A group of Yellowstone National Park Visitors have been called 'foolish' for putting a bison calf into their car – because they "thought it was cold".

Karen Olsen Richardson posted a picture of the animal in someone's car on Facebook, following a trip to the vast national park in Wyoming.

Richardson wrote: “The highlight of the trip,” adding, "Dear tourists: the bison calf is not cold and it is not lost. Put it back! (yes, the park rangers took care of the situation).”

Richardson’s brother, Weston Olsen, added:  “Haha! My sister was in Yellowstone and caught some idiots doing this.”

Richardson told East Idaho News that the visitors in question put the young bovine into their vehicle because they thought it was cold.

The individual who was concerned about the calf's welfare was one of a team of parents on a school field trip, accompanying 10 and 11 year olds.

Ms Richardson told EastIdahoNews.com that a father and son drew up outside Lamar Buffalo Ranch, demanding to see a ranger.

Richardson said: “They were demanding to speak with a ranger. They were seriously worried that the calf was freezing and dying.”

“They didn’t care,” added another parent called Rob Heusevelet, "They sincerely thought they were doing a service and helping that calf by trying to save it from the cold.”

The pair were then told to remove the animal from the car and that they could be in trouble for making contact with the bison.

They were then ticketed and told to return to where the bison was found and release it.

Yellowstone National Park officials warned visitors not to approach the wildlife:

“Bison can run three times faster than humans can sprint and are unpredictable and dangerous. Visitors must give the animals enough space and alter their own behaviour to avoid interacting with an animal in close proximity.”