Hydrodynastes gigas is a New World species of large, rear-fanged, Dipsadidae snake endemic to South America. It is commonly and alternatively known as the false water cobra and the Brazilian smooth snake. The false water cobra is so named because when the snake is threatened it "hoods" as a true cobra (Naja species) does. Unlike a true cobra, though, it does not rear up, but remains in a horizontal position. No subspecies are currently recognized as being valid. Common names Hydrodynastes gigas is commonly referred to as the false water cobra, false cobra, South American water cobra, and Brazilian smooth snake. In South America, it is sometimes referred to as boipevassu. Additional South American common names include mboi-peba, ñacaniná, surucucu-do-pantanal, vibora ladradora (barking snake), and yacanina. Description H. gigas is a large dipsadidae snake that may exceed 3 m (9 to 10 ft) in total length (including tail) when adult. Most H. gigas snakes reach about 2 m (6 to 7 ft) in total length. It is of medium body, so is neither particularly heavy nor slender-bodied, Weights can average around and exceed in mature adults, with some specimens weighing . Males are typically much smaller than females in this species. The common name false water cobra is an allusion to its ability to flatten its neck, similar to a cobra, as a defensive reaction to make it look larger and more intimidating. Unlike the true cobra, though, the false water cobra stays in a horizontal position when it hoods, rather than rearing into a vertical position. H. gigas can flatten not only its neck, but also lower down its body, which is not possible for a true cobra. Additionally, the pattern and coloration of this Hydrodynastes species superficially resemble those of true water cobras (Boulengerina). The false water cobra has large eyes with circular pupils, allowing good daytime vision. The tongue is black, and of the typical snake fashion. The background colour of a mature specimen is an olive green or brown, with dark spots and bands covering much of its body. The background coloring and banding generally become darker towards the tail. This colouring gives the false water cobra effective camouflage in its natural rainforest environment. The ventral scales are yellow or brown, spotted with dark flecks that make three dotted lines, which appear to merge towards the tail. Mehrtens, 1987, suggested females are brown ventrally, whilst males