A bison put in a car by tourists because it looked like it was cold had to be put down, officials at Yellowstone National Park said on Monday.
The tourists loaded the animal into their trunk last week and drove it to a ranger station after taking a photograph that prompted a backlash on social media.
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The newborn had to be euthanized because its mother had rejected it as a result of "interference by people," officials said.
"In this case, park rangers tried repeatedly to reunite the newborn bison calf with the herd. These efforts failed," the park said in a statement on its website.
"The bison calf was later euthanized because it was abandoned and causing a dangerous situation by continually approaching people and cars along the roadway."
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The park berated visitors for taking selfies and recording video near the bison, flouting regulations demanding people stay at least 25 yards (23 meters) away.
"In a recent viral video, a visitor approached within an arm's length of an adult bison in the Old Faithful area," it said of the park's famous geyser. "Another video featured visitors posing for pictures with bison at extremely unsafe and illegal distances."
Five visitors were seriously injured last year after approaching bison too closely, the park said.
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In the latest incident, a father and son transported the bison calf in their Toyota Sequoia to a ranger station in the park's northeast corner, a witness told the online East Idaho News.
"They were demanding to speak with a ranger. They were seriously worried that the calf was freezing and dying," said Karen Richardson, one of several parents chaperoning a group of fifth-graders on a field trip.
The website quoted another parent who told the tourists to remove the bison from their car, warning they could be in trouble.
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"They didn't care," he said. "They sincerely thought they were doing a service and helping that calf by trying to save it from the cold."
Critics shared the picture of the calf in the car on Twitter, scolding the "dumbass," "stupid" and "idiotic" tourists.
Yellowstone is the only place in the United States where bison have lived continuously since prehistoric times.
The park is home to 4,900 of the animals, which it says injure more people than any of its other animals.