How Hurricane Maria transformed a society of macaques

MARY LOUISE KELLY, HOST:

We remember Hurricane Maria for the devastation it wrought on Puerto Rico in September 2017. But it also tore through a tiny island less than a mile offshore called Cayo Santiago. Although no people were living there, it was inhabited by hundreds of monkeys. The hurricane laid waste to their home, and in doing so, it seems to have fundamentally changed their society. Here's science reporter Ari Daniel.

ARI DANIEL, BYLINE: The rhesus macaques of Cayo Santiago have a reputation for being intolerant and aggressive.

LAUREN BRENT: They're notoriously competitive in the way of, I form alliances with, you know, a small number of members of my group, and we go after what we want.

DANIEL: Lauren Brent is a behavioral ecologist at the University of Exeter in the UK. She says the macaques were brought to this little tropical island in the 1930s to create an experimental field site.

BRENT: It's the source of most of what we know about the species. And unfortunately, since it is based in Puerto Rico, it's hit by hurricanes on occasion.

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DANIEL: A few days after Maria ripped through Cayo Santiago, one of Brent's colleagues recorded this video of the island from a helicopter. Most of the 1,800 monkeys had survived somehow, but the island was devastated.

BRENT: And then you see the damage to the trees.

DANIEL: Almost two-thirds of the vegetation was destroyed, which meant the monkeys had way less shade to find relief from the sweltering, hundred-plus-degree temperatures. Camille Testard remembers just how desperate the macaques were. She's a behavioral ecologist and was in grad school at the University of Pennsylvania at the time.

CAMILLE TESTARD: You'd have scenes of, like, you know, one dead tree, and you'd have the shade behind it. It's just this one line, and the monkeys would all line up. You'd even have some animals following us in our shade.

DANIEL: But Testard and her colleagues noticed something else. Despite the limited shade, the macaques weren't squabbling over it. So Testard compared their social interactions five years pre-hurricane to five years post, and she found that the macaques had become more likely to sit closer to one another in the puddles of shade and to do so in larger groups.

TESTARD: So it's not just that I sit next to my favorite monkey more. It's that I sit next to a lot of new monkeys that I didn't use to sit next to before.

DANIEL: This behavior occurred at other times of the day as well. In addition - and this was a big surprise - overall aggression levels dropped. Testard's got a theory.

TESTARD: What you're trying to do is lower your body temperature as efficiently as possible. Being aggressive - that really increases your body heat.

DANIEL: So playing it cool is a way of keeping cool. Lauren Brent was struck that, in the face of an altered habitat, the monkeys alter their social structure.

BRENT: Not only that, yes, animals are using their social lives to cope with challenges, but, two, they're flexible in how they go about that. They can change what their social networks look like.

DANIEL: And get this. The macaques who had more social partners on average, which meant more shade access - they were 42% less likely to die. Brent, Testard - who's now at Harvard University - and their colleagues published the results in the journal Science.

JORG MASSEN: So I would have liked some speculations about the mechanism behind this. How is it that these normally quite intolerant macaques suddenly become so tolerant?

DANIEL: Jorg Massen is an animal behaviorist at Utrecht University who wasn't involved in the study. He says the research aligns with an emerging understanding of some primates' social plasticity. And given that climate change is transforming habitats the world over, Camille Testard says the macaques show us one way that some species might try to adapt, at least initially.

TESTARD: So this need for rapid change is increasingly common with the increasing of natural disasters and other types of ecological changes.

DANIEL: Making, in certain cases, shady social interactions the safest.

For NPR News, I'm Ari Daniel.

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