Delacour’s langurs are among the world’s rarest primates, found only in northern Vietnam. Van Long Nature Reserve protects the largest remaining population, living among its dramatic limestone karst mountains.

I searched for them from a small boat, rowed quietly along the river by a local woman as we watched the cliffs above us. The langurs tend to remain high on the limestone, and at first the mountains seemed almost impossibly steep for an animal to navigate.

Eventually I spotted a family. Watching them move across the cliffs was extraordinary. They clung effortlessly to sheer rock faces and ran along the sharp, jagged ridges with an ease that seemed completely at odds with the terrain.

I began thinking of them as the original rock climbers.

What fascinated me most was not simply how rare they are, but how completely they belong in this landscape. Their movements, their bodies and even the way they rest seem perfectly adapted to these severe limestone cliffs.