These monkeys use rocks as sex toys

Masturbation is not as uncommon in the animal kingdom as one might think. Researchers have already been able to demonstrate the behavior in penguins, dolphins and dogs, among others. Masturbation provides sexual pleasure, of course, but it also serves another purpose in most animals: just before mating, it can make them more attractive. The sperm becomes fresher and increases the chances of offspring. In female animals, the vaginal muscles are trained and the vaginal secretion is renewed. In some species, such as the South African ground squirrel, masturbation also serves to prevent sexual diseases.

Monkeys, our closest relatives in the animal kingdom, also like solo sex. However, they do not only use their hands or their mouths, as biologists from the Canadian University of Lethbridge were able to observe. Between 2016 and 2019, the scientists studied the behavior of around 170 cynomolgus monkeys in a nature reserve in Bali, Indonesia. The macaque species therefore prefers specifically shaped stones as sex toys for the act of masturbation. The researchers published their study results in the journal "Ethology".

The assumption is at least confirmed by the observed behavior of male and female Javanese monkeys in youth and adulthood. To climax, they rub the stones on their genitals or use the stone tool to stimulate their private parts and groin area with gentle, repeated tapping. However, the finding is not entirely new: the lead author of the study, Camilla Cenni, a doctoral student at the University of Lethbridge, already put forward the sex toy hypothesis in a study in 2020.

At that time, however, the behavior could only be demonstrated in male monkeys. This time the researchers also focused on the females of the monkey group - and lo and behold, there was a difference. Both sexes use stones as a sex toy. The female monkeys, however, were much more choosy when it came to choosing suitable stones for pleasure. They tend to select sharp-edged rocks with a grainy texture. That they are so picky also indicates that the Javanese monkeys' masturbation cannot be accidental.

The researchers therefore assume that the masturbation of the macaque species has no further evolutionary benefit, but solely serves sexual pleasure - a kind of self-reward, so to speak. Juvenile monkeys used the stones as sex toys more often than older individuals.

For the monkeys, however, rocks are more than just a sex toy: earlier studies from Panama, for example, show that they used rocks to help themselves find food. The Bali monkeys also used stones for non-genital play by throwing or arranging them.

The researchers suspect that the Javanese monkeys must have had the experience that they can use the stones for sexual pleasure satisfaction only in the recent past. Gradually, they seem to have learned how to behave with one another. Tourists have only been observing the behavior of the animals in the Sacred Monkey Forest Sanctuary in Ubud on Bali for a few years. Since they are protected in the nature reserve and have constant access to sufficient food, the monkeys have little to worry about.

What remains is a lot of free time. Now we know how they use it.

Incidentally, most monkeys have another vice, which is not just for fun: alcohol. You can find out what's behind it here: