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Thunder rumbling above the treetops startles a visitor.

Another flinches at the sounds of an animal crashing through the underbrush and turns to look. But no clouds roll overhead, no animal appears.Through 144 speakers, racks of compact disc players, sensors that detect the presence of people and adjust the sound levels accordingly, and a complex computer system that controls all, the San Diego Zoo has infused the real sounds of a tropical rainforest into its newest exhibit, the $11.5 million Gorilla Tropics, which opened in March. Five weeks of jungle noise recordings - bird songs, animals rustling through the underbrush, primate calls, rain and thunder - from the African countries of Rwanda and Zaire have been incorporated into the four aviaries and 30,000-square-foot gorilla enclosure to augment the sounds of the 200 birds and six gorillas that are confined there.

Although the $200,000 high-tech sound system was designed by Australian Les Gilbert to immerse humans in a tropical rainforest setting, it is also being used to take the latest philosophy in zoo management one controversial step further. Animal behaviorists who believe that keeping zoo animals passive is passe, want to induce momentary fear or anger in primates to reduce the stress of severe boredom that most face every day of their pent-up lives.

Researchers at the San Diego Zoo eventually plan to test this theory by using the sounds of approaching rivals or predators to induce an excited response in the gorillas. But they plan to begin slowly, by finding out how the gorillas respond to soothing sounds - the natural nighttime noises of the tropical jungle, such as the buzzing of insects - piped into their sleeping quarters.

"There are two things going on with animals in zoos," said Don Lindburg, an animal behaviorist at the zoo's Center for Reproduction of Endangered Species. "Chronic understimulation, where they are bored, and chronic overstimulation where long-term proximity to humans produces stress or they are in a constant state of fear."

When primates are exposed to too many visitors for too long, for example, they exhibit behavior characteristic of human depression: they become lethargic, won't groom each other or themselves and won't interact with each other.

Over the past 20 years, many zoos and research laboratories have incorporated the concept of "environmental enrichment" to help animals live more as they do in the wild. "Most animals used to live in stimulus-poor environments, in barren enclosures," said Dr. Terry Maple, zoo director and animal behavioralist who specializes in environmental psychology. "Zoos took an entirely new approach in the 1970s."

The cages of animals were constructed to simulate their natural environment. In the case of primates, they were also given objects with which to play. A more natural environment results in healthier, happier lives, which is especially important for endangered species, who must feel comfortable before they will breed. Half of all female gorillas in the worlds' zoos, for example, don't reproduce.

Because boredom is still a major problem in zoos, environmental enrichment is now moving beyond trees and toys. Some scientists think that brief periods of arousal, such as anger and fear induced by predators and rivals that is common to all wildlife, and often absent in captivity should be incorporated into the lives of zoo animals. Zoo Atlanta put four breeding groups of gorillas together in the same enclosure to simulate competitive living conditions in the wild to induce the animals to breed more often, said Maple. This type of stimulation seemed to work. After the first year, three baby gorillas were born.

In the most striking example so far of the new approach to environmental enrichment, animal behaviorists Eleanor Moodie and Arnold Chamove at Sterling University in Sterling, Scotland, made a wooden model of a soaring hawk and passed it over a skylight above a room housing tamarin monkeys. The monkeys chirped in alarm and escaped to tree cover during the flyover. The researchers found that this stimulation resulted in the monkeys showing a more normal behavior pattern, such as touching and grooming each other.

The difference between healthy and unhealthy arousal, noted Moodie and Chamove, is the duration of the stimulus, rather than its intensity. "Mild stimulation, if aversive and prolonged, can become a stress; strong stimulation, if brief and not repeated, will not act as a stress," they noted in a paper published in Zoo Biology last year.

Using sound to stimulate zoo animals is a "tremendously new idea," said Maple. The reason it hasn't been done before is that "the technology wasn't there before to do it. Les Gilbert is one of a handful of people who are making recordings all over the world, to record sounds of animals and places that might not be around much longer, as well as to make tapes and for people to use in research."

"There was quite a bit of opposition on the part of those who thought it was no big deal," said Lindburg. "And when we went to the literature on animal husbandry, we found virtually no information out there."

Because the field is so new, the plan in San Diego is to introduce the stimuli gradually. First, night jungle noises will be piped into the gorillas' sleeping quarters. Jackie Ogden, a research fellow at the zoo who is making the project her doctoral dissertation, will monitor the gorillas' behavior, and then remove the sounds to see if any changes occur.

"The theory is that masking the effect of some of the less-than-wonderful noises, such as the ventilation system, will have an enriching effect," she said. The natural sounds can also muffle the noise produced by the activities of animal keepers. Studies have shown that monkeys become much more aggressive a half-hour before feeding time, because they can hear keepers preparing their food.

Eventually, a system will be incorporated into the areas where the gorillas spend their waking hours. The same sensors used to detect humans around the enclosure will be put inside it. If the gorillas learn to trigger the sensors in order to experience certain sounds, they will feel they have more control over their environment, which has been shown to enhance their sense of well-being, Ogden said.

"We will experiment with how often we let them hear acute stimulation such as the distant howling of a troop of chimps, let's say," said Lindburg. "If no chimps appear and it's too repetitious, then it won't work. It's going to take some study to figure how well we can succeed in fooling them."

Gilbert calls his natural sound system the "psychic glue" that pulls an exhibit together. His system isn't anything like a tape recording that operates on a continuous loop and hisses white noise along with the squawk of a hornbill. He obtains digital recordings in the field, waiting weeks for just 30 minutes of sound, if necessary. He does analog mixing and digitally masters the sound onto compact discs, and uses high quality speakers.

He puts together the sounds in the same way a composer writes a musical composition, but with the added challenge of a spatial dimension. "We make the sounds interactive," said Gilbert, so that as a person moves among environments, the sound in adjacent areas is muted. "We make it very natural, so that you're not aware that it's coming from something other than something that's real."

The computer-controlled system at the San Diego Zoo plays different tracks randomly throughout the day, but many are coded to fit the part of the day in which they naturally occur. For example, early morning brings the sounds of monkeys chattering in the treetops; early afternoon has rising winds and approaching storms; and late afternoon, the croaks of frogs and buzzing of insects.

"The sense of being immersed in a more natural situation is enhanced by what you hear," said Lindburg.

Gilbert related an instance in which this was demonstrated to him. At the Maritime Museum in Sydney, Australia, where he had augmented the exhibit of a wooden ship with sounds of its motion under sail, a man who had often sailed such vessels asked Gilbert how he managed to get the deck to move so realistically. The deck didn't move, Gilbert told him. The man just thought it did because he heard creaks and other noises it would be making if it were in motion. Gilbert is now working on putting natural sounds in the Shedd Aquarium in Chicago and facilities in Tennessee and Genoa, Italy, as well as the civil rights museum in Birmingham.

Although the use of sound to help create a more stimulating environment for zoo animals is exciting, it's too new to make any predictions about how it will affect the animals. Some experts think that using it to arouse anger and fear won't work.

"Most animals are going to catch on and are going to realize that they're living in the middle of Muzak," said Ken Norris, a Santa Cruz, Calif., cetacean expert who designed a sound-dampened tank in which dolphins - also victims of overstimulation from noises reverberating in their environment - seemed to be healthier. "The effects will wear off."

But he thinks some of the environmental sounds might be of value, in the same way that "it makes us happy to sit and listen to the wind at home."

Even Lindburg is cautious. "I don't want to oversell this," he said.

"It's likely that it will add something to their psychological well-being. But I'm not expecting baby gorillas to pop out like manna from heaven."

Nevertheless, directors of zoos around the country are watching San Diego's experiments with great interest. "We didn't do it (at Zoo Atlanta) because it was too expensive," said Maple. "But I'm very interested in knowing how well the stimulus will affect the animals and the public as well. It's a tremendous educational tool. If the effects are as dramatic as I think they will be, then perhaps all zoos will use it to enhance their environments."

Salt Lake's Hogle Zoo takes a similar wait-and-see approach, but zoo director LaMar Farnsworth says he doesn't think their animals are all that bored.

And though Gilbert believes in the value of his system, he worries that the environments created with his sounds will become too authentic, leading people to be satisfied with a false "real" environments and not make efforts to save real ones.

"For example, the African rainforest is rapidly disappearing," said Gilbert. "People might think, like having white rhinoceroses in zoos, that possibly it doesn't matter, because they're able to experience it anyway."