After two misguided tourists tried to rescue a baby bison at Yellowstone last week, the calf ended up euthanised by park officials who said the human intervention had caused its mother to reject it. The tourists were fined $110 for violating park regulations, but they apparently got away without a scratch.

They were probably lucky the calf's adult brethren didn't get involved. Since 1980, bison have been responsible for more animal-caused injuries to pedestrians in Yellowstone than any other species, including carnivorous predators such as wolves and grizzly bears. It doesn't help that peak tourist season is also peak mating season for America's national mammal, when male bulls are feeling particularly frisky - and aggressive.

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The park has carried out intense public information campaigns about the dangers of the massive bovines. It requires visitors to stay 75 feet away from the animals, and it passes out fliers featuring a graphic of a bison tossing a camera-toting visitor in the air. In recent times, those measures have helped keep the bison-inflicted injuries between zero and two each year.

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Then, in 2015, five people were tossed or gored by Yellowstone bison -- four so badly that they had to be hospitalised. So Cara Cherry, who works with the National Park Service as an officer with the Centers for Disease Control's Epidemic Intelligence Service, decided to investigate.

Cherry, a veterinarian by training, pulled data on the incidents over the decades. What she found probably won't surprise most people familiar with smartphones.

From 1980 to 1999, there were 35 of what Cherry called "bison encounters." Of those, 29 percent involved photography, and most of those injured were at least 10 feet from the bison. From 2000 to 2015, 25 people were injured, nearly half of them during some "photography-related activity," Cherry said in an interview.

A herd of bison feed near Tower Junction in Yellowstone National Park. Photo / Erik Petersen for The Washington Post.
A herd of bison feed near Tower Junction in Yellowstone National Park. Photo / Erik Petersen for The Washington Post.

In 2015, however, four of the five people were injured when they approached a bison, and three were taking photos from a distance of between three and six feet. One was using an iPad. Two had turned their back on the bison, and one of them, Cherry wrote in a CDC report in March, was "taking a cell phone self-portrait (selfie), which necessitated getting close to the animal."

"The biggest thing really seemed to be photography in terms of reasons people approach bison too closely," Cherry said, and she theorised that the 2015 increase probably had something to do with smartphone cameras' more limited zooms. Never mind those pesky park regulations: If you want a photo of or with a bison, you've got to get close.

Cherry said her colleagues are studying what makes people think approaching a 1,500-pound animal is a good idea. Does their hulking, statue-like presence make Yellowstone visitors believe they're docile? But she also pointed out that bison and Yellowstone are mere symptoms of a bigger selfie-with-wildlife problem. Last fall, for example, a Colorado park closed because too many people were trying to take selfies with bears.

"It does seem like a selfie phenomenon with wildlife globally has taken off," she said.

Cherry's report is only the most recent to find that when wild animals hurt people, the people are often to blame. In February, researchers reported that about half of attacks by large carnivores - including bears, wolves, coyotes and cougars - on people in North America, Russia and three European countries were the result of "risk-enhancing human behavior."

Mind-blowingly, the most common risks included "parents leaving children unattended" and "approaching a female with young."