Africa harbors three living species of zebras: the plains zebra (Equus quagga), with several subspecies, the mountain zebra (Equus zebra), and Grévy’s zebra (Equus grevyi). The taxonomy of the group is in fact disputed, as distinct species sometimes produce fertile hybrids when they live in the same place, but let’s not worry about that now. The most distinctive feature these species have in common is, of course, their stripes: they are the only fully-striped members of the genus Equus (zebras, horses, and asses), a group containing eleven species. Here’s what the three zebras look like (notice the difference in pattern):
Now why do they have stripes? If you’re asking the question as an evolutionary one, then one way to pose it is to ask, “What were the selective advantages to the ancestor of modern zebras of having the striped pattern?” (Stripes evolved only once, in the ancestor of zebras, so we don’t require a separate adaptive explanation for each species.)
But that presumes that the stripes were or are an adaptation. Perhaps the pattern wasn’t selected for itself, but is a byproduct of some other adaptive aspect of the zebra’s biology. Perhaps, for instance they’re simply a physiochemical result of a developmental constraint in the production of any body color in zebras. That’s possible, I suppose; but I suspect, given the distinct pattern, that it really did confer some reproductive advantage to zebras, or does so now. Let us assume that it did and does enhance the survival and reproduction of zebras, and see if we can find an explanation of how it does so.
I’ve previously discussed one theory for the evolution of stripes: resistance to biting flies. I’ll highlight a recent paper on that soon, but today we’ll examine another widespread hypothesis: stripes provide camouflage that hide zebras from predators, chiefly hyenas (which take young zebras) and lions.
First, though, let’s list all the hypotheses for striping: camouflage; resistance to biting flies, which won’t land on striped objects (more on that this week); “species recognition,” so that the stripes help zebras find other individuals or the herd; “aposematism”: conspicuous coloration that tells predators, “Stay away! I can bite and kick!”; and a way to cool off by reducing thermal load.
The “camouflage” hypothesis is actually several hypotheses: the stripes hide the zebras in grassland or woodland, making them harder to see; or the stripes break up the body outline so predators can’t discern them as “prey”; or that the stripes confuse predators when they’re attacking a herd, making it hard to single out one individual to nom.
A recent paper in PLoS ONE by Amanda Delin et al. (reference and link below) tests the first two parts of the camouflage hypothesis. They did this by determining the distance at which four species (lions, hyenas, other zebras, and humans) could discern the striped pattern under three light conditions: daylight, twilight, and darkness (moonless nights). (Most predators hunt zebras at twilight). They used measurements of these species’ eyes, photographs of mounted skins as well as of live zebras in the wild, and estimates of visual acuity taken from what we know about vision in domestic cats and passing photographs of zebras through filters mimicking cat’s vision.
The researchers wanted to know the distance at which the four target species (especially the hyena and lion predators) could discern the stripes. They used the “discern stripes” criterion for a good reason: predators attack from a distance, and if they can’t see the stripes at that distance, then those stripes can’t really function to hide the animal. (They could, however, still act to confuse a predator in the midst of an attack.)
The upshot: predators are lousy at discerning stripes from even moderate distances, well short of distances at which predators commit themselves to attack. Zebras and humans (especially the latter) are much better at seeing stripes at a distance than are hyenas and lions, and the ability to discern stripes gets exponentially worse as night falls.
Conclusion: at present, stripes don’t seem to camouflage zebras from predators.
I won’t go into all the details, but below are the data tables showing distances at which the four species can resolve stripes in open habitats—under three light conditions. The table below gives the maximum distance in meters at which stripes can be recognized in different body regions (three species of zebras, two body regions for each). The graph went across the page, so the six rows can be identified from the first shot below:
Humans are pretty good at discerning stripes in daylight, but at dusk (and especially at night), you can’t see the stripes more than 170 meters away on any zebra.
ZebraVision:
Zebras aren’t as good as humans at discerning stripes, but can still see them pretty well at daylight (at least as far enough away to see conspecifics within 75-200 meters), so maybe the stripes can help animals find their herdmates.
What the charts below show is that predators are lousy at seeing the stripes, particularly under low-light (hunting) conditions. At dusk, lions can’t see the stripes when zebras are more than 50 m away, and on moonless nights they have to be right on the zebras before they can see the stripes. Since lions commit to attack at distances much greater than 50 meters, it seems as if the stripes don’t protect zebras from being seen by lions. At distances of 50 m or more, zebras look much like unstriped prey: waterbuck and topi.
Hyenas are even more myopic for stripes: beyond about 30 meters, a zebra looks to a hyena just like any uniformly-colored prey.
Here are some photos showing what zebras would look like to humans and lions at only 16.4 meters away. The caption is this:
Fig 3. A small group of plains zebra taken at a real-world equivalent of 16.4 m as they may appear to a human (a,c,e) and lion (b,d,f) under photopic (bright; daylight), mesopic (dim; dusk) and scotopic (dark; moonless night) conditions. Stripe visibility falls off from human vision to lion vision and as ambient light decreases.
This is from the close distance of 16.4 meters. But even at that distance the stripes are not very visible at twilight (“mesopic”) conditions, while under moonless (“scotopic”) conditions the zebras are just gray blobs, looking much like antelope. As I said, lions attack from distances much greater than this, so zebras at twilight would look like any other prey item. In other words, the stripes don’t appear to camouflage the zebras at distances relevant to protecting them from predators.
It’s still possible, though, that the stripes could confuse a predator once it’s in the midst of a fleeing zebra herd. But I don’t find that particularly plausible, as an attacking lion tends to single out only one individual for attack, while hyenas act as a group when taking down an individual.
The authors did find that stripes did render zebras less conspicuous in woodland, as the vertical stripes tend to hide them amidst the vertical saplings. But this still obtains only at close distances, and, as the authors note, “Thus, stripes cannot help zebras blend in with the background except when a zebra is close to a predator, distances at which predators could likely smell or hear zebras moving or breathing as they are particularly noisy herbivores.” (There’s a hint of special pleading here!)
What about the other hypotheses? Aposematism may still be viable, as once a lion is close to a zebra it might shy away it because of the stripes signalling “don’t mess with me.” That could be tested by dyeing zebras in the wild, but I don’t find that theory very plausible.
The “social recognition” hypothesis for stripes is still viable. The authors try to dismiss it, though, by saying this:
We therefore cannot reject the hypothesis that stripes may assist recognition of conspecifics or individuals, although stripes promoting species recognition seem improbable given the limited extent of allopatry in the three species of zebra. Field observations do not support the idea of stripes enhancing allogrooming, social bonding, individual recognition or being an indicator of phenotypic quality or health. Nor is striping related to crude categories of social organization, namely harem defense polygyny or to resource defense across equids where social requirements might differ. Finally, domestic horses are capable of sophisticated individual recognition using visual cues in the absence of stripes and so it seems somewhat implausible that their close relative, the zebra, needs stripes to do this.
Well, “species recognition” is not a particularly viable hypothesis anyway, and I’ll accept the authors’ notes (there’s a reference given) that stripes don’t enhance individual recognition or other forms of bonding. But stripes still could help zebras find their herds more easily, and there are of course many advantages to finding your herd and rejoining it if you wander off. And the fact that horses can find their herds even though they’re not striped is irrelevant: the question is whether zebras’ stripes give them an enhanced ability to stay with their herd. We simply don’t know the answer to that.
Reblogged this on Mon site officiel / My official website.
Fascinating, looking forward to reading the fly hypothesis. I always assumed camouflage was the pressure that gave stripes.
Looking forward to part 2!
Ditto for part 2. Read about this recently via Google News – your coverage is more extensive and clearer, thank you!
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Fascinating post!
Just this month, the PBS show Nature aired an episode that promoted the camouflage hypothesis, saying that the stripes produced an optical illusion called “motion dazzle.”
http://www.pbs.org/video/2365643333/
The zebra part runs from 1:40 to 8:15 (I don’t know if it will play outside the US).
The same idea was recently posed in a BBC-program. It sounded quite convincing.
I’ve heard the motion dazzle idea, or what would seem to be its equivalent before. But I would suggest that is not about camouflage, it’s about sowing confusion at short range – the predator can see the prey (it’s not camouflaged) but it’s hard to pick a target between the moving stripes.
This seems to be different from, for example, the stripes of a tiger, which really do camouflage the animals, particularly for color blind animals, which is most of their prey (tigers can be very hard to find in monochromatic pictures)
I tried to find a pic of a tiger nicely camouflaged in a jungle (as I’m sure they are), but due to a sort of natural selection among photographers, websites, viewers and Google, such pics are extremely rare.
(That is, photogs all try to take pics where their subject stands out, viewers all want a pic of a tiger without all that vegetation in the way, so do webmasters doing their links, and Google’s algorithms reflect that. Natural selection in action on the Internet)
cr
I thought you meant that natural selection made photographers who try to get a good photo of tigers become a threatened species – very few survive 😉
By coincidence I saw this program (Nature PBS) just a couple of days ago and just dug out the reference that idea is based on. I seems to me that this is quite different from the camouflage set of hypotheses and the specific confusion effect hypothesis that Jerry outlined. So, perhaps a different hypothesis?
How, Martin J., and Johannes M. Zanker. “Motion camouflage induced by zebra stripes.” Zoology 117.3 (2014): 163-170.
I’ve puzzled over this for years and always expected someone like Ken Ham to explain that without the stripes lions would be unable to find dinner.
I saw a thing on this in the news recently that suggested the stripes close up could cause the zebra to look like they’re moving in the opposite direction. Trying to find a reference…
Stripes seem to be fairly common among vertebrates and invertebrates [as seen by Google Images], though I don’t know frequencies and ecologies. This intrigued me:
Okapi has analogously striped legs, and I have found claims [Wikipedia] that the trait evolved for camouflage in bushy environments. (Though no references!)
Africa used to be much more forested, didn’t it? If okapi likes bushy environments, is it out of the question that zebra ancestors did as well?
By the way, where did zebra ancestors evolve? The vague memory I have of an anthropology course here is that ape ancestors migrated out of Africa into Asia contemporary with that many ungulates migrated the other way.
The story there is that zebras and donkeys are among the few surviving species of a once great dynasty horse species. This began in the American continent, where horses started out as small forest browsers. Some lines evolved to larger size with fewer toes, and teeth modified for browsing coarse grasses. Lowering sea levels allowed them to cross from North America –> Asia and Europe and eventually into Africa. Meanwhile they went extinct in the Americas. The surviving wild species include the Przewalski’s horse in Asia (which is ancestral to the domestic horse), and the donkey and the zebra species found in Africa. I may be leaving out a species or two. I am not sure.
Horses were of course re-introduced to America by the Spanish explorers. So the Indians never saw horses until then!
Just boosting the comment count on a science post.
Ditto; I read this one. Partly because science is cool, partly because zebras rock, and partly because we need to read the science posts! It put me in mind of a Stephen Jay Gould essay about whether a zebra is white, with black stripes, or black, with white stripes. I don’t recall the reasoning, but I seem to remember the “answer” was black, with white stripes.
As I recall, the black with white stripes is proven through embryology. Zebra embryos are black and when it grows, the pigment doesn’t keep up and the expanding areas turn out white. Probably got this a little goofy but that is essentially how it was determined.
I don’t remember the answer (:() but it is discussed in _Endless Forms Most Beautiful_ as I recall.
Comment number 57, right here!
Lots of animals have various patterns of stripes which clearly do camouflage them. If the zebra is just another striped animal, but the stripes are not for camouflage against mammalian predators, I’d think the selection pressures would have to be quite strong to drive them past normal camouflage all the way to maximum color contrast with sharply defined white and black stripes. With such strong selection pressures, you’d think it would be relatively easy to suss them out!
Evolution–often so tricky to pin down cause and effect…
Sometimes it could be easy to see the explanation one is looking for.
Years ago I was at a local humanist meeting and there were two guest speakers (who got into quite a row): an evolutionary biologist, and a mathematician interested in science side of genetics.
When it came to patterns in a monarch’s wing the mathematically inclined speaker began to discuss the wing patterns, and how they were similar in other species. She then got into discussion of the sequence of growth, and the gene switching in the embryo stage were similar in both species; hence the similarities of pattern. A butterfly wing is NOT like a color printer, there is a very finite limit to the number of different patterns possible, all controlled by a series of genetic events during development.
The evolutionary biologist became quite angry and insisted that this was for protection, the other researcher countered that feeding tests with birds had shown no particular aversion to monarchs or their copycats.
It developed further with people storming out of the room (who says science is boring)?
To me, neither explanation contradicted the other. Certainly not all color variations are even possible, but for one to survive, it would be expected to have at least some value.
But we may be too quick to jump on the most obvious one.
Biological pattern formation and evo-devo are one of my main jams, so I would have loved to have seen that.
Butterfly wing patterns are seen to be different combinations of possible patterns, plus modifiers for an area of color + differences in what colors are used, etc. Of course they are the outcome of conserved networks of interacting genes, and these networks are constrained in what they can do. Natural selection is not likely to come up with a greatly modified pattern b/c doing so would disturb the operations of a lot of other genes in the interacting network. So butterfly wing patterns tend to be variations on a theme, as it were.
But also what comes out needs to provide some degree of fitness or at least be neutral to natural selection.
Monarch wing colors and the wing colors of similar species are aposematic, and in some cases are the result of mimicry.
If it is biting flies, I wonder why the adaption is so rare among large herbivores.
What about that old standby, sexual selection?
If sexual selection was the answer, wouldn’t we expect to see some sexual dimorphism in stripe pattern? Is this dimorphism present (serious question, as I’m not sure)?
At Google U I just learned zebras exhibit no dimorphism, except the mountain zebra where the female is slightly larger.
It always seemed that the camouflage argument was pretty week. We see real camo in insects; and pattern disruption in some other mammals, but the stripes seem ot scream “Hey I’m a zebra” to friend and foe alike. Have the stripes on tigers been shown to be real camouflage?
Tigers tend to be ambush predators, living in places where it is a thick vegetation, so it is logical that their stripes act as a camouflage.
And as Simon Hayward noted above, tiger prey is often color blind so the stripes are indeed effective camouflage.
Here’s a wild-a** layman’s guess: cooling. The variation of light and dark provides a contrasting light/hear absorption-rate, and gradient between the two allows for faster dissipation of the heat. That or speed. They are racing stripes.
It is interesting to note that the southernmost subspecies of the plain zebra, the unfortunately extinct quagga, “removed his pajama” although mammal predators where presents where it did live. A good evidence against the camouflage hypothesis IMHO.
But it was partially stripped (banded)– mostly the front half of the animal. There should be a potential test of various hypotheses here, if we still had any quaggas.
Other random thoughts:
Is there some reason to expect relaxation of some selective pressure as one moves into more temperate regions of S Afr where the quagga lived? Less biting flies?
Donkeys often have one band across the shoulders. What is that about?
Donkey-zebra hybrids have banded legs, and sometimes bodies as well. What does that tell us?
I thought while reading this that if a predator had selected a particular animal from a distance (as they do) then suddenly when they got close their vision resolved into stripes, that might confuse their attack for a few precious seconds, thus increasing the possibility of survival. However, Jerry pretty much dismissed that idea, and I obviously bow to his expertise.
On another note, I read something in a Bill Bryson book years ago that I’ve always thought was an interesting: humans have more of their DNA in common with chimpanzees than dolphins do with porpoises or zebras do with horses. I’ve always assumed that was correct, but would be interested to have it confirmed by people who actually know.
No idea on the DNA content, but (per TimeTree on my phone) the LCA of Homo and Pan was 6.6mya, for horses and zebras it was 7.2mya and for dolphin and porpoise (had to specify, so atlantic bottlenosed dolphin and common porpoise) 18.4mya. Would expect the shared percentage to decrease with increased time and specifically number of generations from the last common ancestor. Horses breed much younger than humans (or chimps) and dolphins somewhat earlier, so at least superficially the comment would make sense.
I always loved Bryson’s description of himself as someone who was never quite sure why electricity didn’t leak out when nothing was plugged into an outlet.
Thanks Simon! Very interesting.
I didn’t know that about Bryson – you’ve got me wondering too now. 🙂
It’s a little frustrating to think that such a bold and spectacular coloration has no understood purpose. Let’s see the next installment – the next spine tingling episode.
Maybe their only other option was plaid.
“Stripes evolved only once, in the ancestor of zebras”.
Do we really know that while
“The taxonomy of the group is in fact disputed”?
What we do know is that all zebras have a common ancestor that lived more recently than their common ancestor with other relatives who are unstriped. Thus, the phylogeny suggests strongly that striping evolved after that common ancestor split off from the relatives, but before the zebras split from each other.
I always thought Zebras had stripes to inspire the uniforms of NFL referees.
Well I have 0 qualifications for judging how other animals see the world, but I thought that predators like lions saw a lot better at night than we do, as they have a reflective layer behind their retinas. Also, I thought that they did not have a uniform visual field as suggested by the pictures. I thought their visual acuity was pretty sharp in the center, and blurry around the periphery.
Perhaps the fact that zebras are hunted at dusk is evidence that its stripes are effective camouflage in daylight.
I always thought Zebra stripes were for camouflage. Thanks for this elucidation. Looking forward to the ‘biting fly’ explanation.
Is there a rule that sys there can be only one selective advantage to a trait? It seems possible to me that all the proposed benefits might play a small part in perpetuating the stripes.
Also, isn’t variation in hair color a pretty inexpensive thing in animals? That is, there isn’t much cost to it, so there isn’t much selection pressure to go back to homogenous coloration.
While we might prefer to have one clear reason for it, it seems likely to me that zebra striping is just a variation that happened to stick. Nearly neutral with maybe a bit of benefit, no big deal either way.
I agree, Steve, but I’ll go one hypothesis further: Stuff Happens. There is little reason, a priori, to pursue adaptive hypotheses that seek to explain objects or features in nature that we humans find unusual or striking. In my view the proper null hypothesis is that such oddities (to us) are evidence that evolutionary outcomes are unpredictable and need not necessarily confer a selective advantage unto the species in question. I appreciate that the authors of the paper have gone to considerable effort to test hypotheses re: Zebra coloration and PCC has kindly summarized their findings (nada, as far as I can tell). In reading the post I was reminded of Kipling’s piece titled “How the Leopard Got His Spots” and googled it. As it turns out, it was among an aptly titled series called “Just So Stories” (http://etc.usf.edu/lit2go/79/just-so-stories/1304/how-the-leopard-got-his-spots/).
I have less than nothing to add to this fascinating post or to the many erudite comments above, other than my thanks for taking the trouble to write it up in such an accessible way. Looking forward to Part 2!
Intriguing post. I’m looking forward to part 2, and to seeing PCC(E) in London in a fortnight.
🐜
Thank you for your account of this fascinating work. I look forward to instalment 2. I wonder if it would work with mozzies? 😉 If so I’m off to buy some stripy shirts”
I’m a little doubtful about reducing the effectiveness of camouflage to merely visual acuity. The chief question is not how the stripes appear to humans but how to the stripes plus movements appear to predators under various lighting conditions. There could be a two part evolutionary ‘strategy’ here. Stripes to blend into the grey of twilight (better than pure black or pure white) at the most risky time for predation plus once the predators close in for the kill a moving vertical set of stripes may be more confusing than plain colours (even grey) to particular predators’ visual fields. We don’t even know (over evolutionary time) which predators are the most dangerous.
So we need some pictures of all black and all white zebras, adult and young) plus various predator species strapped into a MRI scanner…
I agree. I am not convinced camouflage has be excluded. I look forward to reading part II.
It’s Amanda Melin, Dr. C, not Amanda Delin.
Could the stripes cause predators to misjudge distance when up close (e.g. when striking out pouncing) by the same illusion used in the 3-D effect “magic eye” drawings. Stereoscopic vision requires the brain to align images from the left and right eyes. When images have repeated patterns the brain can get it wrong and something is interpreted as further away or closer than it actually is.
I’ve noticed, though, that the stripes on the arse (butt to those in the US) of zebras are aligned horizontally rather than vertically. Vertical would be the orientation to have to potentially cause the effect in the brain of a predator attacking from behind. But perhaps predators end up with their head titled, relative to the ground, due to a sort of side-strike action that avoids them getting hoofed. I should look at some footage.
You should change your headline to read:
“Scientists researched zebra stripes and you won’t believe what they found!”
Hi Jerry,
Thanks for the science post. A small typo: The lead author’s name is Amanda D. Melin, not Amanda Delin.
Best,
Jon
If I were a zebra getting munched by a lion, I would think “How ironic. He spotted me because his damn eyesight’s NO GOOD?”
cr
If correct, all I can think of is, these biting flies must have one hell bite to drive this, coupled I guess with the sheer fly numbers in their habitat.
It could be conspecific recognition and thermal control may be a handy by product.
My interest is piqued, awaiting the second post.
I always thought the stripes were for slimming purposes.
No, everyone in Britain knows they are to ensure safe passage across a busy main road.
I scanned the comments expecting this to have been mentioned already, but no…
There is NO WAY those zebras in the photos are either ‘supposed’ to be or ‘actually’ are 16.4 metres away ! To me, they are 500-800 metres away ! Unless I am misunderstanding what the 16.4 metres actually means, sorry, they are FAR further away than that !
It depends on the focal length of the camera used. These images are, presumably, a simulation and are not necessarily supposed to appear exactly as you expect.
“A small group of plains zebra taken at a real-world equivalent of 16.4 m”
– that states they are presented such that they should indeed ‘appear exactly as you expect’ (for 16.4 metres).
– and they don’t ! They appear to be hundreds of metres away !
Even if the width of the image is your whole field of view?
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Hi Ant
Then that just gives me a mental interpretation that the view is cropped, without changing my distance perception. Just like looking through a letter box does not trick my mind into thinking objects are 10+ times nearer than they actually are !
I see what your saying. I too have trouble estimating how far they “actually” look to me. But I assume the researchers did some measurements. The problem with trying to correlate these images with your intuition is made difficult by the many variables involved. The frame in relation to field of view, as Ant mentions, is just one. The true height of the animals is another. To illustrate, imagine the zebras are actually only 3 feet high. Now how far do they appear?
“The problem with trying to correlate these images with your intuition is made difficult by the many variables involved.”
– I don’t accept that wholly, I simply observe the size of the animal with what it should actually be for 16.4 metres away (the length of an articulated lorry, only ! And it is far too small for that. In fact, just yesterday I passed (me walking my dog) a horse tethered to the rear of an articulated lorry (horse transport trailer and cab, actually). I walked past the front of the lorry (remember, that means close to the very same 16.4 metres as here)and it did not look like a rabbit standing there !
‘3 feet high, now how do they appear ?’
– Still at least 100 metres away…read my reply to Ant for further info on my thinking.
– But come on, look at the visual detail and size for an animal supposed to be just 3-4 car lengths away, absolute nonsense !
I went to the paper to see if I could find their method of establishing real-world distance, but I didn’t find it. One of the references described a similar study, but distance was not used as a variable.
Another thought:
By the description of the method, it could be that the real-world distance is simulated mainly by the degree of contrast in the image, and not by the perceived size of the subject.
Why don’t you contact the authors and see if they can explain the issue?
‘degree of contrast in the image’, not sure exactly what that might mean, but the visual detail is severely lacking for 3-4 car lengths away. You should be able to discern much more detail and contrast at that short distance. I appreciate your effort, but I am not that interested to go to the lengths of contacting them, the issue is minor in the context of the paper.
(assuming people read this post)
I did, which is all I have to say at the moment.
I’m glad this has never been an issue, either teaching or in research, but I have always had the definitions of photopic and scotopic vision backward. I wonder if I was taught it that way by an absent-minded physiology professor when I was an undergrad at UCLA.
here is an alternative that might be interesting
http://www.sekj.org/PDF/anzf44/anzf44-368.pdf
How did this species evolve?
That reminds my of _Daktari_!
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The stripes on young zebras (at least on Plains zebras; I have not seen either of the others) are brown, and I have photographs to prove it. I would find an explanation for stripes more credible if it could account for this.
I was really surprised at this part:
“The upshot: predators are lousy at discerning stripes from even moderate distances, well short of distances at which predators commit themselves to attack….
Conclusion: at present, stripes don’t seem to camouflage zebras from predators.”
If predators were really good at discerning stripes, would that mean the stripes were good camouflage? I’m not really sure how the poor eyesight of the predator for the stripes makes it harder for the prey to hide… And of course background would really play a part.
For my own interest, I find the stripes very interesting as fractal patterns and would love to know how the DNA sequence translates into the math. 🙂
Do the stripes on tigers and Monarch butterflies help them camouflage? What about my cream tabby cat? 😉
Just to be contrary: maybe lions and hyenas evolved to have poor vision so that they wouldn’t be confused by the stripes.
Do closely related species in non-zebrified environments have better vision than lions and hyenas?
Are there other potential predators (e.g. cheetahs) that have better vision and do NOT successfully hunt zebras much, possibly because the stripes confuse them?