Bitis gabonica, commonly known as the Gaboon viper, is a venomous viper species found in the rainforests and savannas of sub-Saharan Africa. This is not only the largest member of the genus Bitis, but also the world's heaviest viperid, and it has the longest fangs (up to 2 inches), and the highest venom yield of any venomous snake. Two subspecies are currently recognized, including the nominate race described here.
The species is also commonly known as butterfly adder, forest puff adder, or swampjack, among others.
Adults average 122--152 cm (4 to 5 feet) in length with a maximum of 205 cm (81 in) for a specimen collected in Sierra Leone. The sexes may be distinguished by the length of the tail in relation to the total length of the body: approximately 12% for males and 6% for females. Adults, especially females, are very heavy and stout.
In their description of B. gabonica, Spawls et al.. (2004) give an average length of 80--130 cm (32 to 51.5 in), with a maximum size of 175 cm (69.3 in), saying the species may possibly grow larger still. They acknowledge reports of specimens over 6 feet (1.8 m), or even over 2 m (6.5 ft) in length, but claim there is no evidence to support this. A large specimen of exactly 1.8 m (5.9 ft), caught in 1973, was found to have weighed 11.3 kg (25 lb) with an empty stomach. Very large specimens may possibly weigh up to 20 kg (44 lb), which would rank them as the world's heaviest venomous snake ahead of the Eastern diamondback rattlesnake, but these masses are not known to have been verified.
The head is large and triangular, while the neck is greatly narrowed: almost one-third the width of the head. A pair of horns is present between the raised nostrils — tiny in B. g. gabonica, but much larger in B. g. rhinoceros. The eyes are large and moveable, set well forward, and surrounded by 15--21 circumorbital scales. There are 12--16 interocular scales across the top of the head. Four or five scale rows separate the suboculars and the supralabials. There are 13--18 supralabials and 16--22 sublabials. The fangs may reach a length of 55 millimetres (2.2 in) : the longest of any venomous snake.
Midbody, there are 28--46 dorsal scale rows, all of which are strongly keeled except for the outer rows on each side. The lateral scales are slightly oblique. The ventral scales number 124--140: rarely more than 132 in males, rarely less than 132 in females. There are 17--33 paired subcaudal scales: males have no fewer than 25, females no more than 23. The anal scale is single.
The color pattern consists of a series of pale, subrectangular blotches running down the center of the back, interspaced with dark, yellow-edged hourglass markings. The flanks have a series of fawn or brown rhomboidal shapes, with light vertical central bars. The belly is pale with irregular brown or black blotches. The head is white or cream with a fine, dark central line, black spots on the rear corners, and a dark blue-black triangle behind and below each eye. The iris color is cream, yellow-white, orange or silvery.
