You've probably heard of pangolins, Sambar deer, and mousedeer -- all native species to Singapore.

It is unlikely though, that you've heard of the Raffles' banded langur, arguably one of the rarest of Singapore's wildlife.

Extremely rare monkeys spotted at nature reserve

These dusky-coloured langurs are so elusive, they were once on the brink of extinction.

Currently, there are only an estimated 40 to 60 individuals left in the wild.

In a surprisingly rare and serendipitous occurrence though, a family of Raffles' banded langurs were spotted at the Central Catchment Nature Reserve (CCNR).

They were spotted by a nature reserve goer, Jia Wei Woo, who had apparently sighted the reclusive monkeys during an evening walk at the nature reserve on Sep. 12, 2019.

According to a follow-up comment, Woo observed them for one-and-a-half hours.

Here are some of the photos Woo took and posted to Facebook page Nature Society (Singapore).

The monkeys can be seen close to the ground and staring curiously in the direction of Woo's camera.

Could have descended to the forest floor to forage

Many people expressed their surprise over Woo's fortuitous sighting of such a rare species.

While some wondered why the langurs, which usually stick to the treetops and higher branches, had descended from the trees and so low to the ground.

In response, Singaporean researcher Andie Ang, who has been studying the Raffles' banded langur since 2008, confirmed that the langurs had likely descended from their arboreal (living in trees) habitat to drink water from a nearby stream, or forage for fruits and seeds on the ground.

Woo also accurately observed that the tree canopy in that area had been rather thin, which prevented the monkeys from simply hopping between tree branches, and forcing them to the ground to cross between trees instead.

More protection for critically endangered langurs

Raffles' banded langurs, also known as banded leaf monkeys, are considered to be critically endangered in Singapore.

Though once common in the 1920s, their populations were slowly decimated by habitat loss due to urban development.

A family used to inhabit Bukit Timah Nature Reserve.

However, the last member of the troop was reportedly mauled to death in 1987 by a pack of wild dogs.

Since then, the langurs can only be found in CCNR.

However, the Raffles' banded langur could have seen a boost in population, after a conservation plan was launched for them in 2016.

A collaboration between Wildlife Reserves Singapore, the National Parks Board (NParks) and universities from both Singapore and Malaysia, the plan involves enhancing the forest habitat of these monkeys, reported The Straits Times.

Reforestation and new nature parks will afford the langurs more protection and allow them to move easily between forest fragments.

A Species Action Plan for the langurs was also coordinated by the Raffles' banded langur working group -- consisting of a team of experts and volunteers -- that will guide conservation efforts.

Top photo from Jia Wei Woo / FB