The Green Keelback (Macropisthodon plumbicolor, with some regional variants classified under related genera) is a slender, non-venomous snake that inhabits the wetlands, rice paddies, and forested waterways of South and Southeast Asia. In Sri Lanka, it is a familiar resident of marshy lowlands and slow-moving river margins, where its leaf-green coloration renders it almost invisible against aquatic vegetation. Despite its wide distribution, the species is rarely aggressive and poses no threat to humans.
What does the Green Keelback look like?
Adults typically measure between 60 cm and 1 metre in length, with some individuals reaching around 1.2 metres. The dorsal scales are a rich green — varying from pale lime to deep olive — while the belly is yellowish or cream-white. Faint dark markings may run along the flanks, though the overall body surface appears relatively smooth to the eye.
The name "keelback" refers to a structural feature rather than colour: each dorsal scale carries a raised central ridge, or keel, that gives the skin a subtly textured appearance. These keels improve grip during climbing and provide additional purchase when swimming. The head is moderately triangular and clearly distinct from the neck, with large, round eyes positioned on the sides of the skull — an adaptation that broadens the visual field for hunting in dense, waterlogged vegetation.
Where does the Green Keelback live in Sri Lanka?
Across Sri Lanka the species favours the wet and intermediate zones, particularly wherever standing or slow-moving water is available. Rice paddies, irrigation channels, river margins, pond edges, and marshy forest clearings all provide suitable habitat. Sightings are most frequent in the lowland wet zone of the south-west, though the snake also occurs in the drier north and east wherever wetland pockets exist.
National parks that protect significant wetland ecosystems offer some of the best opportunities to observe keelback snakes in their natural setting. Udawalawe National Park, with its large reservoir and surrounding grassland marshes, shelters a diverse reptile community that includes this species. Riverine habitats such as the Madu Ganga estuary in Balapitiya are also excellent locations, where mangrove-lined backwaters provide the combination of water and dense cover the snake prefers.
What does the Green Keelback eat?
The diet is largely determined by proximity to water. Frogs and tadpoles form the core of its menu, supplemented by small fish, aquatic invertebrates, and occasionally lizards or small rodents encountered on land. The Green Keelback is an ambush hunter: it waits motionless near the water's edge or partially submerged among reeds, then strikes rapidly when prey moves within range. Its large eyes are well suited to detecting movement in low-light and cluttered environments.
This predatory role has measurable ecological value. By regulating frog and tadpole populations, the species contributes to the balance of aquatic food webs — including in the rice-field ecosystems on which Sri Lankan agriculture depends. Paddy fields (kumbura) across the island serve as de facto wildlife corridors for wetland snakes as well as for the frogs that sustain them.
How does the Green Keelback behave when threatened?
The species' first response to a perceived threat is retreat — typically into water, where it swims with confident lateral undulations, the slender tail acting as a rudder. If cornered on land, it may flatten its body, raise its head slightly, and release a foul-smelling musk from cloacal glands; this chemical deterrent is the primary defence in the absence of venom. Biting is a last resort and, even when it occurs, causes no significant harm to humans beyond minor scratching.
Green Keelbacks are diurnal to crepuscular, most active in the cooler morning and late-afternoon hours. During the hottest part of the day they often rest coiled under bankside vegetation or partially immersed at the water's edge.
Is the Green Keelback venomous?
No. The Green Keelback is classified as a non-venomous colubrid. It lacks venom glands and specialised fangs. Some closely related keelback species in Asia possess mild rear-fanged venom effective against amphibian prey, but even these are considered harmless to people. The Green Keelback should not be confused with the Sri Lankan Green Pit Viper (Trimeresurus trigonocephalus), which is venomous, arboreal, and distinctly stouter with a broad, triangular head.
How does the Green Keelback reproduce?
Like most colubrids, the Green Keelback is oviparous — it lays eggs rather than bearing live young. After mating, the female selects a concealed, humid location such as a rotting log, a crevice beneath a boulder, or a deep leaf-litter hollow. A typical clutch contains several elongated, leathery eggs. The hatchlings are independent from birth and miniature replicas of the adults in coloration and body form.
What is the conservation status of the Green Keelback?
The species is not currently listed as threatened globally, but wetland degradation poses a long-term concern. Drainage of rice paddies for development, pesticide use that reduces amphibian prey, and road mortality near irrigation channels all affect population health. In Sri Lanka, protection of forest reserves and riparian corridors — including the Sinharaja Forest Reserve, one of the island's most biodiverse wet-zone ecosystems — indirectly safeguards the moist-habitat snake fauna alongside better-known flagship species.
Responsible wildlife watching, avoiding handling wild snakes, and reporting sightings to local herpetology groups all contribute to better population monitoring for this and other reptile species.
How can I see a Green Keelback in Sri Lanka?
Patient observation near slow-moving water in the early morning offers the best chance. Guided nature walks in wetland areas, river safaris, and paddy-field excursions all increase the likelihood of an encounter. A Madu River Safari through the mangrove waterways of the south-west coast regularly turns up a variety of reptiles, including water snakes, monitor lizards, and — with a little luck — keelbacks resting on exposed roots. Carrying binoculars and moving quietly along the bank is more productive than searching actively through vegetation.