Mexico's prolific pair of pandas are proud parents once again. Last year they brought forth the first panda ever born outside China. The little cub lived only eight days. But they tried again, and at exactly 4 p.m. Tuesday afternoon, they succeeded once more where all other expatriate pandas have failed.

Chapultepec Zoo officials say they believe this newborn has a much greater chance of survival than the last. Last year, 6-year-old Ying-Ying was as young as a panda mother can be. Upset and anxious, she rolled onto her first baby and crushed it to death. This year, the mature 7-year-old is already sleeping normally and shows no signs of anxiety.

Zoo officials are making sure no one disturbs her.The entire panda section of the zoo has been fenced off and even the newborn's father, Pe-Pe, has not been allowed near his mate since July 12. Human visitors may see the pair only over a closed-circuit television monitor in the director's office. Not even zoo officials approach the baby or its mother.

"Nobody goes in there and nobody will go in there, I'll tell you," said Jean Schoch, the zoo's technical director, "For one thing, she'd kill anyone who took the baby away. Last year she wouldn't give up her baby even after it was dead. She just kept holding it next to her."

As a result, the exact size of the fragile newborn is only an estimate and its gender only a guess. At about four inces long and weighing perhaps 3.2 ounces -- its bare little form almost lost against the 275-pound mass of its mother -- the cub appears slightly smaller than its sister last year, leading zoo officials to believe it is probably a male.

"Normally in carnovires the newborn male is smaller than the female," said Schoch. But in any case, he added, "You can't tell the sex before they're about 6 months old."

In fact, with the total panda population of the world estimated at only about 300, many aspects of the breed -- and breeding itself -- remain mysterious.

Several zoos that have received pandas as gifts from the People's Republic of China over the last 15 years have tried repeatedly to breed them, but without success. One of the more recent efforts was in Washington's National Zoo, where the resident pair have shown little interest in each other and a male brought over from London seemed more interested in making war than love.

Schoch said yesterday that even in the Peking Zoo, where about a dozen pandas have been born in captivity, only about five have survived and few details are known about them.

Mexican zoo officials are so wary of interfering with the panda parents that they never ran any checks on Ying-Ying to see if she had conceived after a mating period. Instead, they relied on changed in her behavior and eating patterns to confirm their hopes.

When Ying-Ying stopped eating Monday morning and started using her bamboo-shoot breakfast to build a nest, Schoch said he and his colleagues figured the moment had arrived. Ying-Ying's gestation period last year was 127 days. This year it was 126.

At about 3:30 Tuesday afternoon Ying-Ying showed the first signs of contractions, and at exactly 4 p.m. she pulled the baby into the light of day with her mouth. Ever since, Ying-Ying has held it close to her chest. She fed it at least twice when its mewling woke her from a tranquil sleep.

The baby's eyes are closed and won't open for a month, said Schoch. It won't really walk for another two or three months after that, and not for several weeks will it even have the characteristic black and white markings of its parents. No name for it has yet been selected.