The thousand long-tailed macaques that live in this hotbed of hedonism have a field day.
Every day, 10,000 tourists wander through the sanctuary on their way to three local temples, feeding the monkeys as they go. Park staff also give the creatures three regular meals of fruits and vegetables.
With full bellies and few other worries in their little Garden of Eden, these monkeys have plenty of time for recreation – and self-indulgence is their main game.
In recent years, male and female macaques have been regularly spotted rubbing and banging rocks on their genitals, leading scientists to come up with a “sex toy” hypothesis.
This hypothesis holds that stones are used by Balinese macaques for self-directed, tool-assisted masturbation, similar to what has been seen in chimpanzees, porcupines, dolphins and, well, us.
Unlike our own species, however, masturbation rarely leads to actual ejaculation for other male animals. It is therefore difficult to determine the pleasure that the monkeys really derive from the stone game.
That said, male macaques get erections when they rub or tap their genitals with stones, which doesn’t happen when they touch stones on other parts of their body.
Additionally, female macaques are quite picky about the shape of the rocks they choose for their personal enjoyment, indicating that they pick the ones best suited for the task at hand, such as those with sharp edges (!) Or a grainy texture.
“Thus,” the researchers conclude, “one can confidently conclude that these actions are not accidental.”
“Overall,” they add, “our data partially supported the ‘sex toy’ hypothesis indicating that directed tapping and rubbing of the stone on the genital and inguinal region are sexually motivated behaviors” .
frameborder=”0″ allow=”accelerometer; automatic reading; clipboard-write; encrypted media; gyroscope; picture in picture” allow full screen>
The study is based on video footage in the Sacred Monkey Forest, collected by scientists between 2016 and 2019.
The authors collected hundreds of examples of tool-assisted masturbation by local groups of macaques during this period.
Most of the monkeys observed rubbing or banging rocks on their genitals were males, although there was no shortage of female monkeys.
In non-human primates, female masturbation is barely documented. This could be due to human bias or the fact that it’s harder to tell when a woman is sexually aroused. Or both.
It’s hard to say why Bali macaques use rocks to touch themselves, but researchers think the simplest answer is probably the right one: it feels good.
After all, these monkeys have few other activities.
“I think we can probably say with confidence that the free time they have from supply, that’s a big explanation for [stone play]“evolutionary ecologist Camilla Cenni told Vice World News.
“But [free time] is not a sufficient explanation,” she added. “I think that’s only part of the story. The other part is difficult to define because you would have to see the first [monkey] I do.”
Long-tailed macaques aren’t the only non-human animals that have been caught using possible “sex toys.”
Some animals even use other animals for sexual gratification. Male and female Japanese macaques have recently been filmed jumping on the backs of deer and pushing vigorously against their mount.
And headlines hit the headlines a few years ago when a male dolphin was caught using a dead fish for his own pleasure.
A hard, inanimate stone is a little easier to manage. It is a tool that macaques are used to handling. Playing with rocks is a regular feature of macaque society, perhaps as a means of training for genuine foraging or feeding attempts.
Over time, the researchers suspect that this adaptive play was co-opted into a non-functional behavior that offered only short-term pleasure.
Now, Bali macaques are using their instinctive affinity for rocks for a different kind of game. And this one is way more fun.
The study was published in Ethology: International Journal of Behavioral Biology.