A ‘mystery monkey’ could mean its parent’s species could be in trouble

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

Six years ago, tour guide Brenden Miles cruised down the Kinabatangan River. He was on the island of Borneo, in Southeast Asia. It was in an area that is part of Malaysia. Suddenly, he spotted a strange-looking primate. He took some photos and looked at them later.

“At first I thought it might be some form of the silver leaf monkey,” says Miles. A morph is a member of a species with rare coloring. But then Miles noticed other details. The monkey’s tail was thicker than it should be. And his nose was long like a proboscis monkey’s (Pro-BOSS-kiss). After posting a photo of the animal on Facebook, he forgot everything.

Scientists have now studied this photo and others. They suspect that this mysterious monkey is a hybrid. It is an animal born from parents of two different species. A male proboscis monkey (Nasalis larvatus) probably mated with a female silver leaf monkey (Trachypithecus cristatus). Scientists fear this could signal that the creature’s parent species is in trouble.

Monkey Mugshots

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

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

Changes to the habitat of the parent monkeys could explain a hybrid, says Nadine Ruppert. She is part of the team that studied the mysterious monkey. A primatologist, she works in Malaysia at the Universiti Sains Malaysia on the island of Penang.

Ruppert’s team compared photos of the puzzling primate with images of the putative parent species. The scientists also assessed the mystery apes’ limbs based on the images. If the unknown animal was one of these two species, its measurements should be similar to either. “But that’s not the case with this animal,” Ruppert says.

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

Scientists aren’t quite sure yet of the mysterious monkey’s ancestry. DNA in the primate’s poo could reveal if it is indeed the offspring of the two suspected species. So far, researchers haven’t been able to get close enough to retrieve a fecal sample.

But other sightings since 2016 bolster the idea that the mystery monkey could be a hybrid. Local tour guides saw a male proboscis hanging around a troop of female silver monkeys. There is even photographic evidence of a male proboscis monkey mating with a female silver leaf monkey.

Oil palm plantations have developed along the Kinabatangan River. As forests turn into agricultural land, wildlife habitats can be damaged or lost. Or they can be cut into small pieces. The Kinabatangan River flows through the Malaysian state of Sabah. And this state lost about two-fifths of its forest cover from 1973 to 2010. Logging and oil palm plantations were the main drivers of this loss.

In some places, the presumed related monkey species have been crowded into small patches of forest along this river, Ruppert says. The monkeys there might have to compete more for mates. Up close, she notes, the male of the larger species can dominate. In this case, the proboscis male could easily displace the silver leaf monkey males.

A hybrid surprise

The mysterious monkey makes local people quite excited. “The hybrid is gorgeous,” says Ruppert. “But,” she adds, “we don’t want to see any more.” Indeed, finding this monkey worries her.

The International Union for Conservation of Nature creates the official global list of endangered animals. The group classifies proboscis monkeys as endangered. Silverleaf monkeys are vulnerable to extinction, he says. Both species can run out of food and space to behave as they naturally would, Ruppert says.

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

The mystery monkey was last photographed in September 2020 holding a baby. Her breasts were also swollen, which they would be when breastfeeding young. This suggests that the primate is a fertile female. Hybrids tend to be sterile – unable to produce offspring. So here is yet another surprise from this extraordinary monkey.

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