Lots of wild animals avoid power lines, often staying up to 3 miles away, but scientists have never quite understood why. Unlike roads, suspended electrical cables don't present much of a physical barrier and aren't associated with human traffic. Why not just walk under them?

The answer, according to a new study, is that many animals see power lines as flashing streaks of ultraviolet (UV) light across the landscape. UV light is invisible to humans, but another recent study reports that lots of other animals do see it. Earth's 43 million miles of elevated power lines could thus have a hidden side effect on wildlife, creating yet another barrier in already-fragmented habitats.

"The loss and fragmentation of habitat by infrastructure is the principal global threat to biodiversity — it is absolutely major," lead author and University of Tromsø ecologist Nicolas Tyler tells the Guardian. "Roads have always got particular attention but this will push power lines right up the list of offenders."

Since open corridors are cut through forests to make space for power lines, past research has suggested animals avoid the areas because they're more vulnerable to predators there. That doesn't apply to treeless places like Norway's tundra, though, where reindeer already live out in the open yet still avoid power lines. And while it's well-known that electric lines are dangerous to birds, this UV threat is subtler, researchers say. Bursts of UV light may not directly harm animals, but they can be a stressful stimulus that disrupts natural behaviors like migration, breeding and grazing.

"There are hundreds of examples of animals avoiding power lines," says Tyler, who studies the reindeer-herding Sami people of Scandinavia. "Now we know that, not only do these clear-cut corridors mean exposure to predators, at the same time there is this damn thing flashing at you."

Electric utilities already monitor their power lines with helicopter-mounted UV cameras, looking for flashes that can reveal conduction problems (see the video below). Yet those cameras only show a limited range of of UV light, merely hinting at the intensity of light seen by animals.

"Most mammals will let some [UV light] into their eye," University College London vision expert Glen Jeffery tells the BBC. "We're weird — us and monkeys — because we don't see UV. Most animals do."

Jeffery, who co-authored both the UV-vision study and the newer one on power lines, says animal autopsies show that a wide range of mammals see UV light, including dogs, cats, cows, bats and ferrets. He also worked on earlier research, published last October, that revealed reindeer eyes change color with the seasons to help them see more UV light in winter and less in summer.

"Reindeer see deep into the UV range because the Arctic is especially rich in UV light," Jeffery explains. "The animals potentially see not just a few flashes, but a line of flashes extending right across the horizon." In Norway, wild reindeer have divided into 23 distinct populations due partly to habitat fragmentation by man-made infrastructure like roads and power cables.

UV discharges can occur as "standing corona" along cables or as irregular flashes on insulators, the researchers note, and while power companies try to minimize such leakage, some amount of discharge is often seen as inevitable. Jeffery says burying all power cables would be impractical, but he adds there may be ways to hide UV flashes by covering cables with non-conducting shields.

And, as Tyler points out to the Guardian, this revelation about corona should at least help improve the placement of newly built power lines, especially in places where UV-sensitive animals like reindeer live.

"Right now, there is a plan to build a 186-mile-long power line in north Norway," he says. "This new work will encourage power companies to negotiate with herders about where they put the power lines."

Russell McLendon is science editor at MNN. Follow him on Twitter and Google+.

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