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Comment by gcanyon | Hacker News Reader

"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?

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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.)

https://journals.plos.org/plosone/article?id=10.1371/journal...

Thanks for the link; that was indeed worth a click.

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.

But I'll add that:

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.

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.

Seems easy to test? Just paint a horse or cow and keep count of the flies.

It doesn't differentiate the flies mistaking the cow for a zebra, compared to the stripes themselves doing something else to keep the flies away.

It does when the flies are circling a cow in the UK or Japan.

Check out this snazzy looking fellow: https://www.smithsonianmag.com/smart-news/scientists-dressed...

Is it repelling or camouflage?

Yes

Regardless of perceived ease, they cover this:

"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."

maybe zebra skin has a different oder. They said they used pelts.

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