Darwin himself pondered the reason for the stripes, and New Scientist describes his hypothesis as contrary to other ideas of his time:

A popular theory, both in the 19th century and today, is that zebras evolved striped coats as camouflage in tall grass… Darwin suggested that zebras developed their unique stripes to recognize each other, which could be particularly important for male and female courtship.

And the latest theory might have surprised Darwin: horseflies. Researchers in Hungary and Sweden published a paper in the recent edition of the Journal of Experimental Biology that blames the zebra’s stripes on simple horseflies (tabanids).

Well, sure… Horseflies deliver nasty bites, carry disease, and distract grazing animals from feeding. But Discover’s 80beats takes it a step further:

There’s good evolutionary reason to escape the ravages of horseflies, at least for horses and their relatives; though flies are just annoying pests from the human perspective, horsefly-bitten horses can grow skinny and have trouble producing milk for their young. And as soon as baby-making is affected by something in the environment, adaptation isn’t far behind.

Scientists have known for a while that these pesky insects are more attracted to dark horses than to white horses. Since zebra embryos start out with dark skin, the European team wondered whether the zebra’s stripy hide might have evolved to disrupt their attractive dark skins and make them less appealing to voracious bloodsuckers, such as horseflies. So they conducted some experiments, minus the actual zebras.

Travelling to a horsefly-infested horse farm near Budapest, the team tested how attractive the blood-sucking insects found black-and-white striped patterns by varying the width, density, and angle of the stripes and the direction of polarization of the light that they reflected. They trapped the attracted insects with oil and glue, and found that the patterns attracted fewer flies as the stripes became narrower, with the narrowest stripes attracting the fewest horseflies.

“We conclude that zebras have evolved a coat pattern in which the stripes are narrow enough to ensure minimum attractiveness to tabanid flies,” says the team. “The selection pressure for striped coat patterns as a response to blood-sucking dipteran parasites is probably high in this region [Africa].”

Darwin would be proud.

Image: Muhammad Mahdi Karim/Wikipedia

Why are humans so crazy about sports?

Well, according to some Belgian researchers, it’s due to evolution. In an article they published last year in Perspectives in Biology and Medicine, cultural evolution plays a part in our love of sports, but if you get down to the brass tacks of it, sports is all about sexual selection.

According to the article, “From a Darwinian perspective, sports may be seen as one of the cultural activities invented to promote the acquisition of status. And acquiring status is—on average, in the long run, and in the ancestral environment to which our species is adapted—beneficial to an individual’s reproductive success.”

Nick Paumgartner of the New Yorker knows this is true. In an article he wrote earlier this month, he talks briefly about the behind-the-scenes sexual successes of athletes during the Olympics. But it’s how he feels as an amateur athlete that we all understand: “I suspect that skiing is, in some respects, an act of vanity. You want to be seen doing it. You make a mark in the snow and ask others to take note.” Sounds a little like a parading male peacock, huh?

In a recent Scientific American blog discussing this matter, Jesse Bering points out that the status is transferred from athlete to fan. “Sports that allow athletes to clearly showcase their most evolutionarily important attributes—strength, intelligence, endurance, speed and litheness, for example—attract the biggest following.” Basketball, check, but how do you explain the popularity of curling at last month’s Olympics?

As a fan, we take on the athletic ideals of the members of our team. Bering sites another study that found that following a World Cup soccer match, “Mean testosterone levels increased in the fans of winning teams and decreased in the fans of losing teams.”

So, your sports fanaticism? Thank evolution. But be careful, it can also be life-threatening. Heart attack rates have been shown to rise during the Super Bowl and World Cup. Go team, but take it easy.

(Update: more recent articles on dangerous sports-watching were published this week here and here.)

Image: Mahanga/Wikimedia

Besides the 1960 and 2010 earthquakes there have been several other large quakes. Science News provides the reason:

Off the western coast of South America, the Nazca plate of Earth’s crust dives beneath the South American plate, pushing up the Andes and building up stress that gets relieved occasionally in powerful earthquakes.

In 1835, Charles Darwin was in the area and experienced a large magnitude-8.5 earthquake. Though there have been five other great earthquakes since then, none have ruptured in the same area that shook in 1835, a locality now known as the Darwin gap. Scientists, aware that the Darwin gap has been accumulating pressure for over 100 years, had been expecting a large quake in that gap.

But the 2010 quake was north of the Darwin gap. According to New Scientist, researchers studying the area after the recent quake

used tsunami, GPS and radar data to assess the amount of land movement during last year’s quake. By feeding this into a model, they were able to estimate the amount of slippage on the fault and the variation in the release and accumulation of stress along it.

And, the results were striking (from Scientific American):

When pressures build up enough, they snap and cause a quake. Some areas, deep below ground to the north of Concepcion, slipped almost 20 meters in the 2010 earthquake but the area of the “Darwin gap” barely moved.

So rather than relieving and reducing the possibility of another large earthquake in the area, this most recent quake may have increased the possibility of another magnitude-7 or -8 earthquake occurring in the near future. The researchers published their findings in this week’s Nature Geoscience:

…increased stress on the unbroken patch may in turn have increased the probability of another major to great earthquake there in the near future.

Image credit: R. Stein, Lorito et al/Nature Geoscience 2011