The twig snakes, also commonly known as bird snakes or vine snakes, are a genus of rear-fanged venomous snakes in the family Colubridae. The genus is native to Africa. All species in the genus have a slender and elongated profile, a long tail, a narrow head, and a pointed snout. The eye of all species has a horizontal pupil, shaped like a keyhole, which gives twig snakes binocular vision. Twig snakes are greyish-brown with faint light and dark markings. When threatened, they inflate the throat to display bold black markings between the scales. Twig snakes are deadly: the venom is hemotoxic, affecting the blood clotting mechanism and causing uncontrolled bleeding and internal hemorrhaging. Bites by twig snakes have caused death in humans; famous herpetologist Robert Mertens died after being bitten by his pet savanna vine snake (Thelotornis capensis). However, envenomed bites are extremely rare when not handling the snake, as the fangs can't reach the skin except in a few places like the web between the thumb and fingers.