Monkeys in Puerto Rico Became Kinder to Each Other After Hurricane Maria

Hard times can sometimes bring people together. For rhesus macaques, a destructive hurricane made their group an altogether friendlier place and helped increase individual survival year over year.

“It’s crazy — things have changed so much since the hurricane,” says Camille Testard, an ethologist at Harvard University. “The monkeys are less aggressive — they form these larger groups and interact with monkeys they’ve never interacted with.”

Rhesus macaques are native to Asia. But primatologist Clarence Carpenter introduced a colony of hundreds of them in Puerto Rico in the 1930s in an effort to study the creatures closer to his home. He set the colony up on Cayo Santiago, a small rocky isle off the east coast of the main island of Puerto Rico. Researchers have studied the colony off and on ever since.

Rhesus macaques typically live in large colonies with multiple males and females. These groups are hierarchical, and macaques on Cayo Santiago used to be quite aggressive to other members of their group. Individuals would even try to climb the pecking order to enforce their position. Social status determines overall access to food and other resources.

“They are really hierarchical,” Testard says.

When she began to study the macaques as part of her graduate studies, it was right after Hurricane Maria had cut a destructive path through Puerto Rico in 2017. Cayo Santiago saw some of the strongest winds on the island during the storm, and many trees were knocked down. Researchers monitoring macaque groups before Testard arrived noticed a change in their social behavior after the storm.

“They started associating with each other more and being nicer to each other,” she says. “We are seeing a change from a very aggressive hierarchical society to a less aggressive hierarchical society.”

Read More: Why Do Animals Fight — And What Do They Fight About?

This was especially notable in their use of shade. After the hurricane, there weren’t many trees left on Cayo Santiago, where daytime temperatures can exceed 100 degrees Fahrenheit in the sun. Since macaques don’t swim, shade is crucial.

“They are actually terrified of the water, so the only way they can lower their body temperature is by accessing shade,” Testard says.

With fewer trees, they had to clump together more to stay cooler. With these crowded conditions, the Rhesus macaques also had to improve their social tolerance. And this behavior didn’t just happen on hot days — they acted generally less aggressive and more tolerant even at night or when it wasn’t so hot.

Testard says this tolerance, even when there was no need for shade, was a good predictor of future access to shade when needed. Macaques that were nicer all the time were more likely not to be driven out of shady spots on the hot days.

Read More: Humans Aren’t the Only Primates Who “Share for Sharing’s Sake”

Long-term work on the study shows that this change in tolerance was linked to survival. Before the storm, monkeys died more often from injuries from fights with others. But after, heat stress was a much bigger cause of death. This was linked to individual behavior.

Before the hurricane, being less tolerant and more aggressive wasn’t related to a monkey’s chances of survival every year. But afterwards, things changed. Monkeys who tolerated 10 or more new partners in society had a 43 percent decrease in mortality risk every year.

“The rules of the game have changed,” Testard says.

This change has lasted for years — the study was based on observations through 2022, and Testard says that the same tolerance existed last year as well. She’s curious to see how long it lasts. She says that this study shows how macaques can adapt their relationships.

“It really highlights the important of social relationships in the context of climate change,” Testard says.

Read More: Yes, Animals Create Culture and Pass It Along for Survival

Our writers at Discovermagazine.com use peer-reviewed studies and high-quality sources for our articles, and our editors review for scientific accuracy and editorial standards. Review the sources used below for this article:

• Ethologist at Harvard University. Camille Testard

• American Journal of Primatology. A 75-year pictorial history of the Cayo Santiago rhesus monkey colony

Joshua Rapp Learn is an award-winning D.C.-based science writer. An expat Albertan, he contributes to a number of science publications like National Geographic, The New York Times, The Guardian, New Scientist, Hakai, and others.