In the misty highland forests of Mount Kenya, there lives a small primate of disputed taxonomy. Known from only one specimen collected in 1938, The Mount Kenya Potto is a subspecies of potto—but whether of the Eastern or Western Potto, scientists are currently debating. Pottos are small, nocturnal primates with tiny, stumpy tails—and they’re exceptionally difficult to study. Their nocturnal, shy, quiet, and well-camouflaged lifestyle makes them cryptic in even the best surveying conditions.

Whether the Mount Kenya Potto is a subspecies of the Eastern or the Western Potto, scientists agree that this subspecies is in trouble. Native only to the forests around and on Mount Kenya itself, the animals are either very rare or extinct. Recent surveys—as far back as 1999—have failed to detect any Mount Kenya Pottos on Mount Kenya. Their limited range and their reliance on wet forest habitat have made them vulnerable to human encroachment, deforestation, and general habitat disruption.

A little red-backed, big-eyed, hairy-eared pale-masked mammal is missing on the island of Banka. The island, nestled between Sumatra and Borneo, is famous for its tin mines and its mineral extraction disputes. The Bangka Slow Loris was, or perhaps still is, native to the island as well as to a little sliver of Borneo itself. But this species has been lost to the annals of science. Slow loris are small, nocturnal, and excellent hiders—all of which makes it difficult to ascertain their presence or absence in any given area. They’re also crimson red.

Recent studies have failed to find almost any of the Bornean slow loris throughout their historic range. And Banka, with its encroaching mining operations and disappearing natural landscapes, is especially at risk.

Perhaps Asia’s most mysterious primate, the Tenasserim Langur was known only from a handful of museum specimens and decades-old field observations. Until the lost langur turned up in an unexpected spot—the Bangkok Zoo. The langur is a small animal who peers at the world through a white mask.

Native to the Tenasserim region of Myanmar from which it gleans its names, as well as from neighboring Thailand, scientists lack the information to even assess what kind of habitat the primate prefers. The hills include both deciduous and coniferous trees, but no one knows where the langurs lurk—or if they even still survive in the wild. Because there is still good forest cover remaining in the area, especially on the Myanmar side, biologists remain hopeful.

Miss Waldron’s Red Colobus came close to claiming the dubious honor of being the first primate to be declared extinct in more than 500 years after repeated failed searches. But evidence from hunters appeared in 2000 and 2001 that suggested that a very small number of these monkeys may be living in the southeast corner of Côte d'Ivoire. It was first discovered in 1933 by a British museum collector who named it after a colleague on the expedition, Miss F. Waldron, with the last conclusive sighting of the monkey in 1978. No photographs or video of the species alive exist.

Although the quest to find Miss Waldron’s Red Colobus Monkey with partners the Swiss Center for Scientific Research in Côte d’Ivoire and Florida Atlantic University has not yet uncovered photographic evidence of the monkey in question, it has produced striking footage of other rare and endangered primates. This includes the Critically Endangered White-thighed Colobus, Endangered White-naped Mangabey, and the first-ever video of a Critically Endangered Roloway Monkey in the wild, captured high in the canopy of Côte d'Ivoire’s Tanoé-Ehy Forest in west Africa.