Why Yellowstone Had to Euthanize a Bison Calf After a Visitor Put It in His Car Euthanization of wildlife in national parks "is an extremely rare occurrence."

 -- A bison calf recently had to be euthanized at Yellowstone National Park after visitors put it in the back of their car, according to park officials.

Now, many are asking, "Why?"

"There are multiple reasons," Charissa Reid, who works for the park's public affairs office, told ABC News today. "First, we want people to understand that our rangers really made a heroic effort to return the calf to the wild. No one ever becomes a park ranger because they want to kill animals. What happened was really unfortunate."

Park rangers spent over two days trying to get the bison calf to return to a herd, but it just kept getting rejected, Reid said. She added that the calf "wouldn't eat" and kept returning to the roadway and seemed to "be very imprinted on cars and people."

"Rangers had to make the tough decision because we didn't want it to get hit or cause an accident that could harm itself or other people," she said.

Reid also explained why the baby bison couldn't just be sent to a wildlife sanctuary or zoo.

"You have to understand that we're not in the business of animal rescue," she said. "It's also illegal to transport wild bison outside the park without having them tested and monitored for brucellosis."

Brucellosis is a disease that causes abortions, infertility and lowered milk production in bison, according to the U.S. Department of Agriculture's Animal and Plant Health Inspection Service (APHIS).

"The park just doesn't have the facility or capacity to quarantine, test and monitor the calf," Reid said, adding that the process also takes months.

"It was just a very inhumane situation," she said. "The calf was either going to starve to death, get sick, get hit or cause an accident, so we had to make the difficult decision to put it down."

Reid also said that wildlife in national parks being euthanized is an "extremely rare occurrence."

"There might be an occasion where a deer or elk gets fatally injured, and a ranger might shoot the animal to end its pain," she said, "but we do not do this lightly or frequently."

In contrast, incidents of illegal visitor interactions with wildlife have become more common in the past few years, Reid said.

"It could be that we've had more people coming than ever or the advent of the cell phone and people wanting to get that picture," she said. "But regardless, visitors need to realize it's important to follow the rules for their safety and the animals' safety."

Reid added, "You should stay at least 25 yards away from any animal and at least 100 yards away from wolves or bears."