For stark, paralyzing terror, few experiences compare with facing the charge of a 450-pound male gorilla.

J. Michael Fay has learned that for himself during two years of research on western lowland gorillas in the dense forests of the Central African Republic.No matter that the charge is a bluff. Silverback lowland males intimidate intruders by roaring ferociously, running within a few feet of them, rearing up on their hind legs, flailing their arms - and, at the last instant, veering off into the bush.

Even Fay's Pygmy guide, who's had numerous confrontations like this, nearly passes out from fright as he sits (one doesn't stand when dealing directly with gorillas in the wild) and waits for the next charge. His body shakes uncontrollably.

"There's no way to get used to a silverback charging in like a freight train and then passing you at one meter," says Fay, 32, a doctoral candidate in primatology at Washington University in St. Louis. "It's really unbelievable."

He emphasizes, however, that gorillas, which are vegetarians, pose a threat to humans, usually poachers, only when provoked. "Normally, there's absolutely no possibility a gorilla would mortally wound you, unprovoked, in these forests."

Fay, whose research has been supported in part by the National Geographic Society, has discovered one major difference in the way lowland gorillas and the slightly larger mountain gorillas of East Africa communicate: Lowland gorillas clap their hands.

He made the discovery when he and a Pygmy came upon 12 or 13 gorillas, females and young ones, in a tree. The appearance of the humans made the great apes nervous. "And then, all of a sudden, one of them perked up on a branch and clapped her hands in kind of a normal, human-type handclapping."

Although he didn't think much about it at the time, Fay paid more attention as, over the next few hours, most gorillas in the group clapped with varying intensity.

The group's silverback leader was on the ground some distance away. Each time a female clapped, he responded differently, depending on the intensity of the clapping.

A short, soft handclap brought forth only a pig-like grunt from the silverback. But a louder, extended clap drew a slight roar and possibly some movement. Fay, who has since observed handclap-ping in other groups of females, concludes "they're encountering some difficulty or some unusual thing they can't quite deal with."

This contrasts with the blood-chilling shrieks of female gorillas in life-threatening situations. Their cries in genuine crises bring males charging to their rescue. Pygmy hunters, at their peril, sometimes imitate the screams to attract silverbacks.

Fay says handclapping is "an extremely rare type of behavior in mountain gorillas," in contrast to the lowland subspecies. The late Dian Fossey wrote about only one mountain gorilla that did it for a few years and then ceased, he says.

Fossey also wrote that lowland gorillas in zoos clapped their hands. "But no one ever really suspected it was common in western lowland gorillas in the wild," Fay says.

Lowland silverbacks sometimes use other unorthodox ways to communicate, Fay has learned. One day he and a Pygmy heard a loud, resonant boom some distance away. At first they thought it was a gunshot.

To their surprise, they found the sound emanated from the buttress of a tree, pounded periodically by a silverback sitting at the tree's base. Fay surmises the male might have been calling his strayed group together. "Or maybe he was just having fun. I don't know. It was a pretty eerie experience."

Although human hunters are lowland gorillas' chief enemy, Pygmies have told Fay that leopards eat gorillas. At one site he studied, Fay confirmed these reports when he found intact gorilla toes in some leopard dung.

But neither leopards nor poachers are Fay's biggest concern when it comes to the ultimate future of the lowland gorillas. Commercial logging of forests in the Central African Republic and surrounding countries is rapidly increasing, insidiously eating away the habitat of the 40,000 or so gorillas that live there.

Although the lowland gorillas far outnumber their slightly larger mountain relatives, they have been studied much less. Their forest habitat makes them extremely difficult to track and observe.

"They might not be threatened right now," Fay says, "but the rate of destruction of their habitat is so rapid that in another 10 or 15 years the western lowland gorilla could face grave problems."