HELENA, Mont. — A bison calf that tourists loaded into their vehicle at Yellowstone National Park because they were concerned for its welfare could not be reunited with its herd and had to be euthanized, park officials said Monday as they reasserted the importance of avoiding wildlife.

The incident last week and several other recent cases led to fresh warnings that park rules require visitors to stay at least 25 yards from all wildlife and 100 yards from bears and wolves.

Visitors brought the newborn calf to a park facility on May 9, which officials called a dangerous move because adult bison are protective and will attack to defend their young. Rangers took the animal back to where it was picked up, but they could not get it back with the herd after several tries.