Dazzling stripes are a way of deterring flies. Photo: Getty
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How the zebra got its stripes

A method for dodging predators? A means of social interaction? Or a way of getting rid of flies?

Zebras' stripes have baffled biologists since Charles Darwin. Many hypotheses have been proposed regarding their purpose but, despite hundreds of years of study, there remains disagreement.

In an attempt to end the debate, researchers have pitted various models against each other and systematically analysed data from past studies. Their results reveal the one reason zebras have stripes: to ward off flies.

A handful of ideas regarding zebras’ stripes have found some support among biologists. One proposed that the dark and light bands change how air flows around a zebra’s body and helps in heat management, which could go a long way in the hot tropical areas that zebras live in.

Another proposed the stripes were used by zebras as a way of social interaction. They may use them to identify other zebras and for bonding as a group in the wild.

A third proposal suggested zebras used the stripes as camouflage. While stripes are clearly visible in the day, there some thought that it helped at dawn, dusk, and in the night.

All these ideas were shot down when tested rigorously. Two others, however, remained intriguing.

The first was that the stripes were used to dodge predators. It is called the “motion dazzle hypothesis”, and it suggests predators are confused by zebras' stripes and cannot understand their movement. Research published in the journal Zoology in 2013 used a simulated visual system to show that zebra stripes do interfere with visual perception. But this is a difficult hypothesis to test in the field.

Martin Stevens at the University of Exeter has researched the motion dazzle hypothesis by getting human subjects to catch moving stripy objects on a computer. “It’s an artificial experimental system,” he admitted.

The second proposal was that stripes helped keep flies at bay. Zebras are especially susceptible to biting flies due to their geographic spread. These flies, which include the tsetse fly, stomoxys stable flies, and tabanid biting flies, are particularly prevalent in areas with high temperature and humidity – exactly the areas where zebras are normally found.

Bites from these flies can be nasty and, quite literally, draining. About thirty flies feeding for six hours on just one horse can draw as much as 100mL of blood. Usually the flies can number in the hundreds around one animal.

Zebras have shorter hair than other equids – the family that includes horses, donkeys and zebras – which may also increase their susceptibility to attack. Also, four diseases which are fatal to equids have been found in Africa. This could mean that investing in anti-biting defenses such as stripes is especially important for zebras compared to non-African equids.

It is possible that the dazzle effect acts on flies, rather than larger predators, and deter them from biting. “Stripes clearly have a number of functions,” Stevens said, “and these could be interacting in zebras.”

Revealing maps

In the new research, just published in Nature Communications, Tim Caro and his colleagues at the University of California in Davis, didn’t perform experiments. Instead they used ecological and observational data on zebras' geographical locations and related factors. It is the first time that a comparative approach has been applied to find the reasons for zebras' characteristic colouration. Caro thinks his findings may have nailed the answer at last.

Caro looked at seven species of equids and scored them for number and intensity of stripes. Just to be sure, they tested all five hypotheses regarding zebra stripes' use: camouflage, predator avoidance, heat management, social interaction, and warding off flies. The extent of overlap between the geographic distribution of striped equids with each of these five measures was calculated.


E. greyvi, E. burchelli and E. zebra have stripes on all their bodies. Other equids don’t. Caro, Izzo, Reiner, Walker and Stankowich

“The results were a shock to me,” said Caro. Of these five proposals, only warding off flies had statistical support. He had not expected such a clear-cut answer to the question. As the map shows, the only places where flies and equids live together are areas that are populated by striped equids.

The exact mechanism by which stripes deter flies remains unknown, but experimental studies performed by researchers at Lund University in 2012 have found support for this proposal. They created striped surfaces and stuck glue on them. Based on the number of flies on the surfaces with different thicknesses of stripes, they concluded that these flies stayed away from stripes as thin as those found on zebras.

“As is normal in science you get a solution that asks more questions,” Caro said. It is time to hand the problem over to vector biologists, who can understand the susceptibility of horses to biting flies.

In Darwin’s days, people didn’t consider animal colouration with respect to fitness advantages. “People thought that animal colouration existed simply to please humans or was caused directly by the environment,” Caro said.

Darwin “would be delighted” that researchers are now considering animal colouration as a functional trait, he said. We might not have all the answers regarding zebra stripes – but it seems we may be looking through the right lens.

The ConversationThis article was originally published on The Conversation. Read the original article.

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Brain training: exposing the myth behind cognitive-enhancement games

A new study indicates that any benefits gained from brain games may be down to the placebo effect.

If you’ve ever searched for a quick-fix to mental lethargy, it’s likely that you’ve browsed through your smartphone app store to take a look at the latest offerings of brain-training games.

I certainly have. These games have been designed to sharpen people’s mental acuity, while offering “scientifically proven” means for improving IQs; through a variety of mini-games and careful documentation of improvements to intelligence parameters, people would wield the tools needed to craft the desired, smarter minds that the apps promise.

And the market for them has showed no sign of slowing down. In the space of a few years, the demand for the apps has made the industry a billion-dollar one, with growth expected to continue. A couple of the most popular apps have included Lumosity, a web-based program boasting more than 50m users seeking to “improve memory, attention, flexibility, speed of processing and problem solving”, and mobile-based Peak, whose similar goals and striking visuals entice potential users.

Though the apps have had huge amounts of success, there is a new body of research emerging to suggest that the successes may not be as a result of the games themselves, but because of the placebo effect.

The placebo effect is a phenomenon in which a dummy treatment or process can cause significant changes in a person – simply because that person believes the placebo (posing as a real treatment) will help them. With medication, it can be the mere presentation of a sugar pill disguised as a medicine which can cause a patient to get better. And in the case of apps and games, it seems that anything which promises users cognitive benefit, is more likely to do so.

In a study entitled “Placebo effects in cognitive training” published on Monday in the journal Proceedings of the National Academy of Sciences, researchers found that participants who engaged in brain-training games for a single, one hour session showed improvements in IQ by up to ten points, but only if they believed the games would benefit them.

The group of cognitive scientists from George Mason University, Virginia, set up the experiment in a particular way to determine whether or not the placebo effect was involved.

50 participants were recruited, after two different posters asking people to sign up to a study were plastered around campus: one labelled “brain training & cognitive enhancement” and the other “email today & participate in a study”. The rewards for the former promised boosts in intelligence, while rewards for the latter granted study credits. Unknown to participants, however, was that both tests were the same, meaning any resulting changes to IQ were as a result of what participants were telling themselves about the tests.

The tests centred around the engagement of working memory and other factors to impact fluid intelligence – a type of intelligence which revolves around the application of logic and reason, independent of acquired knowledge. Those who chose to sign up to the “brain training & cognitive enhancement” study, aka the placebo study, were the ones to show remarkable gains in IQ after completing a post-brain games IQ test; gains of five to ten IQ points being made. Those who signed up for the control showed no signs of improvement.

Speaking to the Huffington Post, researcher and co-author of the study Cyrus Foroughi said: “Placebos are very pervasive and they have to be controlled for in a tremendous number of fields. This field is no different. So we put together the study to actually test whether expectation for a positive effect can lead to a positive outcome.”

Within the scientific community, frustration had already mounted as a result of the falsely promoted uses of brain games, particularly as tools to reverse age-related, cognitive-faltering illnesses such as Alzheimer’s disease. Overstated claims through advertising were enough to encourage scientists to sign an open letter in 2014, condemning the inaccurately purported benefits of brain training games. Earlier this year, Lumosity was fined $2m by the Federal Trade Commission for deceiving consumers with “unfounded claims”.

The recent findings strengthen this position, as the effects of cognitive training games seem less to do with the content of the games themselves, and more to do with what users tell themselves will happen after a session of, brain-training puzzle bonanzas. That’s not to say the games themselves don’t offer some benefit – it’s just that further clarification is needed to understand what they exactly contribute to, with the placebo effect factored in.

While scientists expand on their research to pinpoint the real effects of brain games, it seems for now that the best options to keep our brains active are the ones we are most familiar with: learn a language, do some exercise, or maybe just read a book.