Researchers Find Monkeys in Bali Using Stones to Masturbate: A Curious Case of Animal Tool Use
Deep within Bali’s Sacred Monkey Forest Sanctuary, long-tailed macaques are not only known for their playful antics and interactions with tourists but also for a surprising and rather intimate behavior. A recent study published in the journal Ethology: International Journal of Behavioural Biology has uncovered that these monkeys use stones as tools for sexual stimulation, a phenomenon that has intrigued scientists and challenged traditional understanding of animal behavior.
Researchers studying the macaques between 2016 and 2019 discovered that the primates—both male and female—engage in frequent stone play . What began as an innocent pastime involving collecting, tapping, and biting stones soon revealed a more complex pattern of behavior: the monkeys were rubbing or tapping the stones on their genitals, seemingly deriving physical pleasure from the act.
Interestingly, even though the males appeared sexually stimulated during these acts, researchers observed that none of them ejaculated, indicating that the purpose might be more about pleasure and sensation rather than reproduction. Cenni explained to VICE World News, “It really seems that they do it because it feels good. There is some sort of tactile stimulation from the contact of those stones with their genitals, and it feels good—and there is no reason to stop.”
While self-stimulation in primates is not an entirely new finding, the use of external tools like stones for the act is extremely rare, making this discovery significant in understanding the evolution of sexual and leisure behaviors in animals.
What stood out was the selectivity in the stones chosen by mature females. They seemed to prefer stones with sharper edges or rough textures, possibly because of the additional tactile stimulation these provided. This level of discernment suggests that the behavior may be intentional and experiential, rather than accidental.
Cenni and her team also speculated that this “stone-assisted masturbation” may have evolved naturally from the monkeys’ existing habit of playing with stones, a common activity among macaque populations in the region.
Cenni explained, “If you stop and start watching those monkeys as they play with stones, you may very likely see it. It’s part of their behavioral repertoire.” This suggests that the macaques’ stone-assisted stimulation could also be spreading socially among individuals in the same group.
The temple staff provide them with fruits and vegetables three times a day, ensuring they have ample nutrition. As Cenni noted, “I think we can confidently say that the free time they have as a result of provisioning is a big explanation for this.”
Unlike their wild counterparts, who spend much of their time searching for food and avoiding predators, the macaques of Ubud’s sacred forest enjoy long periods of leisure—time they use for grooming, playing, and, as it now turns out, exploring self-stimulation with stones.
During the COVID-19 pandemic, when tourist activity dropped dramatically, the macaques of Bali’s Sangeh Monkey Forest even ventured into nearby villages, raiding homes for food—an indication of how closely their behavior is tied to human interaction and availability of resources.
Here, the macaques are using tools purely for tactile pleasure, revealing an emotional and sensory depth often underestimated in non-human primates. “It’s hard to give a solid explanation,” Cenni said, “but it really seems they do it because it feels good—and that’s enough of a reason.”
By studying such behaviors in primates, scientists can better understand how humans, too, may have developed similar curiosity-driven tendencies that extend beyond survival instincts.
Researchers studying the macaques between 2016 and 2019 discovered that the primates—both male and female—engage in frequent stone play . What began as an innocent pastime involving collecting, tapping, and biting stones soon revealed a more complex pattern of behavior: the monkeys were rubbing or tapping the stones on their genitals, seemingly deriving physical pleasure from the act.
The Study and its Findings
The study, led by researchers including Camilla Cenni , a PhD candidate at the University of Lethbridge in Canada, documented hundreds of instances of this peculiar behavior. Using detailed video analysis, they noted that male macaques often displayed this behavior more frequently and for longer durations when they were visibly aroused.Interestingly, even though the males appeared sexually stimulated during these acts, researchers observed that none of them ejaculated, indicating that the purpose might be more about pleasure and sensation rather than reproduction. Cenni explained to VICE World News, “It really seems that they do it because it feels good. There is some sort of tactile stimulation from the contact of those stones with their genitals, and it feels good—and there is no reason to stop.”
While self-stimulation in primates is not an entirely new finding, the use of external tools like stones for the act is extremely rare, making this discovery significant in understanding the evolution of sexual and leisure behaviors in animals.
Female Macaques and Selective Stone Use
For the female macaques, the researchers found it harder to determine signs of arousal, as physical cues are less visible than in males. However, the females also appeared to engage in similar acts, rubbing stones against their genitals.What stood out was the selectivity in the stones chosen by mature females. They seemed to prefer stones with sharper edges or rough textures, possibly because of the additional tactile stimulation these provided. This level of discernment suggests that the behavior may be intentional and experiential, rather than accidental.
Cenni and her team also speculated that this “stone-assisted masturbation” may have evolved naturally from the monkeys’ existing habit of playing with stones, a common activity among macaque populations in the region.
Stone Play as a Cultural Behavior
In macaque communities, “stone play” is a well-documented cultural phenomenon. Populations of monkeys in both Bali and Japan have been seen engaging in such behavior, passing it down across generations. Researchers consider it a socially learned activity—not an instinctive one—meaning young monkeys observe and imitate elders, eventually adopting the habit themselves.Cenni explained, “If you stop and start watching those monkeys as they play with stones, you may very likely see it. It’s part of their behavioral repertoire.” This suggests that the macaques’ stone-assisted stimulation could also be spreading socially among individuals in the same group.
Why Do the Macaques Have Time for Such Behavior?
One key reason this behavior might have emerged is the comfortable lifestyle of the monkeys in the Sacred Monkey Forest. Surrounded by dense greenery, human settlements, and a steady flow of tourists, these macaques are well-fed and face little survival stress.The temple staff provide them with fruits and vegetables three times a day, ensuring they have ample nutrition. As Cenni noted, “I think we can confidently say that the free time they have as a result of provisioning is a big explanation for this.”
Unlike their wild counterparts, who spend much of their time searching for food and avoiding predators, the macaques of Ubud’s sacred forest enjoy long periods of leisure—time they use for grooming, playing, and, as it now turns out, exploring self-stimulation with stones.
Human Interaction and Intelligence
Long-tailed macaques are known for their intelligence and their ability to adapt to human behavior. In Bali, they have famously learned to steal personal items from tourists, such as glasses and phones, and trade them back for food. This kind of problem-solving ability demonstrates their advanced cognitive skills, which may also play a role in their experimental and pleasure-seeking behaviors.During the COVID-19 pandemic, when tourist activity dropped dramatically, the macaques of Bali’s Sangeh Monkey Forest even ventured into nearby villages, raiding homes for food—an indication of how closely their behavior is tied to human interaction and availability of resources.
A Window into Primate Psychology
The discovery that these macaques use stones for sexual stimulation provides fascinating insight into primate psychology and pleasure-seeking behavior. It challenges the long-held belief that tool use in animals is primarily driven by survival needs such as food gathering or protection.Here, the macaques are using tools purely for tactile pleasure, revealing an emotional and sensory depth often underestimated in non-human primates. “It’s hard to give a solid explanation,” Cenni said, “but it really seems they do it because it feels good—and that’s enough of a reason.”
Broader Implications for Behavioral Science
Researchers believe this finding may open new discussions about the complexity of animal emotions, self-awareness, and leisure behaviors. It also highlights how environment, food availability, and social structure can influence the evolution of unusual habits among intelligent species.By studying such behaviors in primates, scientists can better understand how humans, too, may have developed similar curiosity-driven tendencies that extend beyond survival instincts.
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