Wirot’s Palm Viper (Trimeresurus wiroti) — Southern Thailand’s Forgotten Pit Viper

Wirot’s Palm Viper is one of those species nobody outside herpetology had heard of until the 2000s. Trimeresurus wiroti was named in 1989 for the late Thai herpetologist Wirot Nutaphand, who collected the type specimens from the rubber-and-palm landscape of southern Thailand. The species is restricted to the southern peninsula and adjacent parts of Malaysia, where it lives in lowland forest, oil palm and rubber plantations, and is sometimes overlooked because it is easily mistaken for a green form of one of the more familiar pit vipers.

How to recognise one

Adults reach 70–85 cm. The dorsum is a medium to dark olive-green, often noticeably darker than other Thai green pit vipers, and there is usually a series of faint paler crossbars or a pale lateral stripe along the body. The belly is greenish-yellow or whitish. The tail is a dull brown or greenish-brown rather than the bright rust of the White-Lipped. Eye colour is pale green to yellow with a vertical pupil. Body is moderately stout for the genus, head broad triangular, head distinct from neck.

The species is genuinely tricky to identify in life. Wirot’s overlaps in range with the White-Lipped (T. albolabris), Hagen’s Bamboo Viper (T. hageni) and the Sumatran Pit Viper. Practical hints: Wirot’s has darker green than albolabris and a darker tail; it lacks the pronounced white upper lip; and it tends to occupy oil palm and ground-litter habitat more than the more arboreal hageni. Final ID is sometimes only confirmed by scale counts (mid-body 21–23 rows, ventrals around 156–172).

Range and habitat

Wirot’s Palm Viper is a southern-Thailand specialist. Records run from Chumphon south through Surat Thani, Krabi, Phang Nga, Trang, Phuket, Songkhla and into Malaysia. None of the records we trust come from north of the Isthmus of Kra. The species occupies lowland evergreen forest, peat-swamp edge, rubber plantation, oil-palm plantation and disturbed secondary growth. Plantation workers who clear undergrowth around palm bases see them fairly regularly during the rains.

Activity is nocturnal and centred on warm wet evenings. They climb low branches and palm fronds but spend a fair share of time on or just above the ground, particularly in oil-palm habitat where the trunks themselves are not great hunting perches. Activity peaks in the early rainy season and again at the start of the cooler dry months.

Behaviour, diet and reproduction

Diet is mostly frogs and lizards taken from low foliage and ground litter, with a smaller share of small mammals and the occasional bird. Hunting is the standard pit-viper ambush. They are calm in temperament — we have removed adults from house gardens with very little defensive display. The strike, when it comes, is fast and accurate within range.

Reproduction is ovoviviparous; females give birth to 7–13 live young in late wet season, usually November and December in southern Thailand. Newborns are about 17 cm and venomous from birth. Because the southern wet season runs longer than the northern, juvenile encounters can continue into January.

Venom and bites

Bites from T. wiroti are uncommon in the medical literature simply because the species occupies a relatively narrow range. The clinical picture matches the rest of the green pit viper group: severe local pain, swelling, bruising, occasional blistering, and a slowly developing coagulopathy that can take 24–72 hours to manifest. There are no documented fatalities at the time of writing, but tissue damage and prolonged morbidity have been reported. Thai polyvalent antivenom (Queen Saovabha Memorial Institute) is the treatment of record; clinicians manage the case primarily on the laboratory picture.

If you are working in oil-palm or rubber in the south, watch where you put your hands when clearing undergrowth, and use a torch on tracks at night. The species’ calm temperament is its only saving grace; it will not chase you, but a thumb pressed into a hidden viper is the same outcome regardless of what the snake intended. Our reference page on avoiding snakebites in Thailand has the full prevention checklist.

Conservation and where this fits

The species’ conservation status is harder to pin down than for Thailand’s common pit vipers because the records are few. The IUCN currently treats T. wiroti under “Least Concern” but with the caveat that population trends in the heavily transformed lowland palm landscape are poorly known. We see the species adapt reasonably well to oil-palm landscape; the bigger threats are likely habitat conversion to short-rotation crops and direct killing by workers who do not recognise the species.

If you find one in the south, photograph it from a respectful distance and consider posting the record to iNaturalist — restricted-range species like this one benefit enormously from citizen-science observations. For comparison with the rest of the southern green pit viper community, our common Thailand venomous snakes overview covers the lookalike species.

External references: the Reptile Database entry for Trimeresurus wiroti for taxonomy and the IUCN Red List assessment for the most recent conservation review.

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.

Related on Thailand Snakes: how to identify snakes in Thailand, Thailand snakebite first-aid guide, where the snakes are in Thailand.