An autopsy of Ling-Ling's dead cub shows that the infant panda lived for a few minutes, then died from lack of oxygen while the giant mother panda was preoccupied giving birth to or tending to a second, healthier cub, officials at the National Zoo said yesterday.

Also -- and there is no delicate way to put this -- the new panda mom later crushed the firstborn cub when she sat, stepped or "leaned" on it. Zoo authorities say this happened after the cub, a male weighing about 3 ounces, was already dead.

"Nobody could see {on the closed-circuit television monitor} what was going on with this particular cub," said Richard J. Montali, the zoo's veterinarian pathologist, who performed the autopsy. "It clearly breathed, but it was weaker and smaller and didn't survive."

On a happier note, the zoo continued to issue upbeat reports on the health of the surviving 4-ounce, 5-inch-long cub, whose sex has not yet been determined.

"We can hear the infant, and it's as loud as ever," said Robert Hoage, a spokesman for the zoo.

Since Wednesday morning, when Ling-Ling picked the cub up in her mouth and moved into her den, out of camera range, panda watchers have been unable to see the tiny cub. Yesterday, zoo keepers installed a microphone at the entrance to the den and are monitoring the sounds coming from the enclosure.

On Wednesday, a day after heralding the successful birth of one cub, zoo authorities said they had belatedly discovered the body of a second cub, which they announced was stillborn. The subsequent autopsy, however, showed the cub died of hypoxia or lack of oxygen, either because it didn't breathe well enough on its own or because it was accidentally suffocated by Ling-Ling.

"There was evidence that it had breathed but only for a short time," said Montali. The smaller cub didn't move or make any sound to "stimulate the mother to pick it up . . . {and} as soon as the other cub came, the full attention of the mother was toward it."

With a squealing, demanding infant to worry about, Montali said, "Ling totally discarded" the dead cub. "In fact, she leaned on it," he said, citing evidence of "post-mortem injuries."

None of this was picked up on the television monitor. Zoo officials, who have reviewed tapes of the births, say there was no infant vocalization and no sign of movement in the nest until the healthy panda cub arrived at 3:33 a.m. Tuesday. Officials aren't even sure that the sickly cub arrived first, Hoage said, though they think that is what happened.

"I don't think much of anything could have been done differently," Montali said. "Everything was going so well {with the healthy cub}, I don't think we would have disturbed things."

Even if the keepers had been able to get to the sick cub and remove it, "the history of trying to hand-rear these discarded cubs is not good," Montali said.

Pandas are members of the bear family, a conclusion reached only recently in scientific circles -- which once preached that pandas were a family unto themselves or cousins to the raccoon.

A team led by Stephen O'Brien of the National Cancer Institute in Bethesda did an exhaustive biochemical and molecular study of pandas and bears and found a definite kinship.

"The term panda bear is now acceptable," Hoage said.

But what of Ling-Ling? Can she be trusted not to "lean" on her surviving cub? Are panda keepers worried?

"It's really remarkable how careful she is," Hoage said. He noted that a tape of Ling-Ling leaving the nest shows her stepping slowly and deliberately with her right hind foot so as not to injure her cub.

"She's handling the infant with great tenderness," he said.