Hurricane Maria caused widespread destruction in the Caribbean, harming people and wildlife alike, and five years after the storm, its effects are still being felt.
Case in point: Cayo Santiago, a tiny island off the southeast coast of Puerto Rico that was transformed almost overnight from a lush jungle oasis into a desert-like sandbar with mostly skeletal trees.
This posed a major problem for the island's macaque monkeys, who rely on shade to cool off in the tropical midday heat, but with all the trees destroyed by the storm, this resource was in severe short supply.
Rhesus macaques are known as the most combative primates on Earth, maintaining strict social hierarchies through aggression and competition, so it's no surprise that ape-to-ape battle royals would break out for the few remaining patches of shade on the island.
But that wasn't the case. Instead, the macaques did something seemingly inexplicable: they started to get along.
“This was totally unexpected,” says Camille Testard, a behavioral ecologist and neuroscientist at Harvard University. “Far from becoming more competitive, individuals expanded their social networks and became less aggressive.”
A paper published Thursday in Science by Testard and his colleagues describes this unexpected turn of events: They found that monkeys who learned to share shade after a storm were more likely to survive than those who continued to fight.
Scientists have documented many examples of species responding to environmental pressures with physiological or morphological adaptations, but this new study is one of the first to suggest that animals can also respond by permanently changing their social behavior, Testard said.
She and her colleagues drew on nearly 12 years of data collected at the Cayo Santiago Field Station, the world's longest-running primatological field facility. Researchers introduced rhesus macaques to the 38-acre island in 1938 and have been studying them ever since.
The island's roughly 1,000 macaques are free-roaming but are fed by staff at the field station. “Access to food is not the main issue,” says Dr Testard. “It's about shade to avoid heatstroke.”
Daytime temperatures on Cayo Santiago often exceed 100 degrees Fahrenheit, which can be fatal for monkeys left out in the sun.
After Hurricane Maria flattened most of the trees on the island, Testard and her colleagues expected that the macaques would put more effort into forming close alliances so that they could work together to secure shade. But the result was “quite the opposite,” she says: Instead, the macaques formed looser partnerships with more animals and became more tolerant of each other overall.
Dr Testard said he thought this was because fighting was an energy-intensive activity that generated more body heat and was therefore more dangerous to individuals than “monkeys who didn't really care if another monkey was next to them”.
During the hottest parts of the afternoon, the researchers observed the macaques clustering together in narrow strips of shade, but even when temperatures weren't as hot, the animals were still gathering in larger groups than they had before the storm, Testard said.
Not all monkeys took the peaceful route, but those who persisted in their aggressive behavior were likely to pay a heavy price: Overall monkey mortality rate did not change after the hurricane, but monkeys who formed more friendly relationships, and therefore were less likely to suffer heat stress, experienced a 42 percent reduction in mortality.
“It's changed who dies and why,” Dr. Testard said.
Noah Pinter-Wolman, a behavioral ecologist at the University of California, Los Angeles, who was not involved in the study, said the “intriguing” findings were “a great example of how social behavior can buffer the negative effects of environmental change.”
Julia Fischer, a behavioral biologist at the German Primate Center in Göttingen, who was also not involved in the study, added that the “very well-done study” highlights how important behavioral plasticity is for animals to survive when their habitats are upended. “In light of climate change, this is crucial,” she said.
Whether other animals can adjust their social norms to cope with environmental upheaval “depends a lot on the species and the situation,” Testard said, but humans probably fall into that category: People often band together after natural or man-made disasters, for example.
But Testard added that there are limits: if resources become too scarce, humanity could slip into a Mad Max-style dystopia of violent competition. “There's hope that instead of fighting, we can band together and make things work,” he said. “But that's a big guess.”