Around this time last year I paid what turned out to be my final visit to the last remaining buffalo in Chicago. Bison haven’t lived wild in Illinois for well over a century, but beginning in1983 the Lincoln Park Zoo kept a captive pair in their hoofed-mammals exhibit, between the camels and the alpacas. It was a subzero day, and I found the two female buffalo standing with ice on their beards and blank looks on their faces. One of them was missing one of her horns; all that was left was a bloody stump. I walked around the curve of the railing, but their eyes didn’t follow me, even though I was the only creature moving and might have been the only thing in motion for quite some time.

I fed a captive buffalo once when I was a kid. It took grass out of my hand with a tongue as wide as my arm. And last year I almost ran into one. I was driving at night in a park in North Dakota, and standing in the road as I came over a hill was a half-grown, gangly calf. Its hooves slipped on the pavement as it scrambled to escape. I slammed on the brakes, and it loped crazily away. Slowly it registered that this buffalo wasn’t alone. I had driven right into a swarming herd of perhaps 40 animals. They weren’t in a big hurry to cross the road; some were grazing, some walking calmly. Only the juvenile seemed shaken up. An enormous adult was standing a few feet to my left, outside the beam of the headlight. Its dark shape loomed taller and longer than my Honda Civic, and since I’d just read the vital statistics at the nature center I knew it outweighed me and my car combined.

The American bison, called a buffalo by most of us, is a full ton of muscle 6 feet tall and 12 feet long. When pushed or panicked it can run 35 miles an hour. In the old days a herd of buffalo was as uncontrollable a natural force as a tornado. And the herds were enormous. Nowhere else, not even in Africa, did one species of large mammal ever achieve such dominance. Biologists estimate that at their peak, before European settlement, there were around 60 million buffalo in the New World.

Almost every pioneer diary attempts to express the writer’s awe at the vast numbers of bison. They talk about endless seas of the beasts covering the prairie in a thick brown blanket. One mindblower of a report came from an army major who wrote in 1871 that a steadily moving herd along the Arkansas River took five full days to pass his outpost.

Bison ruled North America for many centuries and were able to do so because no animal other than a human could kill an adult, even one separated from the herd. (A grizzly bear could possibly give it a run for its money, but grizzlies weigh only half as much and seldom attack adults.) The bisons’ eons of invincibility help explain why they were such easy prey once Europeans brought guns to the continent. Their evolutionary success lay in their huge bodies and their instinct to travel in groups; genes that might have made them capable of interpreting the sound of a rifle shot as danger hadn’t been selected for. In fact, the bison were so clueless about the hazard guns represented that sometimes a herd would simply stand bewildered, not stampeding or defending itself, while riflemen sat on their horses, calmly shooting one animal, reloading, then shooting the next.

By 1900 hunters had killed off all but 100 of the animals, annihilating the chance for every successive human generation to behold the awesome phenomenon of a bison herd. From those remaining 100, breeders have built up the population to120,000 today. A handful live more or less naturally in Yellowstone and other western parks, but 80 percent are on private farms and ranches where they’re raised for food. While we might be able to watch a free herd of a couple hundred bison in a protected park, none of us will ever witness what the army major did. In 1995 we have enough bison to be amused but not amazed.

I didn’t return to the zoo until mid-October, well past prime visiting season. I arrived on a cold, gloomy Wednesday to find I’d waited too long. The bison had vanished. A sign in the bare display space said the exhibit was under renovation. I walked around the area to see if they’d been moved to another slot, but there was no sign of them.

I wandered north through the main part of the park, searching for them or someone who could tell me where they’d gone. A vendor’s hot-pretzel cart sat abandoned near the penguin exhibit. In the Great Ape House a homeless man sat on one of the benches inspecting the sores around his ankles.

I approached an information booth, where a volunteer stood behind a counter busying herself with rearranging the display of primate skulls. She told me the bison had been moved out and were going to be replaced with something, but she didn’t know what.

At her suggestion I went to the main information desk in the Lion House. There three confused docents tried to convince me the bison were still in their pen. One located a photocopy of a roster of animals at the zoo. He showed me where “American Bison” appeared on the list. I asked if there was somewhere else they could be, since they clearly weren’t in their usual spot. “Oh no,” he said. “They would’ve told us if they moved them.”

But the bison are gone from Chicago for good. John Gramieri, the Lincoln Park Zoo’s assistant curator in charge of hoofed stock and reptiles, said the buffalo had been shipped out a couple weeks before. Mine was the first call he’d received asking about their absence. He said they’d been sent to another zoo in Illinois because they were unmanageable at the Lincoln Park facility. Anytime the zookeepers had to perform a medical procedure or do something else that invaded the bisons’ personal space, it was an ordeal. He added that bison aren’t in danger of extinction anymore. They exist in plenty of other places, so the zoo didn’t feel it had to hold them for conservation reasons.

Of course the zoo has other difficult-to-manage animals. But it turns out the bison had a more fatal flaw, something modern America absolutely cannot tolerate: The two bison were dull to watch. “To be honest with you, the bison were not a big draw,” Gramieri said. “Bison can be a very effective exhibit species if they are in a herd. There’s no such thing as a herd of two bison.”