With Little Else to Do in Bali, Monkeys Have Found a Way to Make Sex Toys

Masturbation is very common in the Sacred Monkey Forest.

There are a lot of long-tailed macaques that live in this area.

Tourists wander through the sanctuary on their way to three local temples and feed the monkeys. Three meals of fruit and vegetables are given to the creatures by the park staff.

With full bellies and little else to worry about, these monkeys have a lot of time for leisure and self-indulgence.

Scientists have proposed a "sex toy" hypothesis after seeing male and female macaques rubbing and tapping stones on their genitals.

The hypothesis states that the stones are being used by the macaques for masturbation, similar to what has been seen in other animals.

Masturbation doesn't lead to actual ejaculation for other males. It's difficult to know how much pleasure the monkeys are getting from stone play.

Male macaques get erections when they rub or tap their genitals with rocks, but not when they touch stones to other parts of their body.

Female macaques are quite picky about what shape of rocks they choose for self-pleasure, which indicates they are picking those most suited to the task at hand.

Researchers conclude that the actions are not unimportant.

The "Sex Toy" hypothesis indicated that stone- directed tapping and rubbing on the genital and inguinal area are sexually motivated behaviors.

Scientists collected video footage in the Sacred Monkey Forest over the course of two years.

Hundreds of examples of tool-assisted masturbation were collected by the authors.

The majority of the monkeys observed rubbing or tapping stones on their genitals were males.

Female masturbation is rare among nonhuman primate. It's not easy to tell when a female is sexually aroused. Either way, or both.

It's not clear why macaques in Indonesia are using stones to touch themselves, but researchers think it's probably the right answer.

These monkeys don't have a lot of other things to do.

"I think we can confidently say that the free time they have as a result of provision, it's a big explanation for stone play," evolutionary ecologist Camilla Cenni told Vice World News

Free time is not enough. It's only part of the story. You have to see the first monkey doing it to figure out the other part.

There are other animals that have been caught using sex toys.

Animals use other animals for sex. The male and female Japanese macaques were filmed jumping on the backs of deer.

The dolphin was caught using a dead fish for self-pleasure.

It's a bit harder to manage a stone. macaques are used to using it. macaque society plays with rocks often as a way to practice for feeding attempts.

Researchers think this adaptive play was co-opted into a behavior that only offered short-term pleasure.

macaques in Indonesia are playing with rocks for a different type of play. This one is much more enjoyable.

The study was published in the international journal of behavioral biology.