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Scientists Solve the Mystery of Why Zebras Have Stripes
Hugh Pickens DOT Com (2995471) writes "There have been many explanations for the zebra's impressive stripes including Darwin who thought that the stripes help males and females make sensible choices about whom they mate with. Now Henry Nicholls reports at The Guardian that Tim Caro at the University of California, Davis, has taken a completely original approach, stepping back from one species of zebra and attempting to account for the differences in patterning across different species and subspecies of zebras, horses and asses to see if there is anything about the habitat or ecology of these different equids that hints at the function of stripes. To answer that question, Caro and his colleagues created a detailed map charting the ranges of striped vs. non-striped species and subspecies. Then they worked on a map for the bloodsuckers that targeted those species — specifically, abanid biting flies (horse flies) and tsetse flies.
'I was amazed by our results,' says Caro. 'Again and again, there was greater striping on areas of the body in those parts of the world where there was more annoyance from biting flies.' Where there are tsetse flies, for instance, the equids tend to come in stripes. Where there aren't, they don't. Biologists who buy into the bug-repellent hypothesis say that, all other things being equal, striped animals would have an evolutionary advantage because they wouldn't suffer from the loss of blood, reduced weight gain and lowered milk production that's associated with bug bites. Tsetse flies are also associated with the transmission of diseases. 'There are a lot of them, such as sleeping sickness, equine anemia and equine influenza,' Caro says. Why would zebras evolve to have stripes whereas other hooved mammals did not? The study found that, unlike other African hooved mammals living in the same areas as zebras, zebra hair is shorter than the mouthpart length of biting flies, so zebras may be particularly susceptible to annoyance by biting flies. 'It's clear that the flies can get through that hair and get to the skin.'"
'I was amazed by our results,' says Caro. 'Again and again, there was greater striping on areas of the body in those parts of the world where there was more annoyance from biting flies.' Where there are tsetse flies, for instance, the equids tend to come in stripes. Where there aren't, they don't. Biologists who buy into the bug-repellent hypothesis say that, all other things being equal, striped animals would have an evolutionary advantage because they wouldn't suffer from the loss of blood, reduced weight gain and lowered milk production that's associated with bug bites. Tsetse flies are also associated with the transmission of diseases. 'There are a lot of them, such as sleeping sickness, equine anemia and equine influenza,' Caro says. Why would zebras evolve to have stripes whereas other hooved mammals did not? The study found that, unlike other African hooved mammals living in the same areas as zebras, zebra hair is shorter than the mouthpart length of biting flies, so zebras may be particularly susceptible to annoyance by biting flies. 'It's clear that the flies can get through that hair and get to the skin.'"
So, this is why very few referees suffer from fly bites? I always wondered.
You know, if you're going to just copy and paste part of the article as your summary, you might as well post the last paragraph, and get to the actual explanation:
Zebras have stripes because biting flies have an aversion to landing on striped surfaces.
Are Zebras Black with White stripes or White with Black stripes?
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Relevant quotes missing from summary:
"researchers built horse mannequins, painted them in a variety of patterns, coated them with sticky stuff, and found that horseflies seemed to avoid landing on the fake horses that were painted with black and white stripes."
"The proposed explanation was that the flies preferred to land on dark surfaces. Such surfaces reflect the kind of polarized light that reminds the flies of the water or mud where they breed. Light surfaces aren't as attractive, but dark-and-light patterns are even worse — perhaps because such patterns confuse the flies' navigational sense."
I thought the stripes broke up the outlines of individuals and made it harder for predators to single one out of a crowd? Or did no actual research go into that claim?
Evolution is like government, sometimes progress or in this case evolution makes little sense.
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The wikipedia article on Zebra's links to the following for a possible explanation to the origin of these stripes:
http://www.bbc.co.uk/nature/16944753
Notice anything similar?
There was a similar story a few months back, also from the BBC, about a study with slightly different conclusions than polarisation of light.It concluded that the stripes cause optical illusions when moving. Link.
Longer hair might have other disadvantages, such as worse heat dissipation and (slightly) more weight. Longer hair might also make them more susceptible to burrs, ticks, lice, or other bugs.
Zebra stripes have traditionally been thought of as an adaptation against detection by vertebrate predators such as lions and hyaenas. A different hypothesis is suggested: that the stripes are an adaptation against visually orienting biting flies and act by obliterating the stimulus presented by a large dark form, which is important in host-finding by many Diptera. This hypothesis is supported by some indirect evidence, and by a field experiment in Zimbabwe in which biting fly catches were compared on moving and stationary black, white and striped models. Striped models caught significantly fewer tsetse (Glossina morsitans) Westwood and other flies (including tabanids) than solid black or white models, but this difference was much reduced in the presence of olfactory attractants.
~Waage, J. K. (1981)
Maybe people studying zebras should start by reading the zebra wikipedia page.
Why does the title claim this is solved? Even the summary calls it a hypothesis
Just playing devils advocate (I am sure there is a joke in there somewhere)...
Anyway, as I understand it, evolution is about the selection of traits for survival, This usually involves environment, eating/not getting eaten, and procreation.
It very well could be that Zebra's with their short hair, developed stripes to hide from biting insects, as their survival was significantly impacted enough to warrant the change. While on the insect side of things, perhaps they have enough of a food source that missing out on the Zebra buffet isn't a significant survival issue, and thus never bothered to evolve any eyeballs capable to seeing them for lunch (or perhaps the Zebra evolution isn't all that effective anyway).
What is more interesting to me however, if this explanation is the case, then why didn't Zebra's just evolve longer hair? Then again, I suppose it is hot, so that might not work out so well. Then again not everything has a lot of hair or is striped in those parts either. I am pretty sure evolution isn't really all that exact anyway, which is partially why it takes so damn long to produce changes over generations. I liken it to randomly programming solutions to a problem, some are better than others, but some are pretty good and stick around for quite awhile, or are just good enough, though over time the best solution will get used more often eventually.
Also mixed into the mess is not only physical things like hot/cold, eat/eaten, procreation, but behavior based on those traits. Basically at which point is a stripey Zebra more sexy to another Zebra as that is perceived as better unconsciously. Oh baby, that's nice stripes you got there... I have to think there is also a significant lag time between physical evolution, and behavioral evolution, as the one pretty much has to occur before the other. Perhaps that is the point, if a trait sticks around long enough, it sort of proves itself a bit, which then kicks in the behavior modification, which further reinforces the trait...
Anyway interesting to try and figure it all out, even if only a thought experiment.
AFAIK the Waage study did not map the respective habitats of zebras and flies; that is what is actually new in this study, and it supports the Waage hypothesis.
Have these scientists demonstrated something about flies vision that the stripes interfere with?
That much has already been shown to be the case - or at least, that flies have an aversion to landing on striped surfaces.
I had heard the theory that zebra striping was a kind of dazzle camouflage [wikipedia.org] which confused larger predators when trying to pick out one animal to pursue.
It can, of course, be both.
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