About Hydrodynastes gigas (Duméril, Bibron & Duméril, 1854)
Hydrodynastes gigas is a large dipsadine snake that can grow longer than 3 meters (10 feet), though most adult individuals reach roughly 2 meters (7 feet) in length. The average weight of this species is around 1.6 kilograms (3.5 pounds), while some fully mature specimens can weigh as much as 4.5 kilograms (9.9 pounds). This species displays sexual dimorphism: males are much smaller than females. The common name false water cobra refers to the species' ability to flatten its neck when threatened, a defensive behavior similar to that of true cobras that makes the snake look larger and more intimidating. Unlike true cobras, however, the false water cobra stays in a horizontal position when it forms its hood, instead of rearing up into a vertical stance. H. gigas can flatten not just its neck, but also the lower portion of its body, a trait that true cobras do not have. In addition, the pattern and coloration of this Hydrodynastes species look superficially similar to those of true water cobras in the genus Boulengerina. The false water cobra has large eyes with circular pupils, which supports good daytime vision, and its tongue is black. The background body color of a mature false water cobra is olive green or brown, with dark spots and bands covering most of the body. Background color and banding typically become darker closer to the tail. This coloration provides effective camouflage for the snake in its natural rainforest habitat. The ventral scales are yellow or brown, marked with dark flecks that form three dotted lines, which tend to merge as they approach the tail. Females have brown ventral scales, while males have yellow ventral scales. It has been suggested that females have lighter body bands and markings than males, but this is not a reliable way to sex H. gigas, since coloration varies slightly between all individuals. Hatchlings and juvenile snakes are much darker in color than adults, and do not have the characteristic dark eyes of mature individuals. Young H. gigas more closely resemble garter snakes or water snakes than they do mature members of their own species. Hypomelanistic H. gigas have been bred in captivity; these individuals vary in coloration, ranging from snakes with only slightly lighter saddles to those that are almost completely patternless. A melanistic population discovered in Tocantins, central Brazil, was originally described as a new Hydrodynastes species called H. melanogigas, but genetic evidence shows this population is only a melanistic color variant of H. gigas. H. gigas occurs in a geographic range that extends from eastern Bolivia to southern Brazil, and is also found in Paraguay and Argentina. H. gigas generally favors wet or humid habitats including marshlands and tropical rainforests, though it has also been observed in drier areas. Its preference for wetland habitats is what gives the species its common name of "false water cobra".
