Survival Through Sociability: How Hurricane Maria Changed the Behavior of Rhesus Macaques

Increased Tolerance and Resource Sharing Among Macaques Leads to Higher Survival Rates Post-Hurricane

In 2017, Hurricane Maria devastated Puerto Rico, wreaking havoc on human populations and natural habitats alike. One notable area affected was Cayo Santiago, a small island home to hundreds of rhesus macaques (Macaca mulatta). These macaques, known for their aggressive behavior, demonstrated an unexpected shift in social dynamics post-disaster, leading to increased tolerance and resource sharing. A recent study published in Science¹ reveals that this newfound sociability significantly boosted their survival rates.

The Impact of Hurricane Maria on Cayo Santiago

Cayo Santiago, often referred to as Monkey Island, experienced severe deforestation due to Hurricane Maria. Prior to the hurricane, the island was densely covered with trees, providing ample shade—a crucial resource given the island's high temperatures, which often exceed 40°C (104°F). The hurricane transformed the landscape into one of bare tree trunks and limited shade, drastically altering the living conditions for the macaques.

Before the hurricane, it was not at all common to see an image like this, with several macaques sharing the shade of a tree. Lauren Brent

Behavioral Changes Among Macaques

Contrary to expectations, the macaques did not become more aggressive in their competition for the scarce shade. Instead, they exhibited increased tolerance towards each other. This shift in behavior was measured by the acceptance of another macaque within two meters—social tolerance tripled after the hurricane, and although it has since declined slightly, it remains double the pre-hurricane levels. Acts of aggression have also significantly decreased.

The composition shows an aerial view of Monkey Island. On the left, the island with all its trees, in an image from 2008. On the right, its condition in 2020, three years after the hurricane. Joyce Cohen/Michelle Skrabut LaPierre

Lauren Brent, a professor of ethology at the Animal Behavior Research Center at the University of Exeter and senior author of the study, noted,

“We expected that the monkeys would become more aggressive by competing for shade, given that this is a society known for its aggressiveness. Instead, they became more tolerant of others and less aggressive.”

Study Findings on Survival Rates

The research team focused on data from the seven groups of macaques, totaling 790 adults, across a ten-year span, comparing figures from five years before and five years after Hurricane Maria. They discovered that social tolerance had a direct impact on survival rates.

Hurricane Maria destroyed 63% of the vegetation on Cayo Santiago, so shade became a scarce and very valuable resource. In the image, a group of macaques on dead trees. Lauren Brent

Camille Testard, a researcher at the University of Pennsylvania and lead author of the study, explained,

“We had the behavioral and survival data for all individuals. We were able to model the relationship between the sociability of individuals and their probability of survival.”

The results showed that post-hurricane, more tolerant macaques had a 42% higher likelihood of survival.

Adaptive Value of Increased Social Tolerance

The need for shade, a critical resource for thermoregulation, led to strengthened social ties among the macaques. This tolerance extended beyond midday interactions, affecting social behaviors throughout the day. Testard said,

“To access the shade, they need to tolerate (and be tolerated by) others, and we found that this tolerance extends to other daily interactions.”

Brent elaborated on the strategic nature of this behavior, noting,

“Competing for shade is different from competing for food. Tolerating others doesn’t cost much. Unlike supporting someone in a fight, you are very unlikely to get hurt or expend a lot of energy tolerating others.”

This low-cost strategy of tolerance facilitated broader social networks, enhancing access to shade and improving survival chances.

Future Implications and Adaptability

The long-term sustainability of this behavior remains to be seen, contingent on the recovery of Cayo Santiago’s vegetation. However, Brent emphasized the adaptability of the macaques, suggesting that their behavior offers valuable insights into how social relationships can help animals cope with environmental changes. Brent said,

“For animals that live in groups, social relationships can allow them to cope with environmental perturbations, including human-induced climate change. The lesson we can learn from this is that we need our social relationships to help us overcome life’s major challenges, and being tolerant of others can lead to biological success.”

Conclusion

The study of the rhesus macaques on Cayo Santiago provides a compelling example of how environmental stressors can lead to significant behavioral adaptations. Increased social tolerance and resource sharing among these primates not only enhanced their survival rates but also underscored the importance of social relationships in overcoming environmental challenges. As climate change continues to impact ecosystems worldwide, such natural experiments offer crucial insights into the resilience and adaptability of social species.

1. Testard, C., Shergold, C., Acevedo-Ithier, A., Hart, J., Bernau, A., Negron-Del Valle, J. E., Phillips, D., Watowich, M. M., Sanguinetti-Scheck, J. I., Montague, M. J., Snyder-Mackler, N., Higham, J. P., Platt, M. L., & Brent, L. J. N. (2024). Ecological disturbance alters the adaptive benefits of social ties. Science (New York, N.Y.), 384(6702), 1330–1335. https://doi.org/10.1126/science.adk0606