We know that many of the traits we see in animals are present because they have evolved to be useful to the species. But the evolution is messy and the supposed benefits of strange features are not always obvious. Take zebra stripes, for example.
The black and white clothing of the three zebra species wandering in Africa has been a biological puzzle for decades, with several contradictory assumptions. Now the researchers have added a new data point – and all it took was to dress up with horses.
Okay, there was more science than that. But the results of the team showed that the bands prevented the flies from landing, thus protecting the zebras from insect bites.
This is not a new idea. Scientists have known for years that fewer flies land on zebras. A 2012 study even found that light and dark bands could reflect polarized light to deter insects in flight.
We are not yet quite sure of the exact mechanism, but American and British scientists have reduced it. They found that flies landed on horses more than three times as often as zebras, because streaks apparently confuse insects and diminish their ability to control their flight.
The team spent several hours observing the horses and zebras to record the number of blood-sucking flies (tabanids) that were buzzing, approaching and landing on the animals.
Then, to make sure there was not something inherent in the zebra odor or movement that changes the behavior of the flies, they turned to disguise to try to dismiss these variables (zerables, if you want).
To do this, the team donned coat horses – plain black, plain white, and black and white striped – and again recorded fly activity. They also took videos of horses and zebras to do a detailed analysis of what the flies actually do; and observe the behaviors of both types of animals in response to flies.
They found that the flies were about equal in number around the two animals and approached zebras as often as horses. But flying to a striped coat? It's at this moment that things get tough.
"We found that tabanids approaching zebras did not slow down in a controlled manner towards the end of their flight path, while they slowed down regularly before landing or touching the horse's coat," wrote Researchers.
"The flies often ran into zebras but did not land or fly."
Interestingly, flies have always landed on the exposed head of the horse wearing the striped coat as often as on normal horses – indicating that the coat was having an effect.
And that's not all. When the flies managed to land, they did not stay that long on the zebras. Videos showing the reaction of animals to flies have shown that zebras spin their tails and escape more actively than horses.
Overall, reducing the time spent on zebras means that flies do not sting and feed on them as much as horses, which shows that scratches play a role here.
Of course, it is possible that bands perform several functions. Until now, studies seem to have excluded camouflage in the environment, social interactions and thermoregulation, but it is still possible that the bands create confusion between closely packaged herd predators.
However, the evidence presented here indicates that one of the advantages of scratching is increased protection against biting flies.
We wonder if this is the last time we hear about it.
The research of the team was published in the journal PLOS One.