A “mysterious monkey” could mean that her parent’s appearance may be in trouble

A puzzling primate may be the offspring of two distant species. And this monkey business worries scientists.

Six years ago, tour guide Brendan Miles traveled the Kinabatangan River. He was on the island of Borneo in Southeast Asia. This was in an area that is part of Malaysia. Suddenly he noticed a strange-looking primate. He took a few pictures and looked at them later.

“At first I thought it might be a monkey morph with silver leaves,” Miles said. The morph is a representative of a species with a rare coloration. But then Miles noticed other details. The monkey’s tail was thicker than it should be. And his nose was as long as that of a pro-BOSS-kiss monkey. After posting a picture of the animal on Facebook, he forgot all about it.

Now scientists have studied this picture and others. They suspect that this mysterious monkey is a hybrid. This is an animal born to parents of two different species. Male Proboscis Monkey (Nasalis larvatus) probably mating with a female monkey with silver leaves (Trachypithecus cristatus). Scientists are concerned that this could signal that the parent species is in trouble.

Pictures of monkeys

Sometimes closely related species hybridize. This only happens occasionally in the wild. What was especially surprising here: the suspected parents of this monkey are distant relatives.

When animals of different species hybridize, the parents both tend to belong to the same genus. This is a group of closely related species. But the proboscis monkey and the silver-leafed monkey belong to different genera. “Hybridization between genera is very rare,” said Ramesh Bunratana. He was not part of this study. He teaches conservation biology at Mahidol University in Nakhon Pathom, Thailand.

Changes in the habitat of the parent monkeys may explain the hybrid, says Nadine Rupert. She is part of the team that studies the mysterious monkey. Primatologist, she works in Malaysia at the Universiti Sains Malaysia on Penang Island.

Rupert’s team compared photos of the puzzling primate to images of the alleged parent species. Scientists also sized the limbs of the mysterious monkeys based on the photos. If the unknown animal is one of these two species, its measurements must be similar to one or the other. “But that’s not the case with this animal,” Rupert said.

a monkey standing on a tree branch, with a mix of black and reddish fur
Brendan Miles took this photo from 2016 of a young monkey in the Malaysian part of Borneo. The animal may be the offspring of distant related species.B. Miles

Scientists are still not entirely sure about the origin of the mysterious monkey. DNA in the primate’s feces may reveal whether it is indeed the offspring of the two suspects. So far, researchers have not been able to get close enough to take a fecal sample.

But some other observations from 2016 confirm the idea that the mysterious monkey may be a hybrid. Local guides have seen a male trunk hanging around a group of female monkeys with silver leaves. There is even photo evidence of a male monkey with a trunk mating with a female monkey with silver leaves.

Oil palm plantations are expanding along the Kinabatangan River. As forests become agricultural land, wildlife habitats may be damaged or lost. Or they can be cut into smaller pieces. The Kinabatangan River flows through the Malaysian state of Sabah. And this country lost about two-fifths of its forest cover from 1973 to 2010. The main drivers of this loss are logging and oil palm plantations.

In some places, the alleged parent species of monkeys have been trapped in small patches of forest along the river, Rupert said. The monkeys there should compete more for friends. Nearby, she notes, males of larger species may dominate. In this case, the male trunk can easily displace the male of the monkey with silver leaves.

Hybrid surprise

The mysterious monkey makes the people in the area quite excited. “The hybrid is gorgeous,” says Rupert. “But,” she adds, “we don’t want to see more of them.” Indeed, finding this monkey worries her.

The International Union for Conservation of Nature has established the official global list of endangered animals. The group classifies proboscis monkeys as endangered. Monkeys with silver leaves are vulnerable to extinction, it says. Both species may lack enough food and space to behave naturally, Rupert said.

In Borneo and around the world, human activities and climate change are damaging animal habitats. As habitat is lost, mating (or at least mating attempts) between different species – including different genera – could increase, Bunratana said.

The mysterious monkey was last photographed in September 2020. She was holding a baby. His breasts were also swollen, as they would be when they breastfed young. This suggests that the primate is a fertile female. Hybrids tend to be sterile – they cannot produce offspring. Here is another surprise from this unusual monkey.

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