Tramway in the Treetops. BIRD'S-EYE VIEW

AT the end of a narrow, secluded trail through a pristine Costa Rican rain forest, by a picturesque waterfall, is a decidedly nonnatural sight. Strung through the trees of the Rara Avis rain forest reserve north of San Jos'e, the series of overhead cables represents an innovative effort to better understand the rain forest by opening up an area that has been largely inaccessible to researchers.

The cables are a tramway, from which is hung a small, open-air car that moves through the rain forest canopy. It allows an unprecedented amount of mobility for study of the area between 10 and 50 meters above the forest floor.

The canopy, says botanist Michael Grayum of the Missouri Botanical Garden, is ``the most important stratum of the forest. It's where the light is landing. There's a greater abundance of fruits, nectar, and other resources there.'' Mr. Grayum, who has been cataloging Costa Rican plant life for 10 years, says that many plant and animal species found in the canopy are not found below.

The problem with studying the canopy has been its lack of accessibility. Aside from greatly increasing mobility and eliminating the chore of studying treetops one at a time, the tramway eliminates such scientific hardships as enduring the attacks of the fiercely protective ants that inhabit many rain forest trees.

The tramway is the creation of American naturalist Donald Perry. A renegade in the scientific community, Mr. Perry loves to share with the news media such ideas as his ``arboreal theory of human evolution,'' which postulates that early man's life in the trees has greatly influenced modern human behavior and attitudes. But he unapologetically refuses to back his theories to the satisfaction of his colleagues. He doesn't need to test what he can see with his own eyes, he says.

They may not like his science, but at least some of his colleagues give Perry credit for helping them study the canopy in new ways. Before building the tramway system, Perry pioneered what is still a much-used method for climbing to the rain forest treetops. It involves the use of a crossbow to shoot climbing lines up to where humans could not otherwise place them.

University of Costa Rica biology professor F. Gary Stiles, a specialist on Costa Rican birds who, like Grayum, has sampled the tramway's delights, calls it a valuable research tool. ``It allows you to get up there and see what's happening, in situ. We have some knowledge of what's up there, but this can help tell us about the biological interactions,'' he said.

Harvard University biologist Edward O. Wilson calls canopy research ``a new frontier with many discoveries ahead'' but says that long-term projects using the system must be initiated if it is to realize its potential in that field.

As for Perry, he is writing a book on his theory and working on a new tramway that will move laterally as well as up, down, and along one cable. He's also planning a camera-toting robot and a mechanical collecting arm for use in canopy work.