Three rare sightings of young adult mountain gorillas playing on their own in a stream in the wild suggest that these large primates may take pleasure, just as humans do, in splashing around for the fun of it.

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In a rare occurrence, researchers have recorded adult mountain gorillas playing on their own in water, just enjoying themselves.

Play is an important developmental process not just in humans but also in other primates.

Through play, humans and other animals gain more physical and mental acuity and learn behaviors that will serve them well into adulthood.

According to the World Wildlife Fund, "Gorillas share 98.3% of their genetic code with humans, making them our closest cousins after chimpanzees and bonobos."

Like humans and many other primates, gorillas — especially throughout childhood and adolescence — engage in play, which allows them to learn key skills and behaviors. Play also allows young gorillas to strengthen their muscles and become more agile.

So far, researchers have focused mostly on studying play as a social activity, but they have paid less attention to gorillas' solitary play and what it might mean to them.

For this reason, a few recent sightings of mountain gorillas playing on their own in water have caught the attention of a team of investigators, from the Primate Research Institute at Kyoto University, in Japan, the Primate Cognition Research Group, in Lisbon, Portugal, and Conservation Through Public Health, a nonprofit organization in Entebbe, Uganda.

The sightings — which occurred at the Bwindi Impenetrable National Park, in Uganda — were even more unusual because the gorillas playing on their own were subadults and adults: a 9-year-old female, a 10-year-old female, a 7-year-old male, and a 15-year old male.

Gorillas just...want to have fun?

The scientists have published their findings in the journal Primates. First author Raquel Costa and colleagues report that the sightings took place on three occasions at the end of the dry season in January 2018.

At these times, members of the Rushegura mountain gorilla group were seeking refreshment at a shallow stream.

During the first sighting, the 15-year-old male — called Kanywani — played on his own by submerging his fingers into the stream and making back-and-forth motions with his hand. "These movements were calm, and he did not splash the water," the researchers write in their paper.

On the same occasion, the 9-year-old female — whom the researchers call Kamara — played with the water in a similar way, also on her own.

During the second sighting, Kamara started splashing vigorously in the water, making a "play face," until she got completely soaked. She did this on and off for 17 minutes. During this, Kamara also briefly splashed, in a playful manner, at the 10-year-old female, Kanyindo, then went on to play on her own.

The third sighting involved the 7-year-old male, Kabunga, who played by making waves across the water with rotating arm movements.

The team believes that this solitary play may help the gorillas explore a new environment — water — while also allowing them to have fun and relax, pure and simple. As the investigators write:

"We observed a link between stimulation-seeking, exploration, and play behavior. We suggest that the observed behaviors served three direct functions: exploration or acquaintance with water as an environmental variable and resource, consumption of water, and a self-rewarding and positive action (possibly exciting)."

"An indirect function might be enhanced behavioral flexibility and better ability to cope with challenges," they add.

Costa and colleagues are unsure whether this solo water play behavior — which researchers have not observed in other groups of gorillas in the wild — is only specific to this mountain gorilla community or whether other gorillas also engage in it but have avoided observation so far.

In the future, they aim to remain alert to the possibility that solitary play may be more widespread among gorillas in the wild than zoologists had previously thought.

Moreover, the authors note, "Further efforts should be made to explore whether this behavior might be socially transmitted across generations within a group."