Bitis gabonica

Common names: Gaboon viper, butterfly adder, forest puff adder, swampjack,^[2] (more).

Bitis gabonica is a venomous viper species found in the rainforests and savannas of Sub-Saharan Africa.^[1] This is not only the largest member of the genus Bitis,^[3] but also the world's heaviest viperid^[2] and it has the longest fangs and the highest venom yield of any venomous snake.^[2] Two subspecies are currently recognized, including the nominate race described here.^[4]

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Description

Adults average 122–152 cm in length with a maximum of 205 cm 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. One female had the following dimensions:^[2]

  In their description of B. gabonica, Spawls et al. (2004) give an average length of 80–130 cm, with a maximum size of 175 cm, saying the species may possibly grow larger still. They acknowledge reports of specimens "over 6 feet", or even over 2 m in length, but claim there is no evidence to support this.^[5]

The head is large and triangular, while the neck is greatly narrowed: almost one-third the width of the head.^[2] A pair of horns is present between the raised nostrils — tiny in B. g. gabonica, but much larger in B. g. rhinoceros.^[5] The eyes are large and moveable,^[2] set well forward,^[5] and surrounded by 15–21 circumorbital scales.^[2] There are 12–16 interocular scales across the top of the head. 4–5 scale rows separate the suboculars and the supralabials. There are 13–18 supralabials and 16–22 sublabials.^[2] The fangs may reach a length of 55 mm:^[3] the longest of any venomous snake.^[2]

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.^[2]

The color pattern consists of a series of pale, sub-rectangular blotches running down the center of the back, interspaced with dark, yellow-edged hourglass markings. The flanks have a series of fawn of brown rhomboidal shapes, with light vertical central bars. The venter 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.^[5] The iris color is cream, yellow-white, orange^[5] or silvery.^[6]

Common names

Gaboon viper, butterfly adder, forest puff adder, swampjack,^[2] Gaboon adder,^[3] Gabon viper.^[7]

Originally a name given by the Portuguese, Gaboon (Gabão) refers to the estuary on which the town of Libreville was built, in Gabon, and to a narrow strip of territory on either bank of this arm of the sea. Currently, Gaboon refers the northern portion of French Congo, south of the Equator and lying between the Atlantic ocean and 12° East longitude.^[8]

Geographic range

 This species can be found in Guinea, Ghana, Togo, Nigeria, Cameroon, DR Congo, Central African Republic, southern Sudan, Uganda, Kenya, eastern Tanzania, Zambia, Malawi, eastern Zimbabwe, Mozambique, northeast KwaZulu-Natal Province in South Africa. The type locality given as is "Gabon" (Africa).^[1]

Mallow et al. (2003) also list Sierra Leone and Liberia in West Africa.^[2]

Habitat

Usually found in rainforests and nearby woodlands, mainly at low altitudes,^[6] but sometimes as high as 1500 m.^[2] Spawls et al. (2004) mention a maximum altitude of 2100 m.^[5]

In Tanzania, this species is found in secondary thickets, cashew plantations, and in agricultural land under bushes and in thickets. In Uganda, they are found in forests and nearby grasslands. They also do well in reclaimed forest areas: cacao plantations in West Africa and coffee plantations in East Africa. They have been found in evergreen forests in Zambia. In Zimbabwe, they only occur in areas of high rainfall along the forested escarpment in the east of the country. In general they may also be found in swamps, as well as in still and moving waters. They are commonly found in agricultural areas near forests and on roads at night.^[2]

Behavior

Primarily nocturnal, they have a reputation for being slow-moving and placid. They usually hunt by ambush, often spending long periods motionless, waiting for suitable prey to happen by. On the other hand, they have been known to hunt actively, mostly during the first six hours of the night. In Kumasi, Ghana, they were regularly killed around some stables in an open area with the forest some 500 m away — a sign that they were hunting rats in the grassland. They are usually very tolerant snakes, even when handled, and rarely bite or hiss. However, bad-tempered individuals do occur.^[5]

Locomotion is mostly rectilinear, in a sluggish "walking" motion of the ventral scales. They may writhe from side to side when alarmed, but only for short distances.^[2] Ditmars (1933) even described them as being capable of sidewinding.^[9]

If threatened, they may hiss loudly as a warning, doing so in a deep and steady rhythm, slightly flattening the head at the expiration of each breath.^[9][2][5] They are unlikely to strike unless severely provoked.^[2]

There have been numerous descriptions of their generally non-aggressive nature. Sweeney (1961) wrote that they are so docile that they "can be handled as freely as any non-venomous species", although this is absolutely not recommended. In Lane (1963), Ionides explained that he would capture specimens by first touching them lightly on the top of the head with a pair of tongs. Anger was rarely displayed, so that the tongs were usually set aside and the snake firmly grasped by the neck with one hand and the body supported with the other. He said the snakes hardly ever struggled.^[2]

Parry (1975) describes how this species has a wider range of eye movement than other snakes. Along a horizontal plane, eye movement can be maintained even if the head is rotated up or down to an angle of up to 45°. If the head is rotated 360°, one eye will tilt up and the other down, depending on the direction of rotation. Also, if one eye looks forwards the other looks back, as if both are connected to a fixed position on an axis between them. In general, the eyes often flick back and forth in a rapid and jerky manner. When asleep, there is no eye movement and the pupils are strongly contracted. The pupils dialate suddenly and eye movement resumes when the animal wakes up.^[2]

Feeding

Generally, these snakes are sedentary opportunistic ambush predators, but they will hunt actively, usually starting at dusk. Following a strike, they tend to hold on to their prey until it is dead. Prey is also lifted off of the ground to prevent it getting hold of anything. Anything large enough to pose more of a threat is released and searched for after a few minutes.^[2]

These snakes feed on a variety of birds and mammals, such as doves, many different species of rodents, hares and mongooses. There are also reports of more unlikely prey items, such as monkeys, the brush-tailed porcupine (Atherurus) and even the small royal antelope (Neotragus).^[2]

Reproduction

During peak sexual activity, males engage in combat. This starts with one male rubbing its chin along the back of the other. The second male will then raise its head as high as possible. As they both do the same, the necks intertwine. When the heads are level, they turn towards each other and push. Their bodies intertwine as they switch positions. They become oblivious to everything else, continuing even after they fall off of a surface or into water. Sometimes they intertwine and squeeze so tightly that their scales stand out from the pressure. They have also been observed to strike at each other with mouths closed. Occasionsally, the combatants will tire and break off the fight by "mutual consent", resting for a while before resuming once more. The event is settled when one of the two succeeds in pushing the other's head to the ground and raising its own by 20–30 cm. In captivity, combat may occur 4–5 times a week until courtship and copulation ends.^[2]

Gestation takes about a year, which suggests a breeding cycle of 2–3 years. A 5-year breeding cycle may also be possible. Usually, they give birth in late summer. B. g. gabonica produces 8–43 live young. B. g. rhinoceros may produce as many as 60. However, the actual number of offspring rarely exceeds 24.^[2] Neonates are 25–32 cm in length and weigh 25–45 g.^[3]

Venom

Bites are relatively rare, due to their docile nature and the fact that their range is mainly limited to rainforest areas. Nevertheless, when it does occur it should always be considered a serious medical emergency. Even an average bite from an average sized specimen is potentially fatal.^[3]

The venom itself is not considered particularly toxic. In mice, the LD₅₀ is 0.8–5.0 mg/kg IV, 2.0 mg/kg IP and 5.0–6.0 mg/kg SC.^[10] However, the venom glands are enormous and produce the largest quantities of any venomous snake. Yield is apparently related to body weight, as opposed to milking interval.^[2] Brown (1973) gives a venom yield range of 200–1000 mg (of dried venom),^[10] A range of 200–600 mg for specimens 125–155 cm in length has also been reported.^[2] Spawls and Branch (1995) state that from 5–7 ml (450–600 mg) of venom may be injected in a single bite.^[3]

A study by Marsh and Whaler (1984) reported a maximum yield of 9.7 ml of wet venom, which translated to 2400 mg of dried venom. They attached "alligator" clip electrodes to the angle of the open jaw of anesthetized specimens (length 133–136 cm, girth 23–25 cm, weight 1.3–3.4 kg), yielding 1.3–7.6 ml (mean 4.4 ml) of venom. Two to three electrical bursts of five seconds each were enough to empty the glands. The snakes used for the study were milked 7–11 times over a 12-month period, during which they remained in good health and the potency of their venom remained the same.^[2]

Based on how sensitive monkeys were to the venom, Whaler (1971) estimated that 14 mg of venom would be enough to kill a human being: equivalent to 0.06 ml of venom, or 1/50–1/1000 of what can be obtained in a single milking. Branch (1992) suggested that 90–100 mg would be fatal in humans. Marsh and Whaler (1984) wrote that 35 mg would be enough to kill a man of 70 kg (1/30 of the average venom yield).^[2]

In humans, a bite causes rapid and conspicuous swelling, intense pain, severe shock and local blistering. Other symptoms may include uncoordinated movements, defecation, urination, swelling of the tongue and eyelids, convulsions and unconsciousness.^[2] Blistering, bruising and necrosis may be extensive. There may be sudden hypotension, heart damage and dyspnoea.^[5] The blood may become incoagulable with internal bleeding that may lead to haematuria and haematemesis.^[5][3] Local tissue damage may require surgical excision and possibly amputation.^[3] Healing may be slow and fatalities are not uncommon.^[5]

Subspecies

Taxonomy

Lenk et al. (1999) discovered considerable differences between the two conventionally recognized subspecies of B. gabonica described above. According to their research, these two subspecies are as different from each other as they are from B. nasicornis. Consequently, Lenk et al. (1999) regard the western form as a separate species, B. rhinoceros.^[11]

See also

• List of viperine species and subspecies
• Viperinae by common name
• Viperinae by taxonomic synonyms
• Snakebite

References

Further reading