ST. LOUIS (AP) — The world’s most endangered wolf species got a big boost at a Missouri wildlife reserve — four little puppies born this spring.

The April 26 birth of a female American red wolf pup named Otter was followed by a litter of three other pups — Molly and her brothers Finn and Obi — on May 4, the St. Louis Zoo announced Monday. All four were born at the zoo’s Sears Lehmann Jr. Wildlife Reserve, which sits about 40 miles (64 kilometers) southwest of St. Louis. Zoo officials said all four are healthy and thriving.

The cuteness overload, however, won’t be open to the public. Visitors aren’t allowed so that the wolves learn natural behaviors and survival skills without much human interaction in case they can eventually be released into the wild, zoo officials said.

Otter was born to 8-year-old Lava and 9-year-old Tyke. The triplets were born to first-time parents Ladybird, age 3, and 8-year-old Wilber.

“When you consider how few red wolves remain, each birth is an achievement,” Sabarras George, director of the St. Louis Zoo WildCare Park, said in a news release.

The red wolf is the only large carnivore solely native to the United States. It’s smaller than a gray wolf, but larger than a coyote, according to the National Wildlife Federation. It differs in appearance from the gray wolf with reddish fur often found around its head, ears and legs.

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The red wolf once ranged from central Pennsylvania to southeastern Texas. Populations were decimated by the early 20th century due to predator control programs and loss of habitat, the U.S. Fish and Wildlife Service said.

Red wolves were declared extinct in the wild in 1980, but the Fish and Wildlife Service captured the remaining 14 and started a captive breeding program. The National Wildlife Federation said it became the first animal to be successfully reintroduced after being declared extinct in the wild. Today, the wild red wolves live in North Carolina’s Albemarle Peninsula.

Still, the numbers are small. About 20 live in the wild. Another 290 red wolves live in human care, including 17 at the St. Louis Zoo reserve.

In September, the Fish and Wildlife Service released an updated recovery plan that calls for spending nearly $328 million over the next half-century to get the red wolf off the endangered species list. The agency said at the time that the American red wolf can only survive with “significant additional management intervention.”

“Hunting, habitat loss and human misconceptions about wolves have all played a role in the plight of the red wolf today,” said Regina Mossotti, the zoo’s vice president of animal care. “But every new birth offers hope for future reintroduction efforts for this vital national treasure.”

Zoo officials said the pups will stay with their parents at least two years. After that, they may be sent to other institutions that are part of the Association of Zoos and Aquariums’ American Red Wolf Saving Animals From Extinction Program to start their own packs. They could also be released into the wild through the Fish and Wildlife Service, the zoo said.