It doesn’t look like a zoo. Indeed, on Cayo Santiago, a 38-acre tropical island off Puerto Rico’s coast, the only mammals in cages are human beings. Edmundo Kraiselburd, the affable director of the Caribbean Primate Research Center here, quickly scoots behind bars to check a few messages on his Blackberry.

In this free-ranging monkey colony with a population of 1,022 rhesus macaque monkeys, it’s the monkeys who are kings (and queens). Visitors to the island must be tested for tuberculosis before arrival. “We know the monkeys are cute, but don’t make eye contact with them,” sternly warns colony manager James Ayala. “I’m being serious.”

Cayo Santiago staff are busy trapping monkeys to obtain scent and DNA to determine paternity lines, marking each monkey for identification purposes. Everything has to be done before hurricane season in June. Adaris Mas, the Research Center’s first Puerto Rican resident scientist, points out the different monkey groupings. Some monkeys are grooming themselves, while others patiently groom each other. They are indeed cute.

Before ducking into his protective cage-office, Kraiselburd tells his visitor, “The whole concept of National Research primate centers came from Cayo Santiago. The subject of sociobiology got its start here also with E.O. Wilson.”

Kraiselburd, a virologist by training, explains that the first monkeys were brought to Cayo Santiago from India in 1938. All the monkeys on the island are descended from the original 409 monkeys, providing a specialized gene pool. Supported by the National Institutes of Health and administered by the Unit of Comparative Medicine at the University of Puerto Rico, the island is a researchers’ paradise.

The latest “hot theme” to be studied is stress, according to Ayala. “The question is whether dominant animals are more stressed than others,” he observes. “You can get information from a naturalistic setting that you can’t elsewhere. The environment in a lab is stressful for the animals in and of itself. That’s what makes this place unique.”

Research on the island, according to Kraiselburd, has resulted in the malaria vaccine, the Hulka Clip that controls reproduction, and tetanus advances, not to mention discoveries in the fields of psychology and sociobiology. The research on the island is multidisciplinary, ranging from cognition and communication to morphology and physiology.

The island now confronts two challenges. One is overpopulation. Monkeys on the island don’t face hungry lions or tigers, and they have ready access to food. This means that the survival rate is considerably greater than in the jungle. The challenge is how to thin the monkey population through sales and donations without upsetting the balance among the groups of monkeys.

The second challenge is a human one. The island is less than a mile off Puerto Rico’s coast; Kraiselburd is initiating projects to integrate the Research Center with the community, a low-income village of fishermen. Even though tourists are not permitted on the island, a monkey-themed museum and library on the mainland could attract visitors. He’s currently involved in a project to improve community schools.

“It’s all about giving back,” says Kraiselburd.

A large rhesus emerges seemingly from nowhere, ignoring the visitors. The sky is as blue as it could possibly be. An iguana slinks by, his green skin bending with the lush vegetation.

“When E.O. Wilson was here recently to participate in a documentary, we tried to find him some ants, because we know he’s interested in ants,” comments Ayala, reminded by the iguana of the island’s spectular biodiversity. “But we couldn’t get enough ants in one place, so we found him a termite mound.”

Try doing that in a zoo.

June Carolyn Erlick is the editor-in-chief of ReVista. She is the author of Disappeared: A Journalist Silenced (Seal Press 2004) and Una gringa en Bogotá (Aguilar, 2007).