speculates on this in his book.
Have a look at my suggestion in comment 21.