How to Identify the Fan Si Pan Horned Pit Viper (Identification Guide)

Key identifying features

The Fan Si Pan Horned Pit Viper stands out among Asian pit vipers for the small, raised, hornlike scales projecting above each eye, a feature it shares with a handful of other "horned" pit viper species in the region. Beyond this distinctive trait, it displays the usual pit viper hallmarks: a broad triangular head, heat-sensing pits, and vertical pupils.

Coloration & pattern

The body coloration typically consists of brown, gray, or olive tones patterned with darker blotches or crossbands, providing camouflage against leaf litter and mossy forest substrates rather than the bright green seen in many lowland relatives. This more cryptic, earth-toned pattern is typical of high-elevation, terrestrial-leaning pit vipers. The belly is generally paler, often cream or grayish, sometimes with faint mottling.

Head, eyes & scales

The head is broad and triangular, clearly distinct from the neck, with the signature small hornlike scale projections above each eye that give this species its name. Heat-sensing pits are located between the eye and nostril on each side. The eyes have vertical pupils, typical of pit vipers, and the dorsal scales are keeled, adding to the rough, textured look consistent with a forest-floor and low-vegetation lifestyle.

Size & body shape

This species is generally small to moderate in size, with adults often reaching roughly 40 to 70 centimeters in total length. The body is relatively stout compared to slender arboreal green pit vipers, reflecting a more terrestrial or low-perching habit, though it retains some climbing ability.

Range & habitat where you'll see it

This species is associated with the Fan Si Pan massif and surrounding high-elevation areas of northern Vietnam, one of the highest points in mainland Southeast Asia. It inhabits cool, moist montane forest, often near leaf litter, mossy logs, and low vegetation, and is most active at night.

How to tell it apart from look-alikes

The hornlike supraocular scales are the most reliable field mark separating this species from most other regional pit vipers, since relatively few Asian pit vipers share this trait, and those that do tend to be geographically separated. Its cryptic brown-and-gray patterning, rather than bright green, also helps distinguish it from the many green arboreal pit vipers found at lower elevations nearby. Careful attention to the eye-horn feature, overall coloration, and known high-elevation locality together provide the strongest basis for field identification.