ST. LOUIS (AP) — The St. Louis Zoo is planning an $11.5 million expansion of its area for primates, adding 35,000 square feet of outdoor space that will include trails and tunnels for observers to get a close-up view.

Monkeys, chimps and their cousins have long been among the most popular of the 17,000 animals at the zoo, even if the Primate House where they live is aging.

Curator of primates Heidi Hellmuth said the new Primate Canopy Trails, expected to open in 2021, achieves two purposes: It allows the animals to spend much of their time outside instead of inside the Primate House, and it gives visitors an almost face-to-face opportunity to see the animals.

“We have animals that have never been out in the sun, never felt the wind in their face, never had the sights and sounds and smells and even textures of being outside,” Hellmuth said.

Primate Canopy Trails actually will be made up of eight outdoor homes for different species, though some that get along well enough will be able to mingle. Visitors will be able to get remarkably close.

Those entering the primate area will walk through a ground-level glass tunnel.

“Imagine them (the primates) just leaping from side-to-side over your head,” said Hellmuth, who has worked with primates for three decades.

Even more unusual will be overhead mesh tunnels, about a dozen feet in the air, allowing visitors to move between existing sycamore trees and observe the animals as they move and play in the trees. Visitors can “get eye-to-eye and nose-to-nose” with monkey, lemurs and other primates, Hellmuth said.

The Primate House, built in 1925, is currently home to about 40 primates representing 14 species. It was last renovated in 1977 and will continue to be used so the animals can be inside when the weather calls for it, or if they just need a break.

The current setup has just six outdoor habitats, and they are available only to a handful of primate species — lemurs, sifaka and saki monkeys.

Hellmuth said the new habitats will allow the zoo to increase its participation in cooperative breeding programs because it will be able to accommodate larger groups as primate families grow.