Borneo has a hybrid ‘mystery monkey’ and researchers are concerned

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An unidentified monkey spotted in Borneo is a rare hybrid between two different species fighting over forest space, a new study has found.

Researchers have concluded that the ‘mystery monkey’ is likely the offspring of a proboscis monkey (Nasalis larvatus) and a silver langur (Trachypithecus cristatus) — two distant species that share the same habitat.

Coronavirus restrictions prevented researchers from investigating the forest where the monkey lives, so scientists instead analyzed photos that began to appear on social media in 2017. The monkey was first photographed as a juvenile, but more recent photos from 2020 reveal the animal is now an adult female and may have her own baby.

“She seemed to be breastfeeding a baby,” study co-author Nadine Ruppert, a primatologist at Universiti Sains Malaysia (University of Science Malaysia), told Live Science in an email. “We were all amazed, it was quite surreal.”

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Although different species generally do not produce viable offspring if they mate, very closely related species can sometimes interbreed in the wild to create hybrids. For example, the northern pig-tailed macaques (macaca leonina) and southern pig-tailed macaques (macaca nemestrina) intersect in some parts of Thailand, according to the International Union for Conservation of Nature (IUCN). However, interbreeding species are usually similar and belong to the same evolutionary group, or genus – proboscis monkeys and silver langurs do not.

The hybrid monkey was spotted near the Kinabatangan River in Malaysia’s Borneo (the island is split between three nations: Malaysia, Brunei and Indonesia), where ranges of proboscis monkeys and silver langurs overlap. But these two species of monkeys are obviously very different.

Adult proboscis monkeys have pinkish faces with elongated noses, while adult silver langurs have black faces with shorter, flatter noses. Probosicis monkeys are also larger. A male proboscis monkey can be up to 30 inches (76 cm) long and weigh 44 to 53 pounds (20 to 24 kg). Silver langurs only reach about 22 inches (56 cm) long and weigh 14.5 pounds (6.6 kg) on ​​average, according to the New England Primate Conservatory.

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A family of proboscis monkeys in a tree in Borneo.

A family of proboscis monkeys in a tree in Borneo. (Image credit: USO via Getty Images)
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A group of silver langurs, also known as silver lutungs, sit on a branch in Borneo.

A group of silver langurs, also known as silver lutungs or leaf monkeys, sit on a branch in Borneo. (Image credit: Anup Shah via Getty Images)

Both species live in groups consisting of a dominant male and several females and their offspring. Males born into these groups are forced to leave once they reach maturity to create their own groups or take over another group. However, habitat decline limits the areas where these dispersing males can go, according to Ruppert.

“We have concluded from observations that photographers have made that male proboscis monkeys mate with female silver langurs in the area and that there are mixed groups where female proboscis monkeys even care for baby silver langurs,” Ruppert said.

Male proboscis monkeys can use their larger size to crowd out male langur and take over langur groups. Researchers suspect the ‘mystery monkey’ in the photos is the offspring of a male proboscis monkey and a female langur as it shares characteristics of both species. For example, her nose is pronounced like a female proboscis monkey, but not as elongated, and her face has a gray tint.

Most hybrids born of different species are sterile and unable to produce offspring, which makes the so-called mystery monkey and its baby even more unusual. It’s possible she was a mother – or cared for another woman’s baby – but photos showed she had the swollen breasts associated with lactation, indicating the offspring was the his.

As unique and intriguing as the discovery seems to be, there is a downside. “It is tragic that the two species are now close together in the narrow patches of riparian forest surrounded by Palm oil plantations, where they compete for food and mating opportunities,” Ruppert said. “I hope people start talking about her, not as an attraction, but as a ‘flagship’ animal of the region. which must be protected, and with it, its two parent species and their habitat.”

The study was published on April 26 in the International Journal of Primatology.

Originally posted on Live Science.

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