Hurricane Maria caused widespread devastation in the Caribbean, not only for people but also for wildlife. Five years after the storm, some of the effects still linger.
Cayo Santiago, a small island off the southeastern coast of Puerto Rico, is a prime example. It transformed almost overnight from a lush jungle oasis to a desert-like spit of sand with mostly skeletal trees.
This posed a big problem for the island’s resident macaques. The monkeys depend on shade to keep cool in tropical daytime heat, but, by wiping out the trees, the storm had rendered that resource in very short supply.
Rhesus macaques are known for being some of the quarrelsome primates on the planet, with strict social hierarchies maintained through aggression and competition. So it would follow that a simian battle royale would break out over the island’s few remaining patches of shade.
Yet that’s not what happened. Instead, the macaques did something seemingly inexplicable: They started getting along.
“This was really not what we expected,” said Camille Testard, a behavioral ecologist and neuroscientist at Harvard University. “Instead of becoming more competitive, individuals widened their social network and became less aggressive.”
A paper by Dr. Testard and her colleagues, published on Thursday in the journal Science, offers an explanation for this unexpected development. Monkeys who learned to share shade after the storm, they found, had a better chance of survival than those that remained quarrelsome.
Scientists have documented numerous cases of species responding to environmental pressure with physiological or morphological adaptations. But the new study is one of the first to suggest that animals can also respond with persistent changes to their social behavior, Dr. Testard said.
She and her colleagues took advantage of around 12 years of data collected at the Cayo Santiago Field Station, the world’s longest-running primatology field site. Researchers introduced rhesus macaques to the 38-acre island in 1938 and have been studying them ever since.
The approximately 1,000 macaques that live on the island are free-ranging but are fed by the field station staff members. “Access to food is not the main point of contention,” Dr. Testard said. “Shade to avoid heat stress is.”
Daytime temperatures on Cayo Santiago often soar above 100 degrees Fahrenheit, or about 38 Celsius, which can be deadly for monkeys stranded in the sun.
After Hurricane Maria took out most of the island’s trees, Dr. Testard and her colleagues expected that macaques might invest more in building close alliances so they could join forces to secure shade. But the “complete opposite” happened, she said. Monkeys instead invested in looser partnerships with a larger number of animals, and they became more tolerant of each other overall.
Dr. Testard said she suspected that this was because fighting is an energy-intensive activity that generates more body heat and poses more danger to individuals than “just caring less if another monkey is next to me or not.”
During the most sweltering hours of the afternoon, the researchers observed macaques crowded together in thin strips of shade. But even when temperatures were less stifling, the animals gathered in larger groups compared with their habits before the storm, Dr. Testard said.
Not all the monkeys jumped on the peace train, but those who adhered to aggression were more likely to pay a steep price. The macaque population’s overall death rate did not change after the hurricane. But monkeys that had more friendly relations experienced a 42 percent decrease in their odds of mortality because they were less likely to suffer heat stress.
“Who dies and for what reason is what has changed,” Dr. Testard said.
Noa Pinter-Wollman, a behavioral ecologist at the University of California, Los Angeles, who was not involved in the research, said that the “fascinating” findings were “a wonderful example of how being social can buffer negative effects of environmental change.”
Julia Fischer, a behavioral biologist at the German Primate Center in Göttingen, who also was not involved in the work, added that the “extremely well-done study” highlighted the importance of behavioral plasticity in helping animals survive when their habitat is upended. “In light of climate change, this is extremely important,” she said.
Whether other animals can also respond to environmental upheaval by adjusting their social norms “is going to be very species- and context-dependent,” Dr. Testard said. Humans probably fall into that category, though. People often band together, for example, after natural and human-caused disasters.
However, Dr. Testard added, there are limits. If resources become too scarce, then humans could descend into a Mad Max-like dystopia of violent competition. “There is hope that we would band together to make things work rather than fight,” she said. “But that’s a big speculation.”