The long-tailed macaques that inhabit and around the Bali Sacred Monkey Forest Reserve have a strange behavior, with one or two stones on hand or around both male and female monkeys, and they are polished.
At the beginning, the tourists who came here wondered if this group of monkeys was afraid that they would not become sperm, would they "rob" with stones? It really echoes the old saying, "There are no tigers in the mountains, and monkeys are called kings", so tourists will feed melon seeds, fruits, peanuts and rice from time to time as road money.
But after a long time, they found that the stones in the hands of the monkeys did not seem to be used for "threat of force", but were smashed according to their own butt mounds...
It looks a bit like that.
When we talk about an animal using tools, it's often thought of as a means of survival, such as sea otters using stones to break clams, chimpanzees (Pan troglodytes) using stones to break nuts, etc., so they can eat delicious food hidden under a hard shell.
Even human beings are only a few points decent compared to it.
However, as the study of animal behavior deepens, scientists have found that the use of objects as tools is not necessarily to solve survival problems, and perhaps physiological problems.
The "weird phenomenon" of monkey colonies on Bali caught the attention of Camilla Cenni, a Canadian evolutionary ecologist whose research team captured hundreds of fragments of male and female macaques rubbing or tapping on their bodies with stones between 2016 and 2019...
Through careful observation, they found that the place where the monkeys struck the most was the genitals and its surrounding areas, and based on this behavior, the scientists proposed a hypothesis of a "sex toy".
That said, these macaques in Bali are using the stones in their hands for self-directed masturbation, most of which are observed by male macaques, and of course, there is no shortage of female macaques curious to try.
In addition, in the choice of stones, compared with the stones used to play and open the shells every day, the stones used to make "sex toys" are very different in texture and shape, especially the female macaques, who are more picky in choosing the shape of the stone, and in general, they will choose rocks with grainy textures or more protruding edges.
However, compared to the pure sexual motivation of the estrus period, rubbing or tapping the genitals with stones is more for sexual pleasure and does not cause male ejaculation or female orgasm, so it is difficult to determine how much pleasure the macaque monkeys get from the stones.
Prior to this, there were few cases of sexual pleasure with the aid of foreign objects in non-human primates, but the more famous one is probably in Japan, where macaques were photographed striking a fawn, jumping onto the deer's back and quickly shrugging their bodies.
A few years ago, in the British "Guardian", it was reported that a male dolphin was holding a dead fish and "entertaining himself".
It's hard to explain their motivation for doing this, but for macaques in Bali using stones to masturbate, researchers believe the simplest answer may be the most correct: it feels cool!
Because this group of monkeys has almost no other pursuits.
Every day, tens of thousands of visitors pass through the reserve to three local temples and feed the monkeys as they go, while the park staff regularly provides them with vegetables and fruits.
In this little Garden of Eden, macaques have a full stomach but an empty mind, because they don't have to worry about danger or no food at all, and they have a lot of time for self-indulgence.
What's more, young males spend much more time on this activity than mature males, and in fact, mature males are very reluctant to use stones because they can reach mature females.
Forced young males to turn their attention to the most handy tool at hand, the use of stones is a common feature of macaque society, and in southern Thailand and the coastal areas of Myanmar, stone fossils used by ancient long-tailed macaques have been unearthed, and they will use the stones in their hands to prey on clams, crabs and mussels.
This suggests that they play with stones not entirely to provide a short-term entertainment, but may also be to practice the way of foraging. The tools created by this foraging behavior may also serve as evidence of the life of human ancestors in the wild, after all, the use of the tool behavior of living primates as a model to test the use of tools by early Stone Age humans can also be regarded as a kind of cross-domain space-time fantasy linkage.
Resources:
https://doi.org/10.1111/eth.13324
https://doi.org/10.1016/j.physbeh.2020.112938