Henry, a 12-pound orangutan at the St. Louis zoo, wakes from a nap in his incubator.
Caption from LIFE. Henry, a 12-pound orangutan at the St. Louis zoo, wakes from a nap in his incubator. Born at the zoo last year, he was removed to an incubator when his mother neglected him. Orangs, which come from Sumatra and Borneo, must be treated like human babies, with a formula diet, oil baths and plenty of personal attention to keep them happy.Nina Leen—The LIFE Picture Collection/Getty Images
Henry, a 12-pound orangutan at the St. Louis zoo, wakes from a nap in his incubator.
Orangutan, 1964
Oryx, 1964
With her floppy ears cocked, the newest addition to San Diego zoo's okapi herd of five stands protectively near her mother.
Ocelot, 1964
Hilda, the 15-year-old grande dame of Detroit's bears, cradles her latest set of twins, her fourth pair.
Growling hungrily, polar bears at Detroit's zoo wait for their next meal while one impatient female gets up on her hind legs for a better look.
Przewalski's wild horses, believed extinct in their habitat on the Mongolian steppes, are bred at the Catskill Game Farm, a private zoo in Catskill, N.Y. There are 120 of these horses in the U.S. and Europe, and in 20 years breeders hope to release some back into the wild.
Tusk to tusk, two white rhinos eye each other at the Oklahoma zoo. The largest of all rhinos, they came from Zululand in South Africa where only 300 survive. No white rhino has yet been born in the U.S. through several zoos have pairs today and hope to mate them.
Whooping Crane, 1964
The cheetah, which is native to Africa and India, grows lethargic in captivity and does not mate. At the Oklahoma zoo the docile male (above) is being given plenty of exercise in the hopes of solving the problem. In fact the zoo believes the female (rear) may be pregnant.
Cheetahs, 1964
Koala, 1964
Two young tortoises crawl by 500-pound, century-old adult at the San Diego zoo. These huge tortoises, one of the most ancient of animals, are facing extinction in their native homes. San Diego, the zoo that has most successfully bred them, is raising 18 young ones and has 92 eggs in their incubators waiting to hatch.
The only pair of captive bongos in the world butt each other at the Cleveland zoo. Though the 18-month-old male (left) is not yet fully mature, he already as a sparing interest in the female. His horns eventually will be 36 inches long. Bongos are so elusive in the deep Central African forests that no one knows how many are left.
Pygmy Hippos, 1964
Zata, one of three pygmy hippos born this year at Washington's zoo, lies at ease in his daily bath. He is not yet allowed in the zoo's small pool—he might b stepped on and drowned. These hippos which grow only two feet in height, come from the west coast of Africa.
Tapir, 1964
Caption from LIFE. Henry, a 12-pound orangutan at the St. Louis zoo, wakes from a nap in his incubator. Born at the zoo

Nina Leen—The LIFE Picture Collection/Getty Images
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See Photos of Endangered Species from the 1960s

Feb 20, 2015

This week, the Oregon chub was removed from the Federal List of Endangered and Threatened Animals, becoming the first fish ever to shed its endangered status. When it was placed on the list in 1993, there were fewer than 1,000 of the minnow species left. Today there are more than 140,000.

In the years since the first official list of threatened and endangered species was published in 1967, 28 species have been recovered, 10 have become (or were discovered to already be) extinct, and more than 2,000 species have joined the original 78.

Though the notion of extinction entered public awareness at the turn of the 20th century and the federal government began taking steps to protect certain species then, it wasn’t until the 1960s that environmental activism pressured the government to be more proactive in identifying and taking measures to protect threatened species. The first significant piece of legislation, the Endangered Species Protection Act, was passed in 1966, followed by an amendment in 1969 and a reworking in the 1973 Endangered Species Act.

In the period leading up to these legislative acts, the zookeeper community was collaborating on strategies they could undertake toward preservation. An August 1964 article in LIFE reported on the creation of “special survival and propagation centers where pairs and herds of endangered species can propagate in peace and quiet.” The magazine sent photographer Nina Leen on an assignment to document these species in the new habitats being set aside for them.

Nineteen U.S. zoos formed the Wild Animal Propagation Trust, addressing issues that had prevented species in captivity from successfully mating in the past. Rhinos in the wild are undisturbed during mating season; their new protected habitats would ensure the same treatment. Baby orangutans abandoned by their mothers would be raised in special nursery facilities. If the Trust succeeded in regenerating species, they hoped to reintroduce some of the animals into their natural habitats.

LIFE also explored advances in the scientific understanding of mating rituals. A male and female gorilla at the Bronx Zoo, Oka and Mambo, had expressed a “mammoth indifference for one another,” refusing to mate. Researchers came to understand that male gorillas raised in captivity, having no exposure to mating in the wild, had not learned proper mating behavior. Polar bears, which in the wild are accustomed to privacy during birth, were killing their newborn cubs because of the throngs of spectators present at their births. Understanding the animals’ behavior in the wild helped zookeepers create environments more conducive to procreation.

All of the species photographed for the story remain threatened or endangered 50 years later. The attention and, ultimately, funding that certain species get can be linked to the public's awareness of and appreciation for them—the whales, for example, are high on many people's list of animals in need of saving. So if you want to step in and support the underdog, consider the clam, of which more than 100 species are endangered. "Save the Georgia Pigtoe Clam" has a nice ring to it.

Liz Ronk, who edited this gallery, is the Photo Editor for LIFE.com. Follow her on Twitter at @LizabethRonk.

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