Merrem's hump-nosed pit viper

The Merrem's hump-nosed pit viper (Hypnale hypnale), also known as Merrem’s pit viper, is a venomous snake found in Sri Lanka and parts of India’s Western Ghats. It is one of the most common causes of snakebite in Sri Lanka, responsible for 22% to 77% of cases. Initially, it was believed to be a relatively harmless species, but in 1821, reports described swelling and bleeding from its bites in animals. Over time, additional studies revealed cases of kidney damage and even fatalities, leading to a better understanding of the severity of its venom. Despite being widely recognized for its medical significance today, detailed studies on its venom and effects only gained attention in the late 20th century.

Sri Lanka is home to three species of hump-nosed vipers: Hypnale hypnale, Hypnale nepa, and Hypnale zara. While H. hypnale is found throughout the country except in Jaffna, H. nepa is restricted to the central hills, and H. zara is confined to lowland rainforests and foothills of the southwestern wet zone. H. nepa and H. zara are endemic to Sri Lanka, whereas H. hypnale is also present in India. These snakes thrive in coconut, rubber, and tea plantations, increasing the likelihood of human encounters. Though they look quite similar, they can be distinguished by differences in their scale arrangements.

This viper has a typical stocky viper-like body with a broad head and grows to an average length of 30 to 45 cm (12 to 18 inches), though some sources suggest it can reach up to 60 cm (2 feet). Its snout is sharply pointed and curves slightly upward, forming a distinct hump. The scales on the head vary, with large shields on the frontal and parietal regions and smaller, irregular ones on the snout. The body is usually grayish with dark brown blotches along the back, while the belly has a mottled yellowish or brown appearance. A unique feature is its brightly colored tail tip, often yellow or reddish, which may be used to lure prey.