In 2017, Hurricane Maria became the largest catastrophe in the history of Puerto Rico, killing over 3,000 people, knocking out power to nearly all of the island and causing more than $100 billion in damages.

The hurricane also destroyed 63% of the vegetation on Cayo Santiago, a small island off the coast of Puerto Rico known as Monkey Island, which is home to about 1,800 rhesus macaque monkeys.

Remarkably, only 2.75% of the macaque population died in the immediate aftermath of the storm.

In a new study on the monkeys, led by scientists at Exeter University and the University of Pennsylvania, in collaboration with Arizona State University and New York University, researchers found that the storm damage has altered the evolutionary benefits of tolerating others and sharing shade — giving a survival advantage to more tolerant macaques.

The studyStudy funders included the National Institutes of Health, the European Research Council, the National Science Foundation and the Royal Society. has garnered the cover of the prestigious journal Science.

‘A Goldilocks research system’

Cayo Santiago has been home to an isolated, free-roaming population of rhesus macaques since 1938. The island serves as a natural laboratory with decades of data, which study co-author Noah Snyder-Mackler describes as “a Goldilocks research system — the perfect mix of the wild and the lab — with a complex social environment where we’re able to pair rich behavioral data with detailed physiological data.”

A 2022 study of monkeys on Cayo Santiago found that survivors of weather-related disasters may have accelerated aging, “with immune systems that looked like (they had) aged an extra two years,” Snyder-Mackler said. “What we are trying to do right now is look at what makes some of these individuals more resilient to the effects of the hurricane.”

Even now, five years after the catastrophe, the tree cover remains far below pre-hurricane levels and — in this hot part of the world — that makes shade a scarce and precious resource for the macaques.

Hurricane Maria destroyed 63% of the vegetation on Cayo Santiago, also known as Monkey Island. Without the tree cover, the monkeys learned to cooperate to share the precious shade. Image courtesy of Lauren Brant/University of Exeter

“In response to the drastic changes caused by the hurricane, macaques persistently increased tolerance and decreased aggression towards each other,” said lead author Camille Testard, from the University of Pennsylvania and now a junior fellow at Harvard.

“This enabled more macaques to access scarce shade, which is critical for survival.”

Testard added: “We examined 10 years of data on the strength and number of macaques’ social ties, before and after the hurricane. Before the hurricane, tolerating others had no impact on risk of death. Afterwards, macaques that displayed more than average social tolerance — and were therefore better able to share shade — were 42% less likely to die than those that were less tolerant.”

This represents a sudden change in “selection pressure” — the evolutionary benefits or costs of different traits or behaviors.

Changing ‘the rules of the game’

Social behavior was assessed by recording aggression and how often individuals were seen sitting together.

“Macaques aren’t the best at sharing resources — be they food or shade. They are well known to live in an aggressive, highly competitive society,” said Lauren Brent, senior author and professor from the University of Exeter. “But in the heat caused by ecological changes, often around 40 degrees Celsius (104F), the macaques had to share space or die.

“In effect, the hurricane changed the rules of the game in the monkeys’ society.”

What this study shows, Brent said, is that “for group-living animals, social relationships may allow them to cope with disturbances in the environment, including human-induced climate change.”

In the heat of the day, a group of macaques clings to a thin strip of shade in order to survive. Image courtesy of Lauren Brant/University of Exeter

“We were surprised the macaques’ social behavior was so flexible, making them resilient to this environmental change, but some species may not display this same flexibility,” Brent added.

Additionally, Testard said, “To access shade, they need to tolerate — and be tolerated by — others, and we found that this tolerance spills over into other daily interactions.”

Ultimately, the hope is that the lessons learned from the latest macaque study can also be used to help humans better adapt to weather-related and similar catastrophes.

“The people in Puerto Rico sort of gelled and increased their support of one another in the face of this event,” Snyder-Mackler said. “Given the strong similarity between these monkeys and us, we know that a lot of the work that we are doing and the things that the monkeys might do to be more resilient to this might be translatable to us and might provide ways for us to intervene and help buffer against the negative effects of these traumatic events.”

These monkeys were ‘notoriously competitive’ until Hurricane Maria wrecked their home

toggle caption Lauren Brent

Hurricane Maria devastated Puerto Rico in September 2017. But it also tore through a tiny island less than a mile offshore called Cayo Santiago. Although no people were living there, it was inhabited by hundreds of rhesus macaques.

These monkeys had 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 go and study their behavior, their genetics, their cognition,” says Lauren Brent, a behavioral ecologist at the University of Exeter. “It’s the source of most of what we know about this species.”

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On Cayo Santiago, an island so small it takes only about half an hour to walk its length, the rhesus macaques have a reputation for being intolerant, hierarchical and aggressive — “despotic and nepotistic” is how researchers describe them.

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

The macaques’ island life didn’t really change much over the years. But in September 2017, Hurricane Maria laid waste to their home. Now, in research published in the journal Science, Brent and her colleagues report that the devastation seems to have fundamentally altered the monkeys’ society.

A ravaged home

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

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

The island itself, however, was devastated. Almost two-thirds of the vegetation was destroyed. And this meant that the monkeys had way less shade in which to find relief from the sweltering 100-plus-degree heat. “You’re just exposed to the full onslaught of the sun,” says Brent. All that remained were “little puddles of shade.”

Camille Testard, a neuroscientist and behavioral ecologist who was in graduate school at the University of Pennsylvania at the time, remembers just how desperate the macaques were.

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

toggle caption Lauren Brent

But Testard and her colleagues in Puerto Rico noticed something else. Despite the limited shade, the macaques weren’t squabbling over it. They seemed to be more tolerant of one another.

Testing tolerance

That led Testard to compare the monkeys’ social interactions in the five years pre-hurricane with those in the five years post-hurricane. And she found that the macaques had become more likely to sit closer to one another in the puddles of shade and to do so in larger groups.

“So it’s not just that I sit next to my favorite monkey more,” she explains. “It’s that I sit next to a lot of new monkeys that I didn’t use to sit next to before.”

Testard observed that the animals hung out closer to one another at other times of day as well and not just when sharing shade.

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

Testard’s theory is that being aggressive makes a monkey hotter.

“What you’re trying to do is lower your body temperature as efficiently as possible,” she says. “Being aggressive — that really increases your body heat.” So playing it cool is a way of actually keeping cool.

Brent was struck that in the face of an altered habitat, the monkeys altered their social structure. “Yes, animals are using their social lives to cope with challenges,” she says. “But two, they’re flexible in how they go about that. They can change what their social networks look like.”

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In addition, the macaques that had more social partners on average — which meant more shade access — were 42% less likely to die. The mortality rates haven’t changed. Rather, “it’s what predicts their survival that has changed,” says Testard. “These partnerships, which help you access shade to lower your body temperature, [are] really key for these animals.”

Jorg Massen, an animal behaviorist at Utrecht University, who wasn’t involved in the study, says the research aligns with an emerging understanding of some primates’ social plasticity — up to a point.

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

Massen is curious to know about the mechanism that drove this change. “How is it that these normally quite intolerant macaques suddenly become so tolerant?” he wonders. “What’s the hormonal, maybe even genetic or epigenetic, underpinnings of that behavior?”

Climate change is transforming habitats the world over, challenging animal populations all over the globe.

“This need for rapid change is increasingly common with the increasing of natural disasters and other types of ecological changes,” Testard explains.

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

Since 1938, the island of Cayo Santiago in Puerto Rico has been home to a unique population of free-ranging rhesus macaques, creating a popular natural laboratory among researchers. “Monkey Island,” as it’s come to be known, has provided an unparalleled opportunity to study these primates in a semi-wild environment.

The devastation dealt by Hurricane Maria in 2017 significantly impacted Puerto Rico, destroying more than two-thirds of vegetation on Cayo Santiago. Even now, tree cover remains far below pre-hurricane levels, resulting in shade becoming scant and a precious resource for the macaques.

“Monkeys don’t sweat like humans do, so they need alternative strategies to try deal with the heat,” says University of Pennsylvania neuroscientist Michael Platt, who is part of a collaborative that has been studying the macaques on the Island for more than 17 years. “One of the biggest environmental changes is that a lot of the trees were killed so there’s much less leaf cover, and it’s about eight degrees hotter on Cayo Santiago, so these monkeys are approaching their physical limit at times.”

In a new study led by Platt and Lauren J.N. Brent of the University of Exeter and published in the journal Science, the team found that the storm damage altered the evolutionary benefits of tolerating others and sharing shade.

View large image The following comic is a collaboration between Camille Testard and Caroline Hu. Comic art by Hu, based on the research paper published by Testard and co-authors, “Ecological disturbance alters the adaptive benefits of social ties”, in the journal Science, on June 21, 2024. View large image View large image View large image Previous Slide Next Slide

“In response to the drastic changes caused by the hurricane, macaques persistently increased tolerance and decreased aggression towards each other,” says first author Camille Testard, a recent Ph.D. graduate from the Perelman School of Medicine at Penn. “This enabled more macaques to access scarce shade, which is critical for survival.”

On the ground

Platt, a Penn Integrates Knowledge University Professor, explains that the researchers were motivated by a finding reported in their previous work on the same macaque population: The animals became more accepting of one another and built new relationships despite scant resource availability. But this time, the team wanted to explore whether the macaques were simply passively engaging in these prosocial shade-sharing behaviors due to the fact they were being “squeezed” into limited space.

From 2013 to 2022, the researchers collected behavioral data across seven social groups encompassing 790 unique adult individuals. They constructed proximity and aggression networks for each group in each year to draw comparisons before and after the hurricane.

Additionally, they recorded ecological measurements, tracking the temperature differences between shaded and exposed areas. They found that shaded areas were consistently cooler, which was crucial for the macaques’ thermoregulation.

“We had our team on the ground watching the monkeys, documenting everything they did for 10 minutes and noting all the monkeys in close proximity,” Platt explains. “This provided a clear snapshot of social dynamics among the monkeys by telling us who they were hanging out with at different times of the day, when it’s hot out versus when it’s cooler.”

Cooler heads prevail

“Before Maria, tolerating others had no impact on risk of death,” Testard says. “Afterwards, macaques that displayed more than average social tolerance, and were therefore better able to share shade, were 42% less likely to die than those that were less tolerant.”

This represents a sudden change in “selection pressure,” the evolutionary benefits or costs of different traits or behaviors, the researchers explain. Social behavior was assessed by recording aggression and how often individuals were seen sitting together.

“To access shade, they need to tolerate and be tolerated by others, and we found that this tolerance spills over into other daily interactions,” says Testard. “Macaques that began sharing shade also spend time together in the mornings, before the day’s heat forces them to seek shade. In effect, the hurricane changed the rules of the game in the monkeys’ society.”

Brent, a former postdoctoral researcher in the Platt Labs, adds that “macaques aren’t the best at sharing resources—be that food or shade.” They are known for living in an aggressive, highly competitive society. But in the heat caused by ecological changes, often around 40°C (104°F), the macaques had to share space or die.

Platt says the hurricane’s impact was so severe that it effectively aged the monkeys rapidly, similar to aging eight years in human terms, as the extreme conditions turned on genes involved in inflammation and bodily wear, while genes responsible for repairing DNA were turned off. “What we see in this latest work is the physical toll made it crucial for the monkeys to adapt quickly to survive.”

“For group-living animals, social relationships may allow them to cope with disturbances in the environment, including human-induced climate change,” says Brent. “We were surprised the macaques’ social behavior was so flexible, making them resilient to this environmental change, but some species may not display this same flexibility.”

Michael Platt is a Penn Integrates Knowledge Professor and the James S. Riepe University Professor, professor of neuroscience, professor of psychology, and professor of marketing, with appointments in the Perelman School of Medicine, School of Arts & Sciences, and Wharton School at the University of Pennsylvania.

Camille Testard is a recent Ph.D. graduate in the Neuroscience Graduate Group in the Perelman School of Medicine and a former member of the Platt Labs at the University of Pennsylvania.

Lauren Brent is a professor in the Department of Psychology in the College of Life and Environmental Science at the University of Exeter and a former postdoctoral researcher in the Platt Labs at Penn,

This work was supported by the National Institutes of Health (Grants R01MH118203, U01MH121260, R01MH096875, P40OD012217, R01AG060931, R00AG051764, R56AG071023, R01AG084706, and R24-AG065172), European Research Council (Grant 864461 – FriendOrigins), the National Science Foundation (Grant 1800558), National Center for Research Resources (Grant 8-P40 OD012217-25), and the Royal Society (Grant RGS/R1/191182).