The National Zoo had given its pregnant lowland gorilla, Mandara, a choice: deliver her baby in a private, secluded area of the Great Ape House or be out with the other members of her group in public view.

Saturday, to the amazement of zoogoers, Mandara, 17, chose to have her fourth baby in the public viewing area--letting passersby witness an event that in the wild unfolds only in the fastness of the African jungle.

Mandara had her baby about 4:15 p.m., said Lisa Stevens, the zoo's associate curator of mammals. It was the fifth gorilla born at the zoo since 1991.

Stevens said zoo officials were excited because while it was Mandara's fourth baby, it was the first for the father, 16-year-old Kuja, thereby adding a healthy new batch of genes to the nation's captive gorilla community.

Visitors who witnessed the event were excited, too.

"I watched it happen," said Kris McWilliams, 21, of Fairfax, who was at the zoo working on a college anthropology report. "I was sitting there watching them, and all of a sudden all the gorillas started screaming and running around."

He said the mother vanished from his sight for a moment but reappeared seconds later holding her tiny offspring in one hand. Gorilla babies usually weigh 4 to 5 pounds. "Everybody was in awe," he said.

McWilliams said the commotion among the giant creatures--some of whom can weigh 400 pounds--was a little unsettling, and it was not clear at first whether the baby was alive.

But Stevens, the curator, said all was apparently well, except that Mandara had not yet brought the baby close enough for keepers to determine its gender. "We basically have to wait for mom to come over and give us a look in the right places," she said.

"We're very excited about it," Stevens said. "Not only have we had another baby born, we have added a new male's genes to the population."

She said the nation's zoos regularly swap male and female gorillas to try to generate a healthy, genetically diverse population among the captive animals.

Mandara, who weighs about 135 pounds, is owned by the Milwaukee County Zoo, and Kuja is the property of the Brookfield Zoo, near Chicago, Stevens said.

Gus, the father of Mandara's other offspring, has been moved to a bachelor's group at another zoo because his genes are now well represented in the national population, Stevens said.

She said Mandara's water broke about 3 p.m. Saturday. Gorillas, which live into their mid-fifties, have an 8 1/2-month gestation period.

Stevens said that, much like with human childbirth, the zoo no longer isolates pregnant mothers but gives them the option of being with other group members.

"Mandara has typically given birth in the [public] exhibit enclosure," Stevens said. "She chooses where she's most comfortable. It's where the group spends their time. It's being close to the group as well. The phenomenon of isolating oneself when giving birth is a human misperception."

CAPTION: Mandara, shown in a 1997 photo, gave birth Saturday. It was the father's first offspring.