In Borneo, a “hybrid monkey” worries researchers

An unidentified monkey spotted in Borneo is likely the offspring of a proboscis monkey and a silver langur, according to a new study. These two species, which share the same habitat, are genetically distant. Researchers are concerned that this type of forced hybridization is multiplying in this region where human activities are taking up more and more space.

Interspecific hybridization in primates is common. For example, the northern pig-tailed macaques (macaca leonina) and southern pigtailed ones (macaca nemestrina) occasionally interbreed in parts of Thailand. However, these species involved are usually similar and belong to the same evolutionary group or genus.

As part of a study published in the International Journal of Primatology, a team of researchers presents several evidences testifying to a possible hybridization event Between Nasalis larvatus (Proboscis) and Trachypithecus cristatus (Silver Langur) in Lower Kinabatangan Wildlife Sanctuary, Sabah. However, these two species which do not belong to the same genus.

A rare event in nature

Researchers rely solely on photographic evidence. In these pictures, a mature female observed as a juvenile three years earlier was carrying a baby. The hybrid monkey was reportedly spotted near the Kinabatanga River in Borneo, where populations of proboscis monkeys and silver langurs overlap.

As a rule, these matings do not produce viable offspring, hence the interest of these observations. It’s possible this mom was mothering (taking care of another female’s baby), but the photos show swollen breasts (associated with lactation), indicating that the offspring was indeed his.

Physically, the two species differ widely. Adult proboscis monkeys have pinkish faces with elongated noses, while adult silver langurs display black faces with shorter, flatter noses. The former are also larger. A male proboscis can indeed measure up to 76 cm long and weigh from 20 to 24 kgwhereas the langurs do not exceed the 56cm long and the 7 kg on the scale. However, the female monkey here presents characteristics of these two species.

A family of proboscis monkeys in a tree in Borneo. Credits: USO
A group of Silver Langurs. Credits: Anup Shah

Proboscis monkeys in the process of imposing themselves?

Both species live in groups consisting of a dominant male. The others are thus forced to leave once mature to create or take charge of another group. However, the decline of their habitat limits their dispersal abilities.

Based on these photographic observations, researchers believe that male proboscis monkeys mate with female silver langurs. These males could indeed use their larger size to crowd out smaller competitors and thus take control of the groups of langurs. Researchers suspect the presence of several mixed groups in the area.

As fascinating as this discovery is, researchers are concerned about it. It is indeed tragic that these two species are finally forced to jostle to share increasingly narrow patches of forest surrounded by oil palm plantations. Food and mating opportunities are increasingly scarce today, which could ultimately harm both species.

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