After hurricane María, Puerto Ricans had to adjust to sudden and dramatic changes in their environment—but they weren’t the only ones. A new study published in June in the journal Science uncovered that the macaque monkeys on cayo Santiago, close to Humacao, have been adapting to the impacts of the devastating cyclone. The research shows that the monkeys have become more social and tightly bonded in response to the challenges posed by the storm. This study is the first to demonstrate that macaques can alter their social behavior to cope with climate-related disruptions in a natural environment.
Led by researchers from the University of Pennsylvania and the University of Exeter, the study found that Hurricane María, which made landfall on Puerto Rican soil in September 2017, significantly altered the social dynamics of the rhesus macaque population on Cayo Santiago. While the storm did not result in a large number of monkey fatalities on the island-laboratory, it devastated 63% of the vegetation. This loss had a profound impact on the macaques, who rely on the shade provided by trees to protect themselves from excessive heat.
“As very little shade remains, and shade being the only way these monkeys can lower their body temperature, it becomes a really limited resource”, explained primary investigator Camille Testard. She added that for the macaques, who fear the water that surrounds the island, the shade is the only method they have to stay alive in the heat of the Caribbean sun.
Usually, these monkeys are some of the most aggressive primates on the planet. ”They’re incredibly hierarchical, they have very clear positions of social status. There’s no compassion for the poor in macaque society”, Testard indicated.
Typically, the monkeys refuse to share shade with individuals outside their inner social circles, often resorting to aggression to maintain their space. However, after Hurricane María’s devastating loss of trees, this behavior changed—but not in the way scientists had anticipated. Rather than triggering more aggression due to the limited shade, researchers observed a surprising shift: the monkeys became more sociable, sharing shade more frequently with others outside their usual groups.
For the researchers who had been observing and studying the macaques of Cayo Santiago for years, the change was striking. “We began to notice that they would all line up together regardless of hierarchy”, noted Josué Negrón Del Valle, a Puerto Rican research assistant at the University of Arizona who collaborated with Testard.
To build the database used in the study to analyze the macaques’ social patterns, researchers in Puerto Rico— including Negrón Del Valle—have been traveling daily to Cayo Santiago to observe the monkeys and record their behaviors. By analyzing 10 years’ worth of data, Testard’s study found that macaques demonstrating more social tolerance—those more willing to share limited shade—had a 42% lower chance of dying compared to those that were less tolerant. This finding underscores the importance of social coexistence as a survival strategy for the macaques of the island.
With less shade available for refuge, the monkeys now face a difficult choice: share shaded spaces in solidarity or risk overheating through aggression. “The theory is that if I get aggressive and cause trouble, I’m wasting energy and overheating,” explained Negrón Del Valle.
The study revealed that this effect was prominent. Instead of being present only in the hottest part of the day, the monkeys’ new social tolerance was also seen in the mornings, before the sun rose. It was also observed in all groups on the island, showing the extent of macaques’ social flexibility.
Testard highlighted the importance of research groups on the island for carrying out the new study. “(They were) completely essential,” she emphasized, speaking of the Caribbean Primate Research Center, led by Dr. Angelina Ruiz-Lambides, which has facilitated monitoring of macaques on the key for more than 80 years.
“What is needed for these kinds of investigations are people on the island who know the animals, who have been with them for years,” Testard stressed.
As an outdoor laboratory, Cayo Santiago offers a unique opportunity to study primates in their natural environment. This setting allows scientists to gain valuable insights into how animal societies adapt to disasters and climate change, and, within that context, the evolutionary significance of social behaviors like solidarity.
This article was originally written in Spanish for audiences in Puerto Rico. Click here to read my original writing.