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ROBERT SIEGEL, HOST:

From NPR News, this is ALL THINGS CONSIDERED. I'm Robert Siegel. And now, another number of the year. In these waning days of 2013, we've been looking at numbers that tell us something about what's happened around the world and today our number is 42. And we don't mean the answer to the question, life, the universe and everything. Forty-two is the number of giant panda cubs that were born in captivity this year and survived. NPR's Andrea Hsu explains what the number tells us and what it doesn't tell us about pandas.

ANDREA HSU, BYLINE: Madrid welcomed Xing Bao, Vienna got Fu Bao and in Washington, D.C., hello, Bao Bao.

(SOUNDBITE OF BAO BAO CRYING)

HSU: Here she is at a checkup just shy of eight weeks. Forty-two surviving cubs in one year is a record and brings the total number of pandas in captivity to 376, most of them in China, of course. The steady gains in the panda population mean not only more unbearably cute photos to coo over, but also better science for researchers to tap.

So, for example, this summer, Atlanta's Lun Lun made this news.

UNIDENTIFIED WOMAN: This mama panda has her paws full tonight. Workers at Zoo Atlanta were preparing for one cub birth. Well, imagine their surprise when two cubs showed up today.

HSU: And although the U.S. had never seen a successful twin birth before, panda keepers knew exactly what to do.

REBECCA SNYDER: To get them both to survive takes some work.

HSU: Rebecca Snyder is Zoo Atlanta's curator of mammals. She says they used a technique developed by their Chinese counterparts.

SNYDER: Periodically, we swap them.

HSU: When they were newborns, it happened every two hours. The Chinese had observed that panda mothers would not tend to two cubs at once. So in Atlanta, while Cub A was with mom, Cub B was kept warm in an incubator and they both thrived. Snyder says these kinds of discoveries like this have made a difference.

SNYDER: I do think the research has been a key reason that the captive population is doing so well now. Applying that to the wild population is not so easy.

HSU: So for all those hoopla over zoo births, there's still a lot of trepidation about pandas in the wild.

JONATHAN BALLOU: I think the breeding success is exciting and is really positive news. The message is we're not there yet.

HSU: Jon Ballou is population manager at the Smithsonian's National Zoo.

BALLOU: We don't know how many pandas are in the wild. The last survey showed 1600 in the wild. Sounds like a lot, but put 1600 people into a football stadium and it looks like a pretty small group right there, right? And so it's not enough.

HSU: Moreover, he says, their habitat is fragmented. It's not one big forest. Wild pandas live in scattered patches of forest. They cannot roam from one patch to another so they're even more exposed to environmental threats and inbreeding. Ballou's focus now is preserving as much genetic diversity as possible in the captive population. He wants to breed pandas that are, as a group, hardier and less vulnerable to disease.

BALLOU: Up until about four years ago, the focus was really on trying to create as many pandas as possible, which is important when you have a small population because you need to get a large. But as the population becomes larger, then you can focus more on genetic management.

HSU: Ballou came up with a formula to determine each panda's genetic value. Those with fewer relatives are more valuable than those with a lot of relatives. Each year, he crunches the numbers and travels to China to recommend matches. Now, there are pandas that Ballou won't recommend for breeding.

Take prolific Pan Pan, a 28-year old male who has fathered 32 cubs. Pan Pan's descendants, all 119 of them including Bao Bao, remember her?

They all share some genes. So matter how adorable, the birth of another Pan Pan descendent just isn't that interesting to Ballou.

BALLOU: If I know the mom and dad and are wild-caught animals and haven't bred before, that makes me the most excited, getting these genes into the population. Yes, and they're cute. Okay, that's kind of an added bonus.

HSU: In the mountains of Southwestern China, researchers have begun reintroducing captive pandas into the wild. But they've told Ballou they're not releasing the most genetically valuable ones yet, given what little is known about how they'll fare.

BALLOU: But I suspect in the future we will be talking about creating pairs to produce cubs to release into the wild.

HSU: Ones with stellar genes that he hopes will insure the long term survival of the species. Andrea Hsu, NPR News.

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