Less monkey business
Hurricane results in friendlier macaques
A baby rhesus monkey is in its mother’s arms. Photo
© Md Rafayat Haque Khan/ZUMA Press Wire/dpa
Evolution can take strange paths. On one island, it caused monkeys to become more peaceful after a hurricane.
Rhesus monkeys are among the most belligerent primates. This was no different on Cayo Santiago – until a Hurricane swept over the Caribbean island. Much more tolerant monkeys now live there. How did this happen? Evolution weeded out aggressive animals for a very specific reason, as a research team reports in the journal “Science”.
Cayo Santiago is also known as Monkey Island. The uninhabited small island in the Caribbean Sea belongs to Puerto Rico, and temperatures there regularly exceed 40 degrees. In 2017, Hurricane Maria destroyed more than half of the vegetation on Cayo Santiago, including many of the shade-giving trees, according to a team led by Camille Testard from the University of Pennsylvania in the US city of Philadelphia. The tree population is still far smaller than before the hurricane.
Life-saving shadow
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 was previously no problem for monkeys to aggressively insist on their shady spots, since “Maria” more tolerant rhesus monkeys that share shady spots have had a survival advantage – and the number of nastier members of their species has decreased.
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.”
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 for living in an aggressive, highly competitive society,” said co-author Lauren Brent from the University of Exeter in the UK.
That’s why the monkeys aren’t 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 the space or die.”
A completely new society – but not necessarily forever
According to the researchers, the increased tolerance also affects other areas of life. The monkeys, who willingly 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 study also states that the population of Cayo Santiago will not necessarily remain so peaceful if the tree cover returns to its original state. Other factors, such as increased disease transmission between the more sociable members of the group, could then steer evolution in the other direction, towards less social tolerance.