Rhesus monkeys are among the most quarrelsome primates of all. This was no different on Cayo Santiago, also known as “Monkey Island”, an uninhabited island in the Caribbean – until a hurricane swept over it. Now, considerably more tolerant monkeys live there. Because, like a Research team now in the science magazine Science reportedevolution has weeded out the more aggressive animals there.
Cayo belongs to Puerto Rico, where temperatures regularly exceed 40 degrees Celsius. In 2017, Hurricane Maria destroyed more than half of the vegetation on Cayo Santiago, including many of the shade trees, as the team led by Camille Testard from the University of Pennsylvania in Philadelphia explains. The tree population is still far smaller than before the hurricane.
But in the heat, the shade of treetops is a valuable, life-saving resource. Previously, it was not a problem if monkeys aggressively insisted on their shady spots, but since “Maria” more tolerant rhesus monkeys who share shady spots have had a survival advantage. Through natural selection, the number of nastier members of the species decreased.
Intolerant monkeys were less able to cope with the changed conditions
The team examined ten years of data on the social behavior of the island monkeys. “Before the cyclone, tolerating others had no influence on the risk of death,” explained Testard. “After the cyclone, macaques that showed above-average social tolerance – and were therefore better able to share the shade – had a 42 percent lower risk of death than those that were less tolerant.”
Rhesus monkeys of the species 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 fur and tails. “They are known for living in an aggressive, highly competitive society,” said co-author Lauren Brent from the University of Exeter in the UK. This is why the 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 is also evident in other areas of life. The monkeys that share shady spots also spend 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 population of Cayo Santiago does not necessarily have to remain so peaceful when the treetop cover returns to its original state, the study says. Other factors such as more disease transmission between the more sociable members of the group could then lead to less social tolerance.