In the autumn of 1973, the naturalist and writer Peter Matthiessen and the zoologist George Schaller set out on a gruelling trek into the Himalayas. They were headed toward the Dolpo region of the Tibetan plateau. Schaller wanted to study Himalayan blue sheep; Matthiessen hoped to see a snow leopard—a large, majestic cat with fur the color of smoke. Snow leopards, which belong to the genus Panthera, inhabit some of the highest mountain ranges in the world, and their camouflage is so perfectly tuned that they appear ethereal, as though made from storm clouds. Two of them feature on the Tibetan flag of independence, representing harmony between the temporal and spiritual planes.

For Matthiessen, a serious student of Zen Buddhism, the expedition wasn’t strictly scientific. It was also a pilgrimage during which he would seek to break “the burdensome armor of the ego,” perceiving his “true nature.” After it was published, in 1978—first, in part, in The New Yorker, then as a book—“The Snow Leopard,” his account of the trip, won two National Book Awards, becoming both a naturalist and a spiritual classic. It overflows with crystalline descriptions of animals and mountains: “The golden birds fall from the morning sun like blowing sparks that drop away and are extinguished in the dark,” Matthiessen writes. But it’s also an austere Buddhist memoir in which the snow leopard is as alluring and mysterious as enlightenment itself.

When Matthiessen and Schaller set out on their trip, scientists knew virtually nothing about snow leopards. There were no accurate range maps or long-term studies; although the cat had been described as far back as 1775, Matthiessen tells us that only two Westerners had seen a wild snow leopard in the previous twenty-five years. One of them was Schaller, who took the first recorded photograph of a snow leopard, in Chitral Gol, Pakistan, in 1970. Four decades after “The Snow Leopard,” Schaller says that the animal has grown only slightly less mysterious. “To this day, doing a decent census is extraordinarily difficult,” he told me over the phone, from his home in New Hampshire. “There are thousands and thousands of square miles that no one has ever checked. Nobody knows if the population has increased or decreased.”

The snow leopard’s habitat, which stretches across twelve Central Asian countries, is forbidding. The air is thin and temperatures fall to twenty below zero. In 1982, the biologist Rodney Jackson travelled to western Nepal to radio-collar some snow leopards; to get to the field site, he took a small plane from Kathmandu into western Nepal, then hiked for ten days. In four years, he collared only five animals. In 1993, Schaller—who has returned to the Tibetan plateau almost every year since his trip with Matthiessen—invited the biologist Tom McCarthy to Mongolia to study snow leopards. McCarthy, who is now the director of the snow-leopard program at the conservation organization Panthera, brought a copy of “The Snow Leopard” along for the plane ride, then grew alarmed after reading about a previous researcher who had gone into the field with Schaller, only to quit when his boots filled with blood. “I shut the book and didn’t pick it up again for a few years,” he recalled.

Tracking snow leopards with radio is challenging because of the region’s steep, rocky cliff faces, which scatter radio signals in all directions. “In six years, I found the cats’ signals three hundred times,” McCarthy said. Since 2008, researchers in the southern Gobi region of Mongolia, not far from where McCarthy worked, have outfitted more than twenty snow leopards with G.P.S. collars, which send their exact positions to a satellite every few hours. “What I collected in six years, they collect in a week,” McCarthy said. But the new data only underscore the solitude of snow leopards and the vastness of their range. Except for an occasional shared meal at a kill site, the animals hunt and roam alone, and avoid contact by leaving behind scent-markings and scrapes that mark their territory. Every male requires around eighty square miles of home range—an area three and a half times the size of Manhattan.

Gathering blood, tissue, and genetic material from such a diffuse population is extraordinarily difficult. For some species, scat serves as a source of DNA, but in the Himalayas snow-leopard scat often freezes and stays frozen. As a result, “snow-leopard poop from a hundred years ago may appear similar to snow-leopard poop from a year ago,” Evon Hekkala, an associate professor of biological science at Fordham University, said, making it hard to establish present-day population size or genetic diversity. Through limited sampling of the snow-leopard genome, researchers have discovered that, during the Pleistocene, the animals underwent a severe population decline that reduced their genetic variation. “It’s hard to estimate what the impact of that ancient loss of diversity has been,” Hekkala said. “We still don’t fully understand the adaptive potential of their genome.”

In 2016, Matthiessen’s son Alex, an environmentalist, joined Schaller to retrace the route described in “The Snow Leopard.” He wrote about the experience in a special edition of the book, recently published by the Folio Society, that also includes reproductions of Schaller’s original photographs, from 1973. Throughout the trek, the younger Matthiessen noticed how many glaciers seemed diminished since those pictures had been taken. “There are fewer snowy peaks, and less snow on the ones that do have cover,” he wrote. According to John Farrington, a wildlife researcher and conservationist who has studied snow leopards since 2000, climate change is already bringing about “the drying up of surface-water sources, and the transformation of moist alpine meadows into less productive steppe-type ecosystems.”

As their habitat deteriorates, the cats may increase their predation of livestock, which tends to provoke retaliatory poaching. Last year, because the rate of their decline has lessened in some places since the fall of the Soviet Union, when the animals were more often poached, the International Union for Conservation of Nature downgraded the snow leopard’s status from “endangered” to “vulnerable.” But conservationists point out that less than two per cent of the species’s range has ever been rigorously sampled. “What you have to go by is, how fragmented are the populations?” Schaller told me. “You can have a hundred tiny populations and they are all threatened, because of inbreeding, and so forth. There’s a lot of fragmentation in snow-leopard range, and there is going to be more with climate change.”

Farrington had studied snow leopards for sixteen years before he saw one in the wild; Tom McCarthy said it was not unusual for him to go five or six years between sightings. Matthiessen never saw one. “I always sensed that he was slightly embarrassed about it,” Alex told me, in an e-mail, “especially given that Schaller did see a snow leopard days after my father departed for home.” And yet his father, he continued, “well understood that, given the book’s spiritual and philosophical themes, not seeing the snow leopard held more power symbolically than seeing it would have.” For Matthiessen, the animal’s elusiveness became a lesson in the Buddhist idea of nonattachment—the radical letting go of one’s regrets about the past and dreams for the future, in order to be present in the here and now.