A photo of two monkeys sitting on the ground with a baby monkey in between them.

Social Animals

When Hurricane Maria ripped apart their tiny island home, a colony of rhesus macaques became a study in trauma and social behavior that could have a bearing on humans

By Jenny Comita

“What doesn’t kill us makes us stronger” is a cliché that most of us rarely question. But is it actually true? Does overcoming adversity have the power to change us, and if so, how? Those are among the big questions scientists are now exploring on a tiny island off the coast of Puerto Rico, with help from a bunch of monkeys.

A little background: Cayo Santiago, as the small landmass is known, has a population of exactly zero humans and about 1,500 rhesus macaques. The fuzzy, long-tailed, two-foot-tall creatures—who top out at around 30 pounds—are actually native to Southeast Asia. But way back in 1938, primatologist Clarence Ray Carpenter chartered a boat in India, filled it with 500 of the petite primates, and plonked them down on this tropical isle in the name of science. About two decades later, biologist Stuart Altmann tattooed the monkeys for easy identification and started keeping a detailed, longitudinal census, thereby establishing Cayo Santiago as one of the world’s foremost field laboratories. For more than half a century, scientists from top research institutions—including NYU—have sailed the half-mile from the mainland to study the health and behavior of the animals in a naturalistic, free-ranging environment. And then, in September 2017, the island earned another, much less desirable, distinction: it was the first stop on Hurricane Maria’s devastating tear across Puerto Rico.

“The hurricane destroyed all of the research structures, flooded large portions of the island, and pretty much wiped out all of the vegetation,” says James Higham, an associate professor of biological anthropology who has worked on Cayo Santiago for more than a decade. Happily, when staff were able to get to the island, wading in from their boats because the docks had been swept away, they were pleasantly surprised to find that the monkeys had made it through. “There was some higher mortality in the couple of months afterwards relative to previous years, but generally the population of monkeys survived,” says Higham. “We don’t know what they did, whether they huddled or found whatever shelter they could. They’re very low to the ground relative to us, so that also makes them less exposed to the elements in some ways.” Still, just because the animals had lived didn’t mean they hadn’t been affected. The only environment they’d ever known had been turned upside down, and, says Higham, “they were outside throughout the whole of the storm, which was presumably extraordinarily traumatic.”

A photo of four monkeys surrounded by fallen trees and branches. One of the monkeys is holding a baby monkey.

Rhesus macaques sit among the dead, devastated vegetation after Hurricane Maria.

In the months that followed, crews worked to repair the island’s infrastructure, and, before long, scientists were able to get back to their research, this time with an additional—and timely—area of focus. Because they had so much data on the monkeys both before and after the storm—everything from preserved brains and bones to fecal and urine samples and copious behavioral logs—they could make some fascinating comparisons. How was the health and behavior of the macaques affected by living through the type of major natural disaster that, thanks to climate change, seems to be occurring with alarming frequency these days?

Some possible answers are starting to roll in. In a study published in Current Biology, Higham and his coauthors looked at the monkeys’ social relationships before and after the hurricane. Their findings: after weathering the storm, the monkeys were more tolerant of one another and formed new bonds. “In particular, it was individuals that were socially isolated before the hurricane who made efforts to increase their rate of social behavior toward others,” says Higham. “And they didn’t just target their existing social partners; they made new social connections.”

The research, of course, is about more than just primate popularity. Because they share about 93 percent of our DNA, the rhesus macaque is the number one nonhuman primate model for human health in the world. “More than 70 percent of all primate research for human health is done on macaques,” says Higham, who points out that the animals were used heavily in the development of the COVID vaccines. One major goal in looking at the monkeys’ social behavior, then, is to gain insight into our own—and, more and more, scientists are discovering just how important our social connectivity is to our physical well-being. “The number and quality of your relationships is one of the top predictors of your health,” says Higham. “Having a poor social environment is as high a health risk factor as smoking, and degree of social support is one of the top predictors of five-year cancer survival rates.” The interesting takeaway from this study, he adds, “is to see how flexible social behaviors in response to a natural disaster might help individuals buffer their stress and anxiety and might help with their resilience.”

As one measure of that resilience, Higham and his colleagues recently compared the molecular age—basically a measure of the rate at which the body is aging—of monkeys who lived through the storm with that of those who’d died before it hit. What they found is that hurricane survivors are potentially aging at a faster rate. The next logical step: measuring how social relationships affect the pace of that acceleration. “How does sociality get under our skin and become our biology?” says Higham. To put it in entirely unscientific terms, some clichés are apparently truer than others. What doesn’t kill us may actually make us older, not stronger, but we may get by with a little help from our friends.

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