'Mystery monkey' spotted in a Borneo forest could be a rare hybrid of two different species, new study says

• An unidentified "mystery monkey"could be a rare hybrid of two different species, a new study says.
• Interbreeding between distantly related species is "rarely observed in the wild," according to the study.

An unidentified "mystery monkey" seen in Borneo could be a rare hybrid of two different species, according to a study published in the International Journal of Primatology.

The mysterious primate spotted near the Kinabatangan River in Malaysian Borneo, is likely to be the offspring of a proboscis monkey and a silvery langur – two species that inhabit the same forest and are distantly related, researchers said.

While closely related species occasionally interbreed to create hybrids, hybridization between distantly related species is "rarely observed in the wild," according to the study.

The two monkey species do not even belong to the same genus and visually look very different from each other. Adult proboscis monkeys have reddish-brown fur and elongated noses, while adult silvery langurs have gray-tipped dark fur and flatter faces.

The hybrid monkey has physical characteristics of both species – pictured here.

The mysterious primate was first seen by researchers in photos on social media in 2017, while it was still a baby. Photos from 2020 suggest that the monkey is now a grown female and has a baby of her own.

"She appeared to be nursing a baby," study co-author Nadine Ruppert, a primatologist at the University of Science Malaysia, told Live Science. "We were all in awe. It was quite surreal."

Most hybrids born from different species are sterile and unable to produce offspring, according to Live Science, which adds to the mystery of this monkey and her baby.

While it's possible she was taking care of another female's baby, she appeared to have swollen breasts, suggesting she was lactating.

Ruppert told Newsweek that the existence of the unusual hybrid could be evidence of an ecosystem out of balance.

"Seeing this putative hybrid is per se not of concern to the balance of the ecosystem or the two species, however, it is an alarming symptom of an ecosystem that already seems out of balance," she said.

The males of both species typically disperse from their families once they mature to find mating opportunities.

However, habitat decline due to deforestation is limiting mating opportunities for these species, which could explain how the hybrid came to be, Ruppert told Newsweek.