Monkey society changes after Hurricane Maria: NPR

A macaque sits on a rock at Cayo Santiago as a rainbow stretches across the sky in February 2022.

Lauren Brent


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Lauren Brent

Hurricane Maria devastated Puerto Rico in September 2017. But it also tore through a tiny island less than a mile off the coast called Cayo Santiago. Although it was devoid of people, it was home to hundreds of rhesus monkeys.

These monkeys have roamed the island since 1938, when an American primatologist brought their ancestors from India to create an experimental field site for studying these primates in the wild.

“It's a fantastic location to study their behavior, their genetics and their cognition,” he says Lauren Brenta behavioral ecologist at the University of Exeter. “It's the source of most of what we know about this species.”

On Cayo Santiago, an island so small that it takes only half an hour to walk across it, the rhesus macaques are known for being intolerant, hierarchical and aggressive. According to researchers, they are “despotic and nepotistic.”

“They're notoriously competitive,” Brent says. She imagines herself as one of the monkeys, then adds, “I form alliances with a small number of my group members, and we go after what we want against our other group members or against another group.”

The macaques' island life hasn't really changed over the years. But in September 2017, Hurricane Maria devastated their home. Now, in research published in the journal ScienceBrent and her colleagues report that the devastation appears to have fundamentally changed the apes' society.

A destroyed house

A few days after Hurricane Maria tore through the Caribbean, one of Brent’s colleagues shot video of the island from a helicopter. “It was the first time anyone had seen the devastation that had occurred on Cayo Santiago,” she says.

Most of the 1,800 monkeys had survived — somehow. “We don't know where they went,” Brent says, “or how they did it.”

The island itself, however, was devastated. Nearly two-thirds of the vegetation had been destroyed. And this meant that the monkeys had far less shade to find relief from the stifling 100-plus degree heat. “You're just exposed to the full onslaught of the sun,” says Brent. All that was left were “little puddles of shade.”

Camille Testarda neuroscientist and behavioral scientist then studying at the University of Pennsylvania, remembers how desperate the macaques were.

“You'd have scenes of one dead tree and you'd have the shadow behind it,” Testard says. “It's just this one line — and the monkeys would all be in that one line. You'd even have some animals following us in our shadow.”

On a patch of bare ground is the shadow of a long, narrow tree trunk. Macaques stand in a row on the shadow. In the background is blue water and other islands.

Macaques gather in the shade of a dead tree in March 2022.

Lauren Brent


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Lauren Brent

But Testard and her colleagues in Puerto Rico noticed something else. Despite the limited shade, the macaques didn't fight over it. They seemed more tolerant of each other.

Tolerance testing

That led Testard to compare the monkeys’ social interactions in the five years before the hurricane with those in the five years after the hurricane. And she found that the macaques increasingly huddled closer together in the shaded pools and that they did so in larger groups.

“So it's not just that I'm sitting next to my favorite monkey more,” she explains. “It's that I'm sitting next to a lot of new monkeys that I wasn't sitting next to before.”

Testard noted that the animals also huddled closer together at other times of the day, not just when they were in the shade.

The big surprise was that the macaques' overall aggression levels dropped. “It's the complete opposite of what we thought this primate would do,” Brent says.

Testard's theory is that a monkey becomes more attractive when it is aggressive.

“What you're trying to do is get your body temperature down as efficiently as possible,” she says. “Being aggressive — that really raises your body heat.” So playing it cool is a way to really stay cool.

Brent was surprised that the monkeys changed their social structure in the face of a changed habitat. “Yes, animals use their social lives to deal with challenges,” she says. “But secondly, they are flexible in how they deal with that. They can change what their social networks look like.”

What’s more, the macaques that had more social partners on average — meaning they had more access to shade — were 42 percent less likely to die. Mortality rates didn’t change. Instead, “the thing that predicts their survival has changed,” Testard says. “These partnerships, which help you get access to shade to lower your body temperature, [are] really crucial for these animals.”

Jorg Massen, an animal behaviorist at Utrecht University who was not involved in the study, says the research fits in with a growing understanding of the social plasticity of some primates — to a certain extent.

“It's not endless, such flexibility, of course,” he argues. “There is some flexibility, but we shouldn't overestimate it.”

Massen is curious about the mechanism driving this change. “How can these normally quite intolerant macaques suddenly become so tolerant?” he wonders. “What is the hormonal, perhaps even genetic or epigenetic, basis for that behavior?”

Climate change is altering habitats around the world, posing challenges to animal populations.

“This need for rapid change is becoming increasingly common as natural disasters and other forms of ecological change increase,” Testard explains.

And she says that the macaques – through their social flexibility – show us one way that some species try to adapt.

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