The herd had been crossing the Lamar River on Saturday evening when the calf got separated from its mother on the river bank, according to a press release from the agency. A man observing the scene approached the animal with apparent rescue intentions.
"As the calf struggled, the man pushed the calf up from the river and onto the roadway," NPS said. "Visitors later observed the calf walk up to and follow cars and people."
Park rangers repeatedly tried to reunite the calf with the herd, but the herd resisted.
The rangers later euthanized the calf, saying its persistence in approaching cars posed a hazard to guests, according to NPS.
NPS is investigating the incident and asking the public to share any relevant information to a tip line. The agency has yet to identify the man behind the incident, describing him as a "white male in his 40-50's, wearing a blue shirt and black pants."
Pending the outcome of the investigation, he could be charged with Class B misdemeanors, including disturbing wildlife, disorderly conduct (creating a hazardous condition) and approaching wildlife, according to Morgan Warthin, a spokesperson for Yellowstone National Park.
If found guilty of those charges, the man could face up to six months in jail and a $5,000 fine, Warthin told NPR.
Yellowstone requires that visitors stay at least 25 yards away from its two breeding bison herds, which collectively contained 5,900 animals at the last count in 2022. The park is the only place in the contiguous U.S. to have maintained a continuously free-ranging bison population since prehistoric times.
Yellowstone's herds were nearly poached to extinction in the late 1800s. Today, park guests can spot the animals nearly year-round and from the roadway in places like Wyoming's Lamar Valley, a confluence of rivers in the park's northeast corner.
NPS has frequently defended its policy of not interfering in the natural death of animals on public lands, including orphaned offspring.
"Our focus is on sustaining viable populations of native wildlife species, rather than protecting individual animals," reads an NPS webpage on the policy. "An animal's survival depends on its own daily decisions and natural selection."
Another Yellowstone bison calf was euthanized following human interference in 2016. Edward O'Brien of Montana Public Radio reported that two tourists placed the calf in their car and drove it to a nearby park facility because they "thought the animal looked cold and uncomfortable."