For years National Geographic photographer Joel Sartore worked far away from home—documenting the astonishing wildlife of Bolivia’s Madidi National Park or scrambling up the three tallest peaks in Great Britain or getting too close to grizzly bears in Alaska. His wife, Kathy, stayed in Lincoln, Nebraska, and took care of the kids. “He never wanted to change diapers or be a stay-at-home dad,” she says.

But in 2005, on the day before Thanksgiving, Kathy was diagnosed with breast cancer. The cancer sentenced her to seven months of chemotherapy, six weeks of radiation treatments, and two operations. So Joel Sartore had no choice: With three kids ages 12, 9, and 2, he couldn’t travel for the stories that were the mainstay of his career. Of that time, he says now, “I had a year at home to think.” He thought about John James Audubon, the ornithologist. “He painted several birds that are extinct now,” says Sartore, who has prints of Audubon’s Carolina parakeet and ivory-billed woodpecker in his home. “He could see the end for some animals, even in the 1800s.” He thought of George Catlin, who painted American Indian tribes “knowing that their ways of life were going to be seriously altered” by westward expansion. He thought of Edward Curtis, who “photographed and recorded, on early movie footage and sound,” threatened native cultures.

“And then I thought about myself,” he says. “I’d done almost 20 years of photographing in the wild, and I wasn’t moving the needle very much in terms of getting people to care.”

He had taken pictures that showed in one frame why a species was struggling—an Alabama beach mouse, for instance, in front of a coastal development that threatened its habitat—but he wondered whether a simpler approach would be more effective. Portraits could capture an animal’s form, features, and in many cases its penetrating gaze. Could they also be used to capture public attention?

On a summer day in 2006 Sartore called up his friend John Chapo, president and CEO of the Lincoln Children’s Zoo, and asked if he could take portraits of some of the zoo’s animals. Even with Kathy’s illness, he could work a little close to home—and the zoo was one mile away. Chapo told Sartore to come on down. “I was mostly humoring him,” Chapo says.

When he arrived, Sartore requested two things from Chapo and curator Randy Scheer: a white background and an animal that would sit still. “What about a naked mole rat?” said Scheer. He put the bald, bucktoothed rodent on a cutting board from the zoo kitchen, and Sartore started taking pictures.

It might seem odd that such a humble creature could inspire what has become Sartore’s lifework: photographing the world’s captive species and making people care about their fate. But launching a planetwide mission with a tiny rodent fits perfectly with Sartore’s philosophy. “I get most excited when I do little critters like this,” he says, “because nobody’s ever going to give them the time of day.”

There are estimated to be between two million and eight million species of animals on the planet. Many of them (forecasts range from 1,600 species to three million) could go extinct by the end of this century, as a result of habitat loss, climate change, and the wildlife trade. “People think we’re going to lose animals in their grandchildren’s time,” says Jenny Gray, CEO of Zoos Victoria in Australia. “We’re losing them now. And those animals are gone forever.”

Zoos are the last hope for many animals on the verge of vanishing—but zoos shelter only a fraction of the world’s species. Even so, Sartore estimates that it will take 25 years or more to photograph most of the species in captivity.

Since 2006, he’s photographed more than 8,000 animals for the passion project he named Photo Ark. He’s taken pictures of small ones: a green-and-black poison dart frog, an El Segundo flower-loving fly. Large ones: a polar bear, a woodland caribou. Marine animals: a foxface rabbitfish, a Hawaiian bobtail squid. Birds: an Edwards’s pheasant, a Montserrat oriole. And on and on and on. Sartore says he won’t stop until he dies or his knees give out. (Read more: “Aquatic Mammal With Snorkel Nose Is 8,000th Animal in Our Photo Ark.”)

Sandra Sneckenberger, a biologist with the U.S. Fish and Wildlife Service, has seen firsthand how Sartore’s photos can move others. A few years ago the population of Florida grasshopper sparrows—a bird Sneckenberger concedes looks “drab brown” from a distance—had plummeted to about 150 pairs at only two locations. After Sartore’s image of the bird raised awareness of its plight, federal funding to help the agency conserve it soared from $20,000 to more than a million dollars.

Sartore has taken portraits of animals that may be saved—but also of animals that are doomed. In 2015, at the Dvůr Králové Zoo in the Czech Republic, he photographed a northern white rhino, one of only five left in the world at the time. The 31-year-old female lay down to sleep at the end of the shoot. A week later it died of a ruptured cyst. In March 2018, Sudan, the world's last northern white rhino died. “Do I think that the rhinos going away is sad?” Sartore says. “It’s not just sad. It’s epic.”

Most of the animals in Photo Ark, which is supported by the National Geographic Society, have never been photographed so distinctively before, with their markings, their fur, and their feathers so clearly on display. If they disappear, this will be the way to remember them. Sartore’s goal “isn’t just to have a giant obituary of what we’ve squandered,” he says. “The goal is to see these animals as they actually looked in life.”

Now millions of people have seen the animals that Sartore has photographed. People have met their gazes on Instagram, in this magazine, in documentaries, and projected on the sides of some of the world’s great monuments: the Empire State Building, the United Nations headquarters, St. Peter’s Basilica in the Vatican, and more.

There are as many ways to photograph an animal as there are animals, but Sartore operates within some basic parameters. All the portraits are taken with a black or white background. “It’s a great equalizer,” he says. “The polar bear is no more important than a mouse, and a tiger and a tiger beetle are exactly the same.”

Large animals are photographed in their enclosures, where Sartore either hangs a giant black curtain to serve as backdrop or paints a wall. At the Houston Zoo he draped 18 feet of black cloth at one end of a giraffe’s stall. The giraffe didn’t even notice, says Peter Riger, vice president of conservation at the zoo. “It just knew it was coming in to get lunch.”

Small animals are placed in a soft-sided box, with Sartore poking his lens through a slit in the side. “Some of them fall asleep or eat in there,” he says. “A lot of them don’t like it at all.” He keeps the sessions short, a few minutes at most.

Sartore doesn’t wrangle the animals himself; he leaves that to the zookeepers. If at any point “the animal shows signs of stress, the shoot is over,” he says. “The safety and comfort of the animals come first.” None has been injured.

Sartore, however, has not been so lucky. “A crane tried to blind me one time,” he says. “That was terrifying.” A mandrill, a burly type of primate, punched him in the face. A white-crowned hornbill—“the nastiest, most badass bird I’ve had to do”—struck him with its beak and drew blood. “But aren’t I asking for it, in a way?” he says.

Joel and Kathy Sartore sit side by side at their kitchen table in Lincoln with the lights dimmed. His arm rests on her shoulders. He had returned from Madagascar the night before (he began traveling again in 2007) and wanted her to help him select photos of rare lemurs and pochards, a kind of duck, to post on Instagram. “The thing that draws people is the human element,” says Kathy, who often serves as his photo editor.

Sartore grew up not too far from Lincoln, in Ralston, Nebraska. His parents loved nature. His father took him mushroom gathering in the spring, fishing in the summer, and hunting in the fall. His mother, who died last summer, gave him a Time-Life book about birds when he was around eight years old that may have changed his life. Toward the back, in a section on extinction, was a picture of Martha, the world’s last passenger pigeon. He remembers returning to that page over and over: “I was amazed that you could go from billions to none.”

Joel and Kathy met as University of Nebraska students at a spot called the Zoo Bar. “Our dates,” Kathy recalls, “were fishing and frog-gigging,” which involves spearing frogs for their legs. The activity can be justified, Sartore hastens to explain: “They were bullfrogs; they’re an invasive species here in Lincoln.”

Kathy’s cancer came back in 2012; she had a double mastectomy. That same year, their son Cole, who was 18, was diagnosed with lymphoma. Both recovered, but the illnesses have left their mark. “We don’t get uptight about too much anymore,” Sartore says.

Photo Ark has changed him as well. “It has made me very aware of my own mortality,” he says. “I can see how long it’s going to take.” If he can’t finish the job—he still has thousands of species to photograph—Cole will take over. “I want the pictures to go to work,” Sartore says, “long after I’m dead.”