Since 1938, the island of Cayo Santiago in Puerto Rico has been home to a unique population of free-ranging rhesus macaques, creating a popular natural laboratory among researchers. “Monkey Island,” as it’s come to be known, has provided an unparalleled opportunity to study these primates in a semi-wild environment.
The devastation dealt by Hurricane Maria in 2017 significantly impacted Puerto Rico, destroying more than two-thirds of vegetation on Cayo Santiago. Even now, tree cover remains far below pre-hurricane levels, resulting in shade becoming scant and a precious resource for the macaques.
“Monkeys don’t sweat like humans do, so they need alternative strategies to try deal with the heat,” says University of Pennsylvania neuroscientist Michael Platt, who is part of a collaborative that has been studying the macaques on the Island for more than 17 years. “One of the biggest environmental changes is that a lot of the trees were killed so there’s much less leaf cover, and it’s about eight degrees hotter on Cayo Santiago, so these monkeys are approaching their physical limit at times.”
In a new study led by Platt and Lauren J.N. Brent of the University of Exeter and published in the journal Science, the team found that the storm damage altered the evolutionary benefits of tolerating others and sharing shade.
“In response to the drastic changes caused by the hurricane, macaques persistently increased tolerance and decreased aggression towards each other,” says first author Camille Testard, a recent Ph.D. graduate from the Perelman School of Medicine at Penn. “This enabled more macaques to access scarce shade, which is critical for survival.”
On the ground
Platt, a Penn Integrates Knowledge University Professor, explains that the researchers were motivated by a finding reported in their previous work on the same macaque population: The animals became more accepting of one another and built new relationships despite scant resource availability. But this time, the team wanted to explore whether the macaques were simply passively engaging in these prosocial shade-sharing behaviors due to the fact they were being “squeezed” into limited space.
From 2013 to 2022, the researchers collected behavioral data across seven social groups encompassing 790 unique adult individuals. They constructed proximity and aggression networks for each group in each year to draw comparisons before and after the hurricane.
Additionally, they recorded ecological measurements, tracking the temperature differences between shaded and exposed areas. They found that shaded areas were consistently cooler, which was crucial for the macaques’ thermoregulation.
“We had our team on the ground watching the monkeys, documenting everything they did for 10 minutes and noting all the monkeys in close proximity,” Platt explains. “This provided a clear snapshot of social dynamics among the monkeys by telling us who they were hanging out with at different times of the day, when it’s hot out versus when it’s cooler.”
Cooler heads prevail
“Before Maria, tolerating others had no impact on risk of death,” Testard says. “Afterwards, macaques that displayed more than average social tolerance, and were therefore better able to share shade, were 42% less likely to die than those that were less tolerant.”
This represents a sudden change in “selection pressure,” the evolutionary benefits or costs of different traits or behaviors, the researchers explain. Social behavior was assessed by recording aggression and how often individuals were seen sitting together.
“To access shade, they need to tolerate and be tolerated by others, and we found that this tolerance spills over into other daily interactions,” says Testard. “Macaques that began sharing shade also spend time together in the mornings, before the day’s heat forces them to seek shade. In effect, the hurricane changed the rules of the game in the monkeys’ society.”
Brent, a former postdoctoral researcher in the Platt Labs, adds that “macaques aren’t the best at sharing resources—be that food or shade.” They are known for living in an aggressive, highly competitive society. But in the heat caused by ecological changes, often around 40°C (104°F), the macaques had to share space or die.
Platt says the hurricane’s impact was so severe that it effectively aged the monkeys rapidly, similar to aging eight years in human terms, as the extreme conditions turned on genes involved in inflammation and bodily wear, while genes responsible for repairing DNA were turned off. “What we see in this latest work is the physical toll made it crucial for the monkeys to adapt quickly to survive.”
“For group-living animals, social relationships may allow them to cope with disturbances in the environment, including human-induced climate change,” says Brent. “We were surprised the macaques’ social behavior was so flexible, making them resilient to this environmental change, but some species may not display this same flexibility.”
Michael Platt is a Penn Integrates Knowledge Professor and the James S. Riepe University Professor, professor of neuroscience, professor of psychology, and professor of marketing, with appointments in the Perelman School of Medicine, School of Arts & Sciences, and Wharton School at the University of Pennsylvania.
Camille Testard is a recent Ph.D. graduate in the Neuroscience Graduate Group in the Perelman School of Medicine and a former member of the Platt Labs at the University of Pennsylvania.
Lauren Brent is a professor in the Department of Psychology in the College of Life and Environmental Science at the University of Exeter and a former postdoctoral researcher in the Platt Labs at Penn,
This work was supported by the National Institutes of Health (Grants R01MH118203, U01MH121260, R01MH096875, P40OD012217, R01AG060931, R00AG051764, R56AG071023, R01AG084706, and R24-AG065172), European Research Council (Grant 864461 - FriendOrigins), the National Science Foundation (Grant 1800558), National Center for Research Resources (Grant 8-P40 OD012217-25), and the Royal Society (Grant RGS/R1/191182).
