Macaques have become much more tolerant since Hurricane Maria

Cayo Santiago is also called Monkey Island. The uninhabited small island in the Caribbean Sea belongs to Puerto Rico; the temperature there regularly exceeds 40 degrees.

Hurricane Maria destroyed more than half of the vegetation on Cayo Santiago in 2017, including many of the shade trees, as the team led by Camille Testard of the University of Pennsylvania reported in the journal Science. The tree population is still much smaller than before the hurricane.

Shade provides a survival advantage
In the heat, the shade of treetops is a very valuable, life-saving resource. And this is exactly where evolution came into play, as the researchers explain: While it wasn’t previously a problem if monkeys aggressively pushed for their shady spots, the more tolerant rhesus monkeys that share shady spots have a survival advantage – and the number of meaner congeners has decreased.

Competitive society
Rhesus monkeys (Macaca mulatta) belong to the macaque group and actually live in Asia, while wild groups also occur in Florida and Puerto Rico. Males in particular usually secure their status by hitting, biting and tearing their fur and tail. “They are known to live in an aggressive, highly competitive society,” said co-author Lauren Brent of Britain’s University of Exeter.

“Share space or die”
That’s why the monkeys aren’t actually very good at sharing resources, whether it’s food or shade. “But in the heat caused by ecological changes, often around 40 degrees, the macaques had to share space or die.”

According to the researchers, the acquired tolerance also has consequences for other areas of life. The monkeys, who liked to share shady spots, also spent time together in the morning, before the heat of the day, Testard explained. “The hurricane changed the rules of the game in the monkey community.”