Less monkey business

Hurricane results in friendlier macaques

However, the population of Cayo Santiago does not necessarily have to remain so peaceful if the canopy cover returns to its original state ...(Bild: Iurii - stock.adobe.com)

Rhesus macaques are among the most belligerent primates. Even on Cayo Santiago, until a hurricane hit the Caribbean island. Now much more tolerant monkeys live there. How did this come about? Evolution has weeded out the aggressive animals for a reason.

Cayo Santiago is also known as Monkey Island. The uninhabited small island in the Caribbean Sea belongs to Puerto Rico, where temperatures regularly exceed 40 degrees.

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

Shade ensures survival advantage
In the heat, the shade provided by tree crowns is a very valuable, life-saving resource. And this is precisely where evolution came into play, as the researchers explain: Whereas previously it was not a problem when monkeys aggressively insisted on their shady spots, since "Maria" more tolerant rhesus monkeys sharing shady spots have had a survival advantage - and the number of nastier conspecifics decreased.

About the study

The team examined a total of ten years of data on the social behavior of the island monkeys. The result: before the hurricane, tolerating others had no influence on the mortality risk. After the hurricane, macaques that showed above-average social tolerance - and were therefore better able to share the shade - had a 42 percent lower mortality risk than those that were less tolerant.

Competitive society
Rhesus monkeys (Macaca mulatta) belong to the macaque group and actually live in Asia, with feral groups also living in Florida and Puerto Rico. The males in particular usually secure their status by hitting, biting and tearing at their fur and tail. "They are known to live in an aggressive, highly competitive society," said co-author Lauren Brent from the British University of Exeter.

Mothers of rhesus monkeys treat their male offspring more aggressively than their daughters in the first year of life.(Bild: APA/dpa, Krone KREATIV)

"Share space or die"
 This is why monkeys are not particularly good at sharing resources, be it food or shade. "But in the heat caused by the ecological changes, which is often around 40 degrees, the macaques had to share space or die."

According to the researchers, the increased tolerance also affects other areas of life. The monkeys, who willingly shared shaded areas, also spent time together in the morning, before the heat of the day, explained Testard. "The hurricane has changed the rules of the game in monkey society."