Vipers are a family of venomous snakes found all over the world. They use their long fangs to inject haemotoxic venom into their victim, which slowly kills the victim (necrosis, or tissue death) and also helps to break down the meat. The snake returns after the kill to eat its meal.

Pit vipers are the largest subfamily of vipers (the second largest subfamily is the 'true vipers', of which none (in particular the deadly Russell's viper) live in Sumatra). They are called pit vipers, not because they live in pits, but because they have pits between their eye and nostril on each side of their head. The twin pits are heat-sensitive and allow the snake to locate prey in darkness (nearly all species hunt by night) by heat, with pinpoint accuracy.

There are a dozen pit viper species currently known in Sumatra, of which four are endemic (Sumatran palm pit viper, Brongersama's pit viper, Gunalen's pit viper and the Toba pit viper), and there is also one endemic subspecies of Pope's pit viper (the barat bamboo pit viper). Most viper species in Sumatra are predominatly green pit vipers. The green pit vipers in Sumatra consist of the following seven species, each of which may appear to be simply a 'green viper', although female Wagler's pit vipers are not green, and Sumatran/Gunalen's pit vipers have black stripes (although these might not be noticeable in juveniles):

• Hagen's pit viper (Trimeresurus hageni), found in lowland forest.
• The white-lipped pit viper (Trimeresurus albolabris), found at 0-1200 metres across SE Asia, Nepal and India. It is a common cause of snakebites in mainland SE Asia but is believed to be rare and/or localised, in Sumatra, and might only inhabit areas close to Java.
• The barat bamboo pit viper (Trimeresurus popeorum barati) - a subspecies of Pope's viper endemic toWest Sumatra (Sumatera barat) and the Mentawai islands
• The Toba pit viper (Trimeresurus toba) - once believed to be a barat pit viper, subsequent analysis showed that it is a distinct species, and another Sumatran endemic. Originally collected at Balige, near Lake Toba, but found also in West Sumatra. It is the montane counterpart to the barat pit viper, living at higher altitudes.
• the Sumatran pit viper (Trimeresurus sumatranus), which inhabits low-land forest.
• Gunalen's pit viper, (Trimeresurus gunaleni), which similar to the barat and Toba pit vipers, is the montane counterpart to the Sumatran pit viper, and once thought to be the same species. Like the Toba pit viper, it differs in appearance in minor ways from the lowland species.
• Wagler's pit viper (Tropidolaemus wagleri), found in lowland forest

A green pit viper antivenom is available in Indonesia, though not necessarily at Bukit Lawang. None of the green pit viper species in Sumatra are considered to be killers, but all are dangerous and medical attention should be sought in the case of a bite.

The other Sumatran pit vipers are all predominantly brown:

• the shore (mangrove) pit viper (Trimeresurus purpureomaculatus), restricted to coastal regions.
• The Indo-malayan mountain pit viper (Ovophis monticola convictus), found at 800-2000 metres
• The Javanese palm pit viper, Trimeresurus puniceus found in southern Sumatra (close to Java), at altitudes 0 - 1200 metres.
• Brongersma's palm pit viper, Trimeresurus brongersmai, found in the Mentawai islands, in lowland forest
• The Sumatran palm pit viper, Trimeresurus andalasensis, found in North Sumatra, West Sumatra, and Aceh, at 500-1200 metres.

Wagler's pit viper, Tropidolaemus wagleri, is a snake found in Malaysia, southern Thailand and Sumatra. They live in trees, and mostly feed on geckos, though females (which are larger) may also eat birds, frogs and rodents (squirrels, rats).

Locally in Bukit Lawang they are usually called 'ular bulan', or moon snake. This is because they do not move for long periods, feeding only around once a month, and then only after dark, striking out to attack passing prey (ambush hunting). There is no truth in the local folklore that, because the snakes appear not to move for long periods, they fed by birds!

Although they might appear to be very docile, their pits allow them to sense passing prey and strike out almost instantaneously. This means that you should not get within striking distance (they do not jump, but they can strike - consider a range of up to half-a-metre), particularly considering that the snakes are arboreal and may be at head height for a human. Bites of humans are common, and painful, but generally not serious (causing swelling and perhaps localised necrosis), although complications and death cannot be ruled out. Treatment protocol is available here,. The Wagleri's viper produces several unique peptides, known as wagerlins, unrelated to other known proteins and toxins, which cause respiratory paralysis in small mammals such as mice. Some anti-ageing creams use synthetic Wagerlin-1, as a form of 'botox' intended to paralyse facial muscles, resulting in reduced wrinkles.

Female Wagler's pit vipers grow up to 1 metre long, but the males only grow to half-a-metre. Young snakes, as well as adult males, are plain green with small spots, as per the image below. Females, meanwhile, are green to greenish black, with yellow stripes. Females give birth to several dozen live young.

Mature female Wagler's pit viper, at Bukit Lawang.

Juvenile Wagler's pit viper, at Bukit Lawang. You can see the prehensile tail, adapted to grip trees.

Female Wagler's pit viper, at Bukit Lawang. Different colouring.