Between morning feedings, the gibbons start to sing — loudly.

Their calls, which range from something like emergency sirens and screeching birds to low grunts and guttural hoots, echo across a sanctuary in the hot, dry hills of the Santa Clarita Valley, north of Los Angeles.

Over the past 50 years, the Gibbon Conservation Center has grown from housing a small assortment of animals into a globally recognized institution devoted exclusively to the care and well-being of gibbons — small apes famous for their singing and counted among the rarest and most critically endangered primates on Earth.

Gibbons are the smallest of the apes — sometimes called “lesser apes” in contrast to gorillas, chimpanzees and orangutans — and they travel through the forest canopy by brachiation, swinging hand over hand on long arms. In the wild, mated pairs sing coordinated duets that help defend territory and reinforce pair bonds.

The center currently looks after some 40 gibbons across five species. In the sparsely inhabited outskirts of Santa Clarita, the few neighbors can sometimes hear their songs from a mile away.

The facility was founded by the late Alan Mootnick, a self-taught primatologist who became a leading expert on gibbons while simultaneously running a home painting and remodeling business. He published papers in scientific journals, hosted researchers from around the world, and fielded questions from other scientists about gibbon care and identification.

“Sometimes they would just send him a picture of a rescued gibbon asking what species, or they will play him a vocalization,” Director Gabi Skollar said.

Skollar moved from Hungary to the U.S. in 2005 to learn from Mootnick and work at the center, and she has carried on his mission since his sudden death from heart surgery complications in 2011. Although she had initially planned to leave the job to continue her education, she said abandoning the gibbons was never an option.

“We have to keep going and take care of this place,” she said.

Today, she lives on-site with another caretaker, waking every morning to the cacophony of gibbons at sunrise. The animals, she said, are her children.

After decades at the center, Skollar knows the gibbons' personalities and individual quirks well. A young northern white-cheeked gibbon named Pepper always leads the group in starting their singing, while a pileated gibbon named Violet finishes the chorus with her partner Truman. Pierre, a rescued gibbon, does not like men.

The small apes sing three to four times a day, in sessions lasting 20 to 30 minutes at a time.

“There's a lot of things that we can learn in a captive setting that can be shared with conservationists in the field and also with people working in sanctuaries rescuing gibbons,” said Skollar, who is studying gibbon vocalizations in a doctorate program at the University of California, Los Angeles. Gibbons have historically received less scientific attention than the great apes, and research in captive settings like the center is one way that gap is being filled.

In their native range — the tropical rainforests of Southeast Asia — gibbons face increasing threats from habitat destruction driven by logging and agricultural expansion. The wildlife trade poses another danger, with baby gibbons captured from their parents and sold as pets. The International Union for Conservation of Nature classifies most gibbon species as endangered or critically endangered, and all gibbons are listed on CITES Appendix I, which bans international commercial trade in the animals.

Mootnick acquired his first gibbon, named Spanky, in 1976 after responding to a newspaper ad from someone selling a pet. Some gibbons at the conservation center were born there, while others were traded with different institutions to ensure diversity in breeding programs — a standard practice in captive conservation, where endangered species are managed across facilities to keep populations genetically healthy. Some of the species at the center have a wild population of less than 2,000.

The center was the first to successfully breed Javan gibbons, an endangered species found only on the Indonesian island of Java, and it is soon sending a pair of northern white-cheeked gibbons — a critically endangered species whose surviving wild populations are concentrated in Vietnam and Laos — to the Pittsburgh Zoo.

On a recent morning, on-site caretaker Jodi Kleier was making the rounds with buckets of washed lettuce, steamed sweet potatoes and bananas. She handed a piece of chopped banana to Rocky, a young gibbon reaching out through the fence.

Rocky was hand-reared by Kleier after being abandoned by his mother at birth at the center — something that can happen with first-time mothers if they went through a difficult birth.

Kleier has worked at the center for a decade, starting out as a volunteer, and has a tattoo of Rocky on her thigh. She fell in love with gibbons after coming to the center for a primatology course in school.

“I love watching any primates play, because I think they're so natural to us, we kind of see ourselves in them,” she said.

Because the gibbons are fed five to six times a day — imitating the pace of their foraging behavior in the wild — Kleier and the volunteers who come by during the week are constantly washing and cooking produce.

“The Javans, they're my favorite species and the male Javan is singing … one of my favorite songs,” Kleier said of a ghostly vocalization. “In Javanese folklore, they're known as these spirits that live in the forest because you can always hear them.”

While the center is open to visitors only for guided tours on weekends, it receives steady interest from schools, tourists and senior centers. It also works with local organizations such as The Teaching Zoo at Moorpark College to help students gain hands-on experience in animal care. The college takes first-year students on a field trip to the Gibbon Conservation Center every year, and many have gone on to work there after graduating.

“When I went out there for the first time in the 1990s, I didn't know what a gibbon was,” said Mara Rodriguez, a development coordinator who has been with the zoo for decades. “Now I've raised four gibbons in my career.”

Anyone who visits “will leave with a lasting impression and begin to care about the species,” she said.

Skollar's long-term goal is to raise more money to build larger enclosures for the gibbons and to construct an education center for visitors. She hopes to continue honoring Mootnick's legacy and dedicating her life to the gibbons — one of which is named after him.

“When he passed away, I just felt that he's around and helping us keep this center going,” she said.

This story was originally featured on Fortune.com.