It's probably the only forest in the world where the thunder cracks precisely every hour, followed by torrents of rain from an otherwise blue sky. While many of the monkeys run for cover, a pygmy hippopotamus, clearly enjoying every minute of the downpour, moves not an inch in her own private pool.
It's the first portion of an ambitious, three-part project at the Brookfield Zoo to replicate the rapidly disappearing tropical rain forest as it now exists in three parts of the world.
The African forest, featuring strong 40-to-50 foot high concrete trees and three generations of lowland gorillas, six species of monkeys, Sassy the hippo, four-foot lizards, and numerous African birds, has just opened to visitors.
By 1984 when the Asian and South American simulations are completed, the building designed by Skidmore, Owings & Merrill and labeled ''Tropic World'' will qualify as the largest indoor animal exhibit in the world.
Most zoos traditionally have provided separate quarters for each species, from birds to elephants. Many are now choosing instead to mix species which normally live together in the wild and get on well. The hope is that by such a simulation of the animals' natural environment, largely without bars, visitors will feel a part of it and gain a more graphic appreciation of the animals as part of their surroundings. Zoo officials also hope that the animals, with more space to roam, will be more apt to behave naturally and to produce offspring -- long considered a special challenge for animals in captivity.
But this move to mixed species is by no means an easy one for zookeepers. In Brookfield's new African tropical rain forest, each animal species -- right down to the two lizards -- first had the run of the exhibit space separately on the theory that once all were put together, each would feel more secure.
''It's a long training process -- it literally takes several months to do,'' explains assistant director Paul Joslin.
Then, too, even though no such zoo exhibits will ever bring together animals which have a predator-prey relationship in the wild such as lions and wildebeests, zoo officials say they must constantly stay alert to individual temperament problems among animals.
''Mixing species really allows zoos to present the animal kingdom as it is, but you have to be extremely careful,'' notes Robert Wagner, executive director of the American Association of Zoological Parks and Aquariums (AAZPA), an organization based in Wheeling, W.Va. ''Even among the same species there can be problems of aggression -- just like humans.''
And sometimes seeing the animals in an open mixed-species exhibit proves more of a challenge than if the animals were up front in a cage.
''People expect the animals to be right there in front of them like they are on TV, but they have to learn to look harder,'' says Dr. George Rabb, director of the Brookfield Zoo. ''They tend to be put off by the slightest camouflage.''
But even as they mix more species in exhibits, most zoos are moving away from the old ''postage stamp'' approach of acquiring at least one of every species for their collections. Animals can no longer be snatched from the wild as they once were. Zoos are accordingly taking more seriously than ever their longtime responsibility to preserve the species they do have and to encourage the production of offspring in captivity, particularly among scarcer varieties.
''We're concentrating on trying to build breeding groups, and in that sense there may be less variety in the future,'' explains Dr. Rabb. ''We're trying to maximize the genetic diversity in each species -- to hang onto as many genes as we can.''
Actually, as the AAZPA's Mr. Wagner tells it, zoos are faring very well in their breeding mission. He notes that before the mid-1950s, no gorillas had been conceived and born in captivity. Now more than 200 have since been produced in zoos around the world.
''Zoos are not net consumers of wildlife as many used to think. . . . They're net producers of mammals and soon will be of birds,'' he says. ''About 80 percent of all animals you see on display were born in captivity, many from parents who were born in zoos. Once we reach the second generation, we'll have it made.''
These are days when most zoos need all the outside financial help they can possibly recruit. Many are finding that one of the most successful ways to both feed the coffers and strengthen the personal link between zoo residents and the general public is by an animal ''adoption'' program focused on feeding costs. Brookfield Zoo pioneered the program six years ago and annually raises more than one-third of its $318,000 animal food bill with the help of some 6,000 contributors.
Brookfield encourages the giving by naming many of its charges and sometimes, as with Olga the walrus who eats $10,000 worth of food a year, offering several adoption ''shares'' on one animal.