Joel Sartore, veteran National Geographic photographer, has made drawing attention to the world’s most vulnerable creatures his life’s work. His vastly ambitious Photo Ark project is a 25-year effort to document every species in captivity—especially those at risk of extinction. “Our job is to give them a voice, to speak for them before they go away,” Sartore says. “The goal isn’t to create a big obituary, but to get the public to act.”

Above is a 70-day-old baby white-bellied pangolin (Phataginus tricuspis) clinging to its mother, photographed at Pangolin Conservation in St. Augustine, Florida.

A pied mossy frog (Theloderma corticale) at the Houston Zoo. Sartore began his project out of concern for endangered amphibians. “Some frogs are now down to single numbers,” he says. “Most people don’t understand that there really is an extinction crisis, and how it will affect humanity. It’s really important to use this little device, of looking animals in the eye, to get people to stop and think—about the material we all consume, what kinds of chemicals we put on our lawns, etc. It’s an easy way to get people to care.”

A hawk-headed parrot (Deroptyus accipitrinus accipitrinus), also photographed at the Houston Zoo. Since all of the creatures Sartore photographs are in zoos, aquariums, and captive-breeding facilities, he can rely on their keepers to choose individual subjects who are likely to tolerate the process. His technique varies according to the size of the animal. Larger creatures have their pens lined with black or white paper or cloth, and keepers use food treats to get them to shift about and face the camera. Birds, insects, and other small animals are transferred first to a crate and then to Sartore’s cloth-lined “shooting tent.”

“The animal goes in there, sees the front of my lens, and probably thinks it’s been abducted by the mother ship,” Sartore jokes. “I work very quickly, to get them back to their enclosures as soon as possible. If a bird poops, we just clean it up in Photoshop.”

A six-day-old Malayan tapir (Tapirus indicus) at the Minnesota Zoo. “Cuteness is huge and cannot be underestimated,” Sartore says. “If something looks very cute or anthropomorphic, people like that. We notice a great uptick whenever we show very charismatic animals: baby tapirs, baby koalas, baby anything.”

A giant snake-necked turtle (Chelodina expansa). So does Sartore have file cabinets full of ugly creatures that no one ever sees? “The Photo Ark is geared to the least among us,” he says. “Our chance to show off uncharismatic animals is when we do books or exhibitions. But when we put snakes and rats and toads on Geographic Instagram, we actually lose followers.”

A Hawaiian bobtail squid (Euprymna scolopes) at the Monterey Bay Aquarium. Sartore likes photographing at aquariums. “They’ve already collected many of the best-looking animals,” he says. He’d love to shoot large marine creatures, like whales, but hasn’t come up with a feasible way to do it yet. “I figure it will take me another 15 years.”

A pygmy marmoset (Cebuella pygmaea) at the Lincoln Children’s Zoo in Nebraska. “We’ve made a pretty good cross-section of what life looked like on Earth at this time in this century,” says Sartore, “but time is of the essence. When I go to a zoo and see the last of something—it’s a big responsibility, and an honor.”