Chicago's famous Brookfield Zoo begins its second half-century boasting a stunning new animal environment, Tropic World, where explorers charting the rain forests of three continents can hear the strange birds cry, sniff the moist air and completely forget that the Chicago Loop is just 14 miles away.

Barless, roofless habitat exhibits, now the zoo industry standard, were pioneered at Brookfield. Sculpted bear grottos date to the infancy of the 51-year-old zoo. The bluffs of Baboon Island provide a tribe of merrily licentious monkeys with ample scampering ground. But the $10.8 million Tropic World exhibit easily surpasses these displays.

Tropic World presents the rain forests of three continents as living dioramas, sans mosquitoes. Under the 55-foot-high sky blue ceiling, three daily rainstorms -- appropriately prefaced by volleys of thunder -- drench all of South America, Southeast Asia and Africa, except the walkway laid for visitors. The temperature stands at 75 degrees.

The safari path starts in South America, at the foot of a cliff washed by twin waterfalls. Flash, the two-toed sloth, dozes in a mossy alcove, and a tribe of spider monkeys hurtles through tangles of vines hung from 50-foot-tall artificial trees. A family of golden lion tamarins, shiny as new pennies, streaks through its own grove of trees. A king vulture arcs across the horizon, perches, preens and flaps casually down to the paved path for his morning stroll.

Southeast Asia is just a footbridge away. While two young orangutans cuddle in the shadow of their mountainous aunt, an adolescent cousin marches up an incline, turns three somersaults down to a stand of trees and clambers back up for an encore. Background music is provided by five species of brightly colored birds.

One continent over, a family of African gorillas keeps house on the sloping banks of a meandering stream. Samson, the silver-backed patriarch of the clan, stands sentry while his wives and children forage across their terraced realm. A pygmy hippo wades in a shaded bend in the river.

Zoo director George B. Rabb says Tropic World is good for both its residents and their human guests. Visitors come closer to seeing primates and birds framed by authentic rain forest scenery than most of them otherwise could. And because the animals feel more relaxed in their new digs than in cramped cages, birthrates are up. Each time captive animals add to their families, they expand the global storehouse of genetic diversity, and everyone benefits.

The rusting cages of the Primate House, where upwardly mobile Tropic World dwellers once lived, memorialize obsolete zoo styles. Although many of the 2,000 creatures on display at Brookfield bunk in modern quarters, others would welcome more elbow room.

The Reptile House is the oldest building in the park, and a 17-foot-long Asian python named Vulcan is its fattest boarder. His body is thicker than your leg. Dinner, served weekly, is either an unplucked chicken or an adult rabbit, complete with fur. Since he would flatly reject a meal redolent of humans, picky Vulcan is served his freshly killed chow by keepers who handle his food with tongs.

Other snakes prefer live mice for supper. The 70,000 adult and baby mice served up annually by the animal commissary are all lean, Grade AAA specimens.

The tiniest creatures living in the glass-walled cases are the Panamanian poison arrow frogs. The thumb-long amphibians are named for the Indian practice of dipping arrowheads in their venom.

Marine mammals live in the Seven Seas Panorama. Unlike coastal zoos, inland Brookfield custom-blends its salt water. Recreating oceanic salinity levels in the 200,000-gallon dolphin tank requires 15 tons of salt weekly.

All five Atlantic bottlenose dolphins display their charming natures and their echo-location skills in daily performances at the heavily visited Dolphinarium. To keep their charges alert and happy, trainers at Seven Seas use rewards ranging from mackerel snacks to play-and-stroke sessions. Why would a dolphin consider interaction with its teachers a valuable perk? "We're just lucky, I guess," figured head trainer Randy Brill. As he spoke, veteran performer Nemo cruised across the tank to retrieve a battered Frisbee for a bonus round of fetch.

At the Predator Ecology exhibit, keepers have swapped day for night. Small, feisty, lantern-eyed felines scale moonlit cliffs. The big cats live next door.

Shima, a 60-pound black rhinoceros baby, was delivered in the Pachyderm House in January, and the keepers were as delighted as parents Embu and Brook. Since wild black rhinos are an endangered species, the birth of a healthy baby was an especially joyous occasion.

At the Children's Zoo, special attractions for city children and their parents range from daily cow-milking sessions to horse and pony spectacles that include cart pulling and obstacle course races.

With its naturalistic habitats and breeding successes, the Brookfield Zoo is more than a museum of living things. Director Rabb likens the place to Embassy Row: "The animals are our guests; they are ambassadors for their species. We are ambassadors for the animals." CAPTION: Pictures 1 and 2, Polar bear cubs at the Brookfield Zoo. Shima, a black rhinoceros born Jan. 18 at the Brookfield Zoo. Photos copyright (c) by Mike Greer, Chicago Zoological society.