"the source of the effect remains unexplained" They list several candidate explanations, but miss one I came up with immediately: maybe zebras are in some other way less attractive targets for the flies -- their skin is thicker, their blood/fluids don't taste as good, etc. -- and the stripes are simply a visual indicator to the flies that they are zebras, and therefore less desirable. This sort of trait has evolved poison frogs, insects, and other plants and animals, so why not zebras?
This is mentioned in the paper, and you can find citations there to previous experiments where the visual effect alone is tested and demonstrated to be very strong. Moreover, the researchers took steps to limit any olfactory effect by salt-curing the hides used in this experiment. Whatever other defenses a zebra has, it's been shown that flies really hate those stripes.
I think the parent is acknowledging that the stripes have an effect, they're whether the authors considered that some other property of zebras is unattractive and the stripes are a good indicator, leading to an evolved preference among the insects against stripes.
Here's an analogy: people often fear and avoid insects that are yellow/black striped, not because the stripes are annoying, but because many such distinctively striped insects have a painful sting, and avoiding the stripes avoids the stinging insects.
I understand what the parent (and you) are saying; what I was trying to diplomatically suggest is that they read the paper, where these issues are all addressed. Biologists who publish in Nature anticipate a surprising number of the objections that might occur to Hacker News readers in the first ten seconds of thinking about a topic.
I have read (quickly) the paper. I see nothing there that addresses the idea that the stripes might be simply an identifier the flies use. Rather, the paper seems focused on the effect the stripes have on the flies rather than the idea I suggested. For example: "If, as our results indicate, stripes that span the range of interspecific stripe width variation repel flies to the same extent, adaptive explanations for stripe width variation would have to pertain to some other selective pressure."
The use of "would have to" completely ignores the potential of stripes to be an identifying feature. If my hypothesis were correct, there would not have to be any other selective pressure; since all stripes identify "zebra," any stripes would work.
For example, to test my hypothesis, it might be sufficient to check whether, in those zebras where some of the hide is tan rather than striped, the tan portion of the hide is attacked by the flies disproportionately to the striped portion of the hide.
In sum: the paper reflects on the mechanism of the stripes in repelling flies, and seems to disregard the possibility that the stripes simply allow the flies to identify "not a good target."
One finding that comes to mind is the fact that of the flies that do land on the zebra pelt, nearly all (75%) land on the black stripes. This suggests that something more is happening than just identifying the zebra as an unappealing target.
You also have experiments like this, where painting Japanese cows in zebra stripes was an effective fly deterrent, even though the flies there have no evolutionary history with zebras. (It's worth clicking just for the photo of the test cow.)
1. It's possible that Japanese flies are repelled for different reasons than African flies.
2. It's possible that flies are repelled both because the stripes interfere with the flies' vision in some way, and because the (African) flies prefer other animals.
Nice, thanks. The original article's claim that "Though the effect of stripes on flies is well-established, the source of the effect remains unexplained." is over-broad and incorrectly pessimistic.
Typically, if a question is super logical and fundamental to the thesis of a finding, you can assume the reviewers asked the authors to address it, and typically they do in the paper. And they do in this paper.
"Controlled experiments have used various landing substrates, including striped and solid oil tray traps, sticky plastic, smooth plastic, cloth (Experiment 2 in [22]), horse blankets or sheets, and paint on live animals."
Our findings confirm that zebra stripes repel biting flies under naturalistic conditions and do so at close range
Which has lead to suggestions to breed this into livestock. Which will certainly change the landscape in US.
As I said before: Somehow driving through South Dakota looking out over a vast field of seaweed eating, zebra striped cows was not the future I anticipated as a youth.
It’s possible this only works when only some animals in the neighborhood have stripes, I.e., the insects have other targets available and they’d just choose the less distracting one. In the absence of any other alternative they might end up still biting the striped animals.
Biting flies have all kinds of food sources- I live in an area of the midwest with no animal farms for several miles (lots of trees and water springs though) and the woods are so thick with deer flies that I can't take my dogs with on a walk- their ears will have 6 flies each before we get 50 feet.
Deer, bears, coyote, wolves, raccoons, mice, otters, beaver, rats, muskrats, cougar, bobcats, chipmunks and squirrels, elk and moose, skunks, badgers, porcupines can all be found in nature in the upper midwest, along with various moles, voles, shrew and weasel family members can be found pretty much anywhere outside of a city center.
Since deer fly (and other biting flies) typically need a bit of standing water to breed, they're going to be found pretty much anywhere mammals live. If they decide they don't like striped cows, there's going to be plenty of other options to munch on.
Now that you mention "not remotely connected to tech" and "black and white" . . .
I like the introduction section of this paper (basically speculating "How the Zebra Got His Stripes") as a modern descendant correcting some of the misconceptions from Rudyard Kipling's How the Leopard Got His Spots[0].
Especially a tale of an imaginary past when leopards and men had not yet been magically given their recognized colorings at the time.
But Zebra and Giraffe already had the patterns we know today:
Zebra moved away to some little thorn-bushes where the sunlight fell all stripy, and Giraffe moved off to some tallish trees where the shadows fell all blotchy.
'Now watch,' said the Zebra and the Giraffe. 'This is the way it's done. One—two—three! And where's your breakfast?'
Leopard stared, and Ethiopian stared, but all they could see were stripy shadows and blotched shadows in the forest, but never a sign of Zebra and Giraffe. They had just walked off and hidden themselves in the shadowy forest.
'Hi! Hi!' said the Ethiopian. 'That's a trick worth learning. Take a lesson by it, Leopard. You show up in this dark place like a bar of soap in a coal-scuttle.'
'Ho! Ho!' said the Leopard. 'Would it surprise you very much to know that you show up in this dark place like a mustard-plaster on a sack of coals?'
I like the disclaimer Disney puts up on some of its streaming items - it's something like
"This content stereotypes some peoples or cultures - it was wrong then and now but we are keeping it here as a historical document (and to spark conversation)"
(If you have Disney+ go look for the original Dumbo)
I think people have always known that "the zebra (or anybody else) can't change his stripes", but its stripes can change on him (over time). Ages before the concepts of genetics or evolution, they still knew the stripes had to come from somewhere.
And the latest findings here can be convincing to an extent, but not entirely conclusive.
So as it stands nobody still knows for sure how the zebra got its stripes.
I recently learned this from BBC's Life in Colour documentary. It explained it, similar to the article (although the paper really dances around this hypothesis), that the flies had a hard time landing due to some visual weirdness from the stripes when the flies are close up. They had a lot of close-up footage of flies hovering above a zebra's skin but seemingly confused on how to land.
Intuitively, it does seem like stripes would make it more difficult to achieve focus for a compound eye, similar to how when looking through a chain-link fence or window screen it's easy to focus the wrong plane due to the spaced repetition.
Maybe the adaptation would require significantly more advanced visual processing with attendant increase in weight, size, and energy consumption, making it not worth it?
Evolution is two-sided race. Insects breed/evolve much faster than large mammals. One must wonder why they haven't evolved the brainpower to overcome this trickery. The difficulty of feeding on zebra must not be enough of an evolutionary pressure ... until all the other animals start evolving stripes too.
Good point. Maybe there's competition from alternative patternings. Like camouflage type patterns for hunters / preppers / military cosplayers. Or high-viz colors for people who worry about safety wrt. various kinds of accidents. Or just whatever the current per se meaningless trend colors are?
Flies have been around since the Triassic [1] so could there be convergent evolution of zebra-like stripes at different points in Earth’s history, similar to how crab-like exoskeletons have evolved multiple times [2]?
The idea that vertical stripes confuse a fly’s visual processing reminds me of Peter Watts’s Blindsight, in which vampire legends are based on a real H. sapiens subspecies that predated on baseline humans in the Paleolithic but went extinct once we started building permanent settlements. This was caused by an (un)fortunate bug in the vampires’ visual cortex that made them unable to perceive right angles without glitching out (this, naturally, would also have been the source of the belief that vampires are repulsed by crosses). As a biologist, Watts was no doubt aware of the zebra stripe hypothesis when he wrote Blindsight.
My idea (consider this public domain / prior art if no one has though of it)
We introduce tree frogs or perhaps geckos or chameleons to live on cows. They would eat the flies and the body heat could probably keep them warm year round.
They’d eventually breed themselves to adapt to the life cycle and be self perpetuating.
This hypothesis has never made sense from an evolutionary standpoint, as it doesn't explain why zebras are the only animal (on a similar environment) that show this traita.
Literally the FIRST two sentences in the abstract:
"The best-supported hypothesis for why zebras have stripes is that stripes repel biting flies. While this effect is well-established, the mechanism behind it remains elusive."
