Hagen’s Pit Viper (Trimeresurus hageni) — Southern Thailand’s Big Green Bamboo Viper

Hagen’s Pit Viper is the biggest of the green bamboo pit vipers we run into in southern Thailand. Trimeresurus hageni (in some recent papers placed in the genus Parias) is named after Bernhard Hagen, the 19th-century German naturalist who first sent specimens back from Sumatra. In Thailand the species is restricted to the deep south, and we have found most of ours in mature lowland forest fragments and large rubber estates in Trang, Phang Nga and Krabi.

Field identification

This is a large pit viper for the genus. Adults regularly reach 90 cm and the largest females we have measured pushed 1.3 m, with weights nearly twice what a same-length White-Lipped weighs — a noticeably heavier-bodied snake. Dorsal colour is a clean leaf-green with a thin pale ventrolateral stripe (more conspicuous in males). The belly is yellowish-white. Tail is a rusty red-brown. The standout feature is the eye colour: hageni typically has a deep brick-red iris with a vertical pupil — much darker than the yellow-bronze eye of the White-Lipped or Pope’s. Once you have seen the red eye in good light, the ID is straightforward.

The other useful character is sheer bulk: a Hagen’s looks heavier than a White-Lipped of the same length, with a thicker neck and a larger triangular head. Mid-body scales are 21 rows, ventrals around 175–185 in our southern Thai material.

Range and habitat

Hagen’s Pit Viper is a Sundaic species. The Thai range is restricted to the southern peninsula, mostly south of Surat Thani, with the densest records from Trang, Phang Nga, Krabi, Satun, Yala and Narathiwat. From there it ranges south through Malaysia into Sumatra and the small islands. It is more reliant on mature forest than the White-Lipped, and disappears quickly when forests are converted to short-rotation crops. We see them in mature rubber estates and lowland evergreen patches, but rarely in oil-palm or open agriculture.

Activity is strictly nocturnal. They climb readily and often hunt 1–4 m off the ground on bamboo or low tree branches. Wet, warm nights after rain are when most encounters happen.

Behaviour, diet and reproduction

Diet is dominated by frogs and lizards but a Hagen’s at full size eats noticeably bigger prey than a White-Lipped — small mammals (rodents, tree shrews) make up a meaningful share of the diet, and we have seen evidence of bird predation in roosting birds taken from the same low branches. The hunting strategy is standard pit-viper ambush. The species is calm in temperament; we have moved several adults with hook and tube without a strike.

Reproduction is ovoviviparous. Females give birth to 13–25 live young in late wet season — clutches are larger than for most other Thai green pit vipers, in line with the species’ larger body size. Newborns are about 22 cm.

Bite and venom

The venom is broadly similar to that of the rest of the green pit viper group — strong local pain, swelling, bruising, blister formation, and a slowly developing coagulopathy. The fact that hageni is a bigger snake matters clinically: the venom yield per bite is higher, the bite holes are further apart, and the local tissue damage tends to be worse. Mortality is still low (under 1% with antivenom and modern care) but the rate of permanent tissue loss is higher than for smaller bamboo vipers. Thai polyvalent antivenom covers the bite.

If you are working at night in southern Thai forest or rubber, the calculation changes from “is it a viper?” to “is it a Hagen’s-sized viper?”. The same first-aid applies regardless: immobilise, do not cut/tourniquet, get to a hospital. Our piece on avoiding snakebites in Thailand has the practical prevention checklist; the Malayan Pit Viper profile is also worth reading because that is the other big-bite viper of southern Thailand, with a meaningfully different envenomation pattern.

If you find one

Stay back two metres, identify positively, and walk past. Hagen’s are not aggressive but they are big and they hold their position. If the snake is on a trail, the right move is almost always to step around it on the same trail rather than try to move the animal. Hagen’s are slow to leave a perch they have chosen for the night.

If the snake is in a building or close enough to people that removal is needed, call professional rescue. The volunteer fire brigade in most southern provinces handles snake removal — see our list of snake removal phone numbers across Thailand. Do not handle a Hagen’s without proper hooks and tubes; the larger size makes hand-handling specifically risky.

External references: the Reptile Database entry for Trimeresurus hageni for the latest taxonomic placement, and the IUCN Red List assessment — the species is listed Least Concern across its global range but local populations are much less stable in the most heavily converted Thai lowlands.

Quick reference card

• Where most often encountered: See the range and habitat section above. Encounter rates rise sharply during the species’ active season — for most Thai snakes, this is the wet season (May–November) with a smaller secondary peak around the end of the cool months.
• Activity period: Whether the snake is diurnal, nocturnal or crepuscular shapes the practical encounter risk. Nocturnal species are more often missed in the dark; diurnal species are more often photographed clearly.
• Bite risk to humans: Determined by whether the species is venomous, how readily it bites when disturbed, how often it is encountered in human-modified landscape, and how potent its venom is. The combination matters more than any single factor.
• Best behaviour on encounter: Stand back, photograph from a respectful distance (two metres or more), do not handle, and let the snake leave under its own power. The great majority of Thai snake encounters resolve themselves without intervention if the human steps back.

Frequently asked questions

Is this species protected under Thai law?

Many Thai snakes are protected under the Wild Animal Reservation and Protection Act. King Cobras, Burmese Pythons, Reticulated Pythons and several smaller species are explicitly listed; killing or trading these species is technically a criminal offence even when enforcement is uneven. For other species the legal status is more permissive, but local rules vary by province and protected-area designation. When in doubt, do not kill — call the volunteer fire-brigade rescue team for free relocation.

What should I do if my pet was bitten?

Take the pet to a veterinarian immediately. Veterinarians in Thailand have access to the same antivenoms used for humans, and treatment success in dogs and cats is reasonable when the bite is recognised quickly. Do not waste time on folk remedies. Photograph the snake from a safe distance if you can — the species ID will help the vet pick the correct antivenom.

How can I keep this species out of my garden?

Three things reduce snake encounters in a garden setting: cut grass and dense ground cover short, store firewood and outdoor materials elevated rather than ground-piled, and reduce rodent populations (snakes follow rats). Lighting walking paths after dark also helps prevent foot-on-snake encounters. None of these are perfect — wild snakes will still pass through — but together they substantially reduce the chance of an encounter.

Are juveniles as dangerous as adults?

For venomous species, yes — juveniles are venomous from birth and the venom is the same potent toxin as in adults. The dose per bite is smaller, but small doses of potent venom can still be life-threatening. There is also a folk-belief that juveniles “cannot control” their venom delivery and inject more per bite than an adult; the evidence for this is mixed but the practical lesson is to treat juveniles with the same caution as adults.