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On the island of Cayo Santiago, about a mile off the coast of eastern Puerto Rico, the typical relationship between humans and other primates gets turned on its head. The 1,700 rhesus macaque monkeys (Macaca mulatta) living on that island have free rein to move around wherever and whenever they please. Humans don’t.
The Cayo macaque population came to be in 1938 when primatologist and explorer Clarence Carpenter captured about 500 rhesus macaques in India. He and the monkeys sailed from Calcutta via Boston and New York to San Juan, and from there to Cayo Santiago, a 15-hectare island that had recently been leased to the School of Tropical Medicine at the University of Puerto Rico.
Lauren Brent began working with macaques on Cayo Santiago when her Ph.D. adviser suggested she consider going down to the island to have a look at the social dynamics of the monkeys. She immediately fell in love with everything about her study subjects, the island, and the questions she was soon addressing. “Cayo is a pretty perfect study system,” Brent says, “and the macaques are such highly social little dudes.”
The more friends of friends a female has, the more offspring she produces.
The macaques on Cayo Santiago can be a handful to work with, in part because of the large size of their social groups. The monkeys are often moving, and even when not, the tattoos and ear-notch marks they have been tagged with may be hidden by the bush or obscured by the angle between monkey and researcher. What that means is that Brent and others working with macaques not only have to maintain records on every monkey’s fur color and pattern, eye features, and other traits that can help distinguish the island’s residents from one another, but they must also memorize that information and be able to make split-second identifications among a group that can number from a few dozen to more than 100 individuals.
One of Brent’s early studies focused on females in two groups of macaques, one of which had 58 females and the other about 20. She’d select a macaque from her list, find it, follow it, record what it was doing, and then select another monkey and repeat the process. Macaques groom one another often, which serves not only to remove parasites from the recipient but to lower stress levels in all parties. Brent was especially interested in grooming networks in females, as well as networks based on the proximity of individuals when they were not grooming. She wanted to see what the networks looked like and whether they changed over time.
When Brent and her team looked at the data from 2010 to 2017, they found that females who had strong friendships (tight connections) with their favored partners in a network had higher survival probabilities than other females. Another way for a female to increase her chances of survival was through weaker connections but with lots of grooming partners. It wasn’t only the number of friends or the strength of specific favored friendships: Friends of friends also matter to female macaques. The more friends of friends a female has, the more offspring she produces.
By 2017, Brent and her colleagues—including research assistants like Daniel Phillips, who was on Cayo Santiago every day—had a deep understanding of the way that the social networks of macaques on Cayo worked and what they meant. There was, of course, always more to learn, but much of that would likely involve filling in the details.
Then Hurricane Maria struck, devastating everything in its path, including the social networks the macaques were rooted in.
When Hurricane Maria hit Cayo full force in September 2017, Brent’s first thoughts were of the staff and research assistants, like Phillips, who live there year-round. “[I] couldn’t get in touch because all the cell towers were down. … No one heard from Danny for two, maybe three weeks after Hurricane Maria. … It was pretty horrible.” Eventually though, she learned that Phillips and everyone on the project were safe. Then her thoughts turned to the macaques and the island itself. “When we saw the satellite tracker … I just thought, ‘This field site is gone; these monkeys are all dead. … It’s a Category 4 hurricane, and these animals were just sitting on the ground on this little chunk of rock in the middle of the ocean.’”
They weren’t all dead, but Hurricane Maria turned the monkeys’ world upside down and meant they needed to reestablish their society, with all its intricate and complex working parts. And quickly. For Brent and her team, Hurricane Maria meant many things, including figuring out new ways to think about the effects of large-scale natural disasters on social networks.
What happened to the networks of the highly social little macaque dudes and dudettes?
More than 60 percent of the green vegetation on Cayo Santiago, along with a lot of the infrastructure that the Cayo team had put in place—including feeders that supplied the animals with some of their food, which they now needed more than ever—was decimated by Hurricane Maria. But not a single macaque was killed during the hurricane, and only about 2 percent of the animals died shortly thereafter, probably due to starvation.
Hurricane Maria had fundamentally altered the social networks of the monkeys.
“It’s completely incredible,” Brent says. “They’re not that big, right? And all their trees were being blown over. It’s not like you can hold on to something.” At first Brent thought that maybe the macaques hid in a place known as Happy Valley, which is partially protected from wind, but normally Happy Valley holds 50 macaques, and there was no way it could have provided shelter for 1,700 monkeys. She and her team are working hard to piece together what happened, but still have not been able to figure out how every single macaque survived the full brunt of a Category 4 hurricane.
About three months after Maria, Brent began thinking seriously again about the dynamics of macaque social networks, primarily because of what she was hearing from Phillips and the other field assistants who were back on the island. They’d tell her, “the monkeys are acting weird,” and when Brent would ask how so, the on-the-ground team told her that they seemed to be especially friendly toward one another. In early 2018, about five months after the hurricane, she went down to check for herself. The island was still reeling from Maria (as was most of Puerto Rico). “But when I got to Cayo,” Brent says, “I was like ‘Yeah, I see it.’ … They look more tolerant. … Monkeys I never expected to be cool just sitting next to each other.”
At the time, very little was known about how animals adjusted their social dynamics after full-scale natural catastrophes, and as awful as the consequences of Hurricane Maria were for Puerto Rico, perhaps, Brent thought, the disaster shed light on those dynamics. When she and her team looked more deeply at the suddenly much more friendly macaques, what they found was that Maria had fundamentally altered the social networks of the monkeys.
For one, when they compared the grooming and proximity networks in two groups of macaques during the three years prior to Maria versus the one year immediately after the hurricane, the data confirmed the anecdotal observations about the monkeys being nicer to each other: Macaques were four times as likely to be found close to one another after the hurricane, and they were 50 percent more likely to groom one another. What’s more, monkeys who groomed least and had spent the least time near others before Maria were the ones who showed the greatest increase in these behaviors after the hurricane.
Focusing their analysis on grooming behavior, Brent and her colleagues thought that the changes in network structure might be due to either an increase in the number of partners or an increase in time spent with specific partners. Or perhaps a bit of each. What they found was that post-Maria, macaques had more social partners, but the average strength of a grooming relationship with a partner had not changed. The monkeys had formed more friendships in their network, not strengthened already existing ones: Maria had brought the macaques in a group closer together, with additional grooming partners buffering them from the devastating effects that the hurricane left in its trail.
And again, friends of friends mattered: Monkeys took the path of least resistance in forming new relationships. If monkey 1 had been in a grooming relationship with monkey 2 before Maria, it was more likely to enter a grooming relationship with one of monkey 2’s grooming partners after the storm. Disaster, in the form of Maria, had brought the monkeys closer to one another, and social network analysis showed how. 
Read Lee Alan Dugatkin’s “3 Greatest Revelations” while writing his new book here.
Reprinted with permission from The Well-Connected Animal: Social Networks and the Wondrous Complexity of Animal Societies by Lee Alan Dugatkin, published by The University of Chicago Press. © 2024 by Lee Alan Dugatkin. All rights reserved.
Lead image: Dmitry Strizhakov / Shutterstock
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