Animals Monkeys on an island have become more peaceful after a hurricane

Monkeys on an island have become more peaceful after a hurricane. How did this happen? Evolution sifted out aggressive animals for a very specific reason, as a research team reports in the journal "Science".

Rhesus monkeys are among the most belligerent primates of all. This was no different on Cayo Santiago before Hurricane Maria in 2017. 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.

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

Life-saving shade

In the heat, the shade provided by tree canopies is a very valuable, life-saving resource. And this is precisely where evolution came into play, as the researchers explain: While it was previously 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.

The team examined a total of ten years of data on the social behavior of the island monkeys. "Before the hurricane, tolerating others had no effect on mortality risk," explained Testard. "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."

True despots

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.

As a result, 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."

A completely new society

According to the researchers, the increased tolerance also affects other areas of life. The monkeys, who willingly shared shady spots, 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."

However, the Cayo Santiago population would not necessarily remain as peaceful if the canopy cover returned to its original state, the study also states. Other factors such as more disease transmission between the more sociable members of the group could then steer evolution in the other direction, i.e. towards less social tolerance.

SDA