Macaques in Puerto Rico learned to share shade after Hurricane Maria

After Hurricane Maria battered Puerto Rico in 2017, monkeys living there forged new bonds to share a suddenly scarce resource: shade.

Rhesus macaques (Macaca mulatta) that were willing to hang out with others boosted their chance of survival in the storm’s aftermath, researchers report July 24 at bioRxiv.org. That newfound sociability may have allowed multiple animals to escape the scorching heat of the day beneath any trees left standing, and any other source of shade they could find.

Amid Hurricane Maria’s heavy rain and furious winds — coming just two weeks after Hurricane Irma dumped a heavy deluge of rain — trees and other plants across Puerto Rico toppled. Nearly a quarter of Puerto Rico’s total forest biomass was demolished (SN: 3/17/20). Cayo Santiago, a once lush key located off Puerto Rico’s coast, was left largely barren after it lost nearly two-thirds of its vegetation.

More than five years later, Cayo Santiago’s flora hasn’t recovered, says Camille Testard, a behavioral ecologist and neuroscientist at the University of Pennsylvania. As of April 2023, the tiny island hosted fewer than 600 living trees.

But a colony of around 1,600 macaques, managed by the University of Puerto Rico’s Caribbean Primate Research Center, also calls Cayo Santiago home. Without much remaining tree cover, the destructive hurricane left many monkeys searching for shady relief from temperatures that regularly exceed 40° Celsius (104° Fahrenheit). Some of that shade now comes in smaller forms: from boulders, water basins or even human shadows.

“If you’re collecting data on the island, you’re gonna have monkeys that are sitting in your shade and following you around,” Testard says.    

Before Hurricane Maria, the macaques, typically hierarchal animals, could often be found fighting over food, status or mates. But immediately after the storm, the macaques expanded their social network to form new relationships, Testard and colleagues reported in 2021. While monkey fights still happen, they aren’t as common as before. These new friendships meant more access to shade in tight places, the team hypothesized. But it lacked the data to back it up.   

In the new study, the researchers examined interactions within groups of monkeys over a 10-year span, five years before and five years after the hurricane. During that decade, the team analyzed monkey groups in 10-minute increments from 6:00 a.m. to 2:30 p.m. (One year, 2020, was excluded because of the COVID-19 pandemic.)

After Hurricane Maria, from 2018 to 2022, the animals spent more time in close company in the heat of the afternoon compared with relatively cooler mornings, the team found. The recorded interactions don’t specifically note whether the monkeys were sitting in shade or sun, says coauthor Lauren Brent, a behavioral ecologist at the University of Exeter in England. But more social monkeys were less likely to die in the five years after the storm, which suggests that afternoon gatherings took place in the shade, so the animals might cool down.

The findings are a “wonderful” example showing how behavior can be an important factor for survival as environments change, says Richard Buchholz, a behavioral ecologist at the University of Mississippi in Oxford. Because researchers on Cayo Santiago ensure macaques have food and water, it’s unclear whether the same might happen in a group of wild monkeys, he says. But “it’s exciting that [the researchers have] shown this impact on mortality.”

It’s possible that the boost in sociality comes not from a benefit of being tolerant of their primate peers after a natural disaster, but because there’s a cost to being intolerant, Buchholz says. “We tend to get lethargic when things get superhot.” The animals may be calmly sitting together in the shade so as not to raise their metabolisms, “avoiding the metabolic cost of chasing after somebody and building up more heat that you’ll have to get rid of somehow.”   

There could also be other consequences to monkey gatherings. In a separate study, posted July 19 to bioRxiv.org, Testard, Brent and colleagues used computer simulations to show that becoming more social may increase the risk that infectious diseases will spread.

“So even if you’re really flexible, and you’re able … to adapt to this new environment, the way that you have to adapt actually makes you more vulnerable to other external shocks to the system,” Testard says. “I think that needs to be kept in mind when we think about the effects of natural disasters on wildlife health.”