When Fermilab’s grazing bison see Cleo Garcia’s silver Ford pickup pull through the pasture gate, they shuffle toward the truck in a snuffling herd. “They think I’m going to give them pellets,” the herdsman says as he climbs back into the driver’s seat after closing the gate behind him.

When Garcia actually has buckets of the nutritional supplement in the bed of his truck, they follow him wherever he steers around their expansive fields. The bison have 80 acres to roam on the bucolic grounds of Fermi National Accelerator Laboratory, with the beaker-shaped main building visible in the distance. There are 22 now, more than Garcia would have predicted just a few months ago.

Expecting no new additions to the herd this season, he worried that a cow showing signs of prebirth was sick. When a full udder suggested otherwise, a call to the vet confirmed that a recently acquired bull, only 14 or 15 months old, could be responsible. So he started checking the others.

“To my surprise,” Garcia says, “I told my boss we had about six cows with signs that they would have calves.” They were born betweenMay 30 and June 20.

That there are bison roaming this pasture at all surprises many visitors. What are they doing here?

There’s a myth that the animals serve as canaries in a physics mineshaft, living measures of radiation levels. But in fact, Robert Wilson, Fermilab’s first director, just wanted to establish a connection to the prairie surroundings. A bull and four cows formed the first herd in 1969.

The tradition has continued, usually on a predictable breeding cycle but with occasional exceptions like this spring’s. By July the newborn calves amble alongside their mothers and nuzzle under their udders to nurse.

The new arrivals don’t change Garcia’s job much, he says. He looks after them the same as the 16 adults, making sure the herd has water and hay and pellets—the latter two matter more in the winter when there’s less grass to eat.

Garcia wants to simulate the wild for them as much as possible, although their grunting interest in the pickup reflects regular contact with civilization. “They know this truck,” he says. “They’re not scared of this truck at all.”

The bison don’t seem to be scared of much, even the coyote that trots past them, attracting about as much notice as a jogger on the street. But the mothers protect their calves with ferocity if they perceive a threat. “The bulls are more docile,” Garcia says. “The cows, especially when they have calves, they try to be a little more aggressive. There’s a couple there that I don’t trust. I don’t walk in there and walk between them.”

As he’s talking, a commotion turns his head. A few bison have stopped to drink at the water trough. To reach it the calves have to put their front hooves on a wooden riser that supports the metal tub. One has lost its balance and fallen into the water.

By the time Garcia notices what’s happened, the calf has scrambled out onto its feet, drenched but probably refreshed in the July heat. “See that?” he says. “Took a swim.”

The visitors who often gather at the fence on the edge of the pasture would have loved that. To school groups and tours, the bison are one of Fermilab’s biggest attractions and surprises. But nobody was more surprised than Garcia to see the herd’s newest additions.