Parents and children alike swarmed the glass exhibit walls, oohing as the massive polar bear Anana belly-flopped into the pool. Her cautious cub watched from the edge.

Eventually the fluffy little bear gathered the courage to splash in, too. She paddled over to her mother and poked a curious nose toward the onlookers.

Joy Endrulas, 49, of Buckeye Lake, watched the swim lesson Wednesday from an ideal spot a few yards away from the crowd. It was the first time the Columbus Zoo and Aquarium showed one of its three new polar bear cubs to the public, so the moment already was special. But it took on new meaning when Endrulas learned how lucky she was to see a cub at all.

In the past 10 years, captive polar bears gave birth to just 14 surviving cubs in zoos across the United States, all but one in Ohio. Four were born at the Columbus Zoo, one at the Buffalo Zoo and the rest at the Toledo Zoo & Aquarium.

"I had no idea," said Endrulas, a bear fan who wore a T-shirt featuring Nora, a beloved polar bear cub who was born at the Columbus Zoo in 2015 and who now lives at the Oregon Zoo.

At a zoo that has reared four cubs in a little more than a year, it's easy to take for granted that it's a rare accomplishment.

The trio of surviving cubs born in November 2016 — one, a female, to Anana and twins, a male and a female, to Aurora — were the only captive cubs born in the country last year. The zoo announced their genders before Anana's cub debuted Wednesday morning.

Aurora's twins are scheduled to go on display for the first time on Thursday, zoo spokeswoman Jennifer Fields said. Because wild mother bears raise their cubs in isolation, the female adult bears will be on a rotating viewing schedule with their cubs, so they'll never be in the exhibit together or with 29-year-old Nanuq, the cubs' father.

Baby polar bears are rare for several reasons. First, there aren't many captive adult bears to breed. Including Columbus' new cubs, there are only 46 total bears in 27 U.S. zoos that are members of the Association of Zoos and Aquariums. But it's also difficult for them to get pregnant, explained Columbus Zoo curator Carrie Pratt and assistant curator Nikki Smith.

After polar bears mate in the spring, a fertilized egg doesn't implant until months later, after the female puts on enough weight to survive winter hibernation in a den. For 10-year-old twins Anana and Aurora, that meant adding 230 and 274 pounds, respectively, to their usual 600 or so pounds.

At zoos, the staff then uses cameras to see if denned bears give birth. There's no way to test if a bear is pregnant and once the cubs are born, they can be viewed only by camera for a few months.

"It's one of the most nerve-racking things we go through," Pratt said.

After birth, fragile newborn cubs still face challenges. They usually weigh only about 1 pound. Their survival rate is just 50 percent, even in captivity, and some mothers neglect them.

Anana, in fact, had twins in November but one cub died a week later.

If a mother bear won't raise her young, keepers typically step in. That happened in 2015 when Aurora gave birth to Nora and another cub that didn't survive. Nora was then hand-raised.

Randi Meyerson, Toledo Zoo assistant director of animal programs and coordinator of the polar bear Species Survival Plan for the Association of Zoos and Aquariums, said it's possible older female bears are having trouble getting pregnant due to side effects of birth-control implants used in the 1990s and early 2000s, when polar bear exhibits and breeding programs were less common because the species was rebounding in the wild.

Today, the programs are high priority.

There are only 20,000 to 25,000 polar bears left in the wild and the species is vulnerable because of climate change and the disappearance of sea ice, according to the International Union for the Conservation of Nature.

Though breeding programs don't aim to repopulate the planet, captive bears can help their wild relatives by providing data for research and inspiring visitors to keep their habitats safe.

"They truly are ambassadors that can help connect people with nature and maybe spark some curiosity or concern," said Geoff York, senior director of conservation for Polar Bears International, an organization that works closely with the zoo.

Officials say the Columbus Zoo cubs are well-prepared to fulfill that role for two years or so, the average time a wild mother bear cares for her young. Then it's likely they could move to another zoo, based on the goals of the Species Survival Plan.

Aurora's twins will be named soon by zoo staff and Anana's cub will be named through an online contest. All three names will be announced on Mother's Day, May 14.

awidmanneese@dispatch.com

@AlissaWidman