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What Can We Learn From a Monkey Water Park?

The study of play has serious implications for social development.

Key points

  • Rhesus monkeys on the island of Cayo Santiago engage in aquatic play, including swimming and diving.
  • Observations collected on the island suggest it is mostly juveniles, and primarily males, that participate in aquatic play.
  • Researchers say this phenomenon offers the potential to learn more about the functions and consequences of social play—in these primates and in humans.
Alyssa Arre, used with permission.
Juvenile rhesus macaques play above water.
Source: Alyssa Arre, used with permission.

The island of Cayo Santiago off the east coast of Puerto Rico is home to an unusual population of rhesus macaques. The Old World monkeys were originally brought there in 1938 to create a place in the western hemisphere for scientists to study these animals in a naturalistic environment. The island now supports around 1,700 free-ranging rhesus monkeys.

Alyssa Arre and Daniel Horschler were conducting cognitive research with the monkeys of Cayo Santiago for their Ph.D.s at Yale University and the University of Arizona, respectively. But during their data collection, Arre and Horschler noticed an interesting phenomenon: On hot days, juvenile macaques played together in a water hole.

“We would stop whatever we were doing to watch the babies swimming and playing because it was such a cute and interesting behavior,” says Arre, who is now scientific director at the Caribbean Primate Research Center at Cayo Santiago.

Soon, Arre was taking photos and videos of the aquatic play each time she observed it. When the journal Behaviour put out a call for anecdotes in animal behavior, she and Horschler knew they had to share their observations.

Playful Primates

In their report, Arre and Horschler describe social play, including swimming and diving, in juvenile monkeys. The researchers observed groups of young macaques gather around a shallow pool surrounded by vegetation and climb the adjacent trees, generally pausing three to seven meters above the water.

Alyssa Arre, used with permission.
Monkeys wrestling in branches above the water and displaying play faces and postures.
Source: Alyssa Arre, used with permission.

Interestingly, the monkeys appeared to take turns jumping into the water. And they would sometimes push each other in the branches above the water.

“If one was on the edge, others would come behind and kind of wrestle on the edge,” says Arre. “A very generous interpretation would be that they are trying to push a monkey to get to their turn. But there are less generous interpretations, too.”

These behaviors were accompanied by play signals, such as specific facial expressions and body postures. Once in the water, the monkeys exhibited play behaviors including gamboling towards one another and wrestling, chasing, slapping, and cuddling.

Arre and Horschler noted that the participants were primarily young macaques between the ages of 1 and 5 — a period between weaning and reaching sexual maturity. In addition, most of the participants were male. Arre says that is one aspect she wants to follow up on, to see if it reflects a real pattern or is just an artifact of their opportunistic data collection.

Diving Into Research Questions

Arre and Horschler propose that these water-loving monkeys present a unique opportunity to investigate questions about the functions of social play and the similarities and differences in play between human children and other animals.

Alyssa Arre, used with permission.
A play interaction between two monkeys involving slapping and chasing.
Source: Alyssa Arre, used with permission.

Previous research on play in humans and non-human animals suggests that it can facilitate social interaction and influence social development. Another aspect of aquatic play that Arre is interested in pursuing involves following individuals as they grow to see how early-life play may affect later social outcomes.

“Some hypotheses suggest that play helps individuals become more socially integrated later in life, with more connections and better social skills,” says Arre.

Arre was also intrigued by another behavior: A single monkey diving into the pool while others watched.

“We would like to investigate if they are following some sort of order,” she says. “When they are watching, are they waiting for their turn? If there is turn-taking happening, is the pushing involved in that?”

Arre and Horschler’s observations raise more questions than they answer, but they are excited for the possibilities for future research. Future studies of these monkeys could reveal more about the functions of social play in primates, including humans.

“When I see the monkeys pausing on the edge of the branch, then going back to the tree, and then back to the edge, it reminds me of little kids on diving boards,” says Arre.

“We obviously don’t know what’s going on in their heads when it is happening, but it looks a lot like the way human children play.”


Alyssa M. Arre and Daniel J. Horschler. Swimming and diving as social play in juvenile rhesus macaques (Macaca mulatta). Behaviour (2021). DOI:10.1163/1568539X-bja10074.

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