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Father-and-son tourists were ticketed and forced to release a baby bison they'd wrangled into the back of their SUV at Yellowstone National Park because they thought it was cold.

The bison calf was released and its 'rescuers' were ticketed May 9 at Yellowstone National Park. Karen Richardson via KECI-TV

Karen Richardson, a teacher in Victor, Idaho, was chaperoning a fifth-grade field trip to the park in Montana last Monday when the two foreign tourists pulled up at a ranger station with the calf, she told NBC station KECI of Missoula, Montana.

Richardson — who took a photo of the calf — said the pair told rangers they were worried that the calf was too cold.

Rob Heusevelet, the father of one of the students on the field trip, told the East Idaho News of nearby Idaho Falls that rangers ordered the men to remove the bison and warned them that they could be in trouble for having the animal, because National Park Service rules forbid visitors from approaching within 25 feet from large animals.

"They didn't care," Heusevelet told the news site. "They sincerely thought they were doing a service and helping that calf by trying to save it from the cold."

The tourists were ticketed, East Idaho News reported.