WE’VE YET TO lay eyes on it, we don’t know whether it’s male or female, and its survival is still touch and go. But the latest addition to the National Zoo has already provoked a full-blown love affair in the District. And yes, count us in on being excited about the birth of a cub to the zoo’s giant pandas, news that is the more thrilling for being so unexpected.

Pregnancies in pandas are notoriously difficult to detect, and scientists at the zoo had given long odds to the possibility that 14-year-old Mei Xiang, artificially inseminated earlier this year, would conceive. Still, as is their routine, zoo employees and volunteers set up a panda watch just in case. At 10:46 p.m. Sunday, they were richly rewarded when Mei Xiang was caught on the panda cam giving birth.

“We’re ecstatic,” Don Moore, associate director for animal-care sciences at the zoo, told The Post’s Michael E. Ruane. Mr. Moore was prepared for disappointment; he greeted news of the birth with, “Yeah, yeah, it’s not April Fools’ yet, so I’m going back to bed.” He said of Mei Xiang, “She’s being a very, very good mom. . . . Every time the kid cries, she cradles it in a different way.”

There is still some danger: The cub is very small — the size of a stick of butter is the popular comparison — and pandas can accidentally crush the newborns.

Ever since Ling-Ling and Hsing-Hsing stepped out of their crates in 1972 — a gift from China to commemorate President Richard Nixon’s historic visit — pandas have had a special spot in Washington’s heart. Mei Xiang and her mate, Tian Tian, on loan from China, filled the void after the death of the first pair, and in 2005 she produced a cub, Tai Shan, who delighted zoo visitors until he was returned to China as part of a research and breeding program. Ling-Ling had five cubs, but none survived more than a few days.

Which brings us to why the birth of this second cub should be celebrated. There’s no question that a thriving cub — and we’re keeping our fingers crossed — will prove to be popular, driving thousands of new visitors to the zoo and, no doubt, spurring sales of its inevitable stuffed likeness. But all that pales beside the significance of the lessons that could be learned and the advances made in the critical work of breeding endangered species in captivity. So welcome, Butterstick II.