Bitis gabonica
Gaboon viper
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 (5 cm), and the highest venom yield of
any venomous snake. Two subspecies are currently recognised, including the
nominate race described here.
The
species is also commonly known as butterfly adder, forest puff adder, or
swampjack, among others.
Description
Adults
average 122–152 cm (4 to 5 feet) in
total length (body + tail) with a maximum total length 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. One female had the following dimensions:
Total
length 174 cm (69 in)
Head
width 12 cm (4.7 in)
Girth
size (circumference) 37 cm (14.65 in)
Weight
(empty stomach) 8.5 kg (19 lbs)
B.
gabonica - note the tiny "horns" between the nostrils and the two
stripes below the eye.
In
their description of B. gabonica, Spawls et al.. (2004) give an average total
length of 80–130 cm (32 to 51.5 in), with
a maximum total length of 175 cm (69.3 in), saying the species may possibly
grow larger still. They acknowledge reports of specimens over 1.8 m (6 ft), or
even over 2 m (6.5 ft) in total length, but claim there is no evidence to
support this. A large specimen of exactly 1.8 m (5.9 ft) total length, 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 colour is cream, yellow-white, orange or silvery.
Common
names
This
snake's common names include Gaboon viper, butterfly adder, forest puff adder,
swampjack, Gaboon adder, and Gabon viper. Originally a name given by the
Portuguese, Gabon (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. As of 1909, Gaboon referred to the northern portion of French
Congo, south of the Equator and lying between the Atlantic Ocean and 12°E
longitude.
Geographic
range
Distribution
of B. gabonica (in black) This species can be found in Guinea, Ghana, Togo,
Nigeria, Cameroon, DR Congo, Central African Republic, South Sudan, Uganda,
Kenya, eastern Tanzania, Zambia, Malawi, eastern Zimbabwe, Mozambique, and
northeast KwaZulu-Natal Province in South Africa. Mallow et al. (2003) also
list Sierra Leone and Liberia in West Africa. The type locality is given as
"Gabon" (Africa).
Habitat
The
Gaboon viper is usually found in rainforests and nearby woodlands, mainly at
low altitudes, but sometimes as high as 1500 m. Spawls et al. (2004) mention a
maximum altitude of 2100 m. According to Broadley and Cock (1975), it is
generally found in environments that are parallel to those occupied by its
close relative, B. arietans, which is normally found in more open country.
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.
Behaviour
Primarily
nocturnal, Gaboon vipers 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 meters 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, unlike most vipers. However, bites by
bad-tempered individuals do occur.
An
ambush predator, its color pattern gives it excellent camouflage as evident
here. This is a B. g. rhinoceros (West African Gaboon Viper).
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. Ditmars (1933) even described them as being capable of sidewinding.
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. Despite
this, they are unlikely to strike unless severely provoked, however they are
one of fastest striking snakes in the world, so care should be taken in
handling them.
There
have been numerous descriptions of their generally unaggressive nature. Sweeney
(1961) wrote 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 he would capture specimens by first touching
them lightly on the top of the head with a pair of tongs to test their
reactions. Anger was rarely displayed, so the tongs were usually set aside and
the snakes firmly grasped by the neck with one hand and the body supported with
the other as he picked them up and carried them to a box for containment. He
said the snakes hardly ever struggled.
Parry
(1975) described 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 forward, 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
dilate suddenly and eye movement resumes when the animal wakes up.
Feeding
Because
of their large, heavy body size, the adults have no trouble eating prey as
large as fully grown rabbits. When prey happens by, they strike with very fast
precision from any angle. Once they strike their prey, they hang on to it with
their large fangs rather than letting it go and waiting for it to die. This
behaviour is very different from the behaviour of other species of vipers.
These snakes feed on a variety of birds and mammals, such as doves, many
different species of rodents, including field mice and rats, as well as hares
and rabbits. There are also reports of more unlikely prey items, such as tree
monkeys, the brush-tailed porcupine (Atherurus) and even the small royal
antelope (Neotragus).
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 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. Occasionally,
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 four or five times a week until courtship and copulation ends.
Gestation
takes about 7 months, which suggests a breeding cycle of two to three years. A
five-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. Neonates are
25–32 cm in length and weigh 25–45 g.
Venom
A
Gaboon viper showing its fangs
Bites
are relatively rare, due to their docile nature and because their range is
mainly limited to rainforest areas. Due to their sluggishness and unwillingness
to move even when approached, people are often bitten after they accidentally
step on them, but even then in some cases they may not bite. However, when a
bite does occur, it should always be considered a serious medical emergency.
Even an average bite from an average-sized specimen is potentially fatal.
Antivenom should be administered as soon as possible to save the victim's life
if not the affected limb.
The
snake's hemotoxic venom itself is not considered particularly toxic based on
tests conducted in mice. 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. However, the
venom glands are enormous and each bite produces the largest quantities of
venom of any venomous snake. Yield is probably related to body weight, as
opposed to milking interval. Brown (1973) gives a venom yield range of 200–1000 mg (of dried venom),[14]
A range of 200–600 mg for specimens 125–155 cm in length has also
been reported. Spawls and Branch (1995) state from 5 to 7 ml (450–600 mg) of venom may be
injected in a single bite.
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 within a space of five seconds apart were enough to empty the
venom glands. The snakes used for the study were milked seven to 11 times over
a 12-month period, during which they remained in good health and the potency of
their venom remained the same.
Based
on how sensitive monkeys were to the venom, Whaler (1971) estimated 14 mg of
venom would be enough to kill a human being: equivalent to 0.06 ml of venom, or
1/50 to 1/1000 of what can be obtained in a single milking. Marsh and Whaler
(1984) wrote that 35 mg (1/30 of the average venom yield) would be enough to
kill a man of 70 kilograms (150 lb). Branch (1992) suggested that 90–100 mg would be fatal in
humans. Due to the rarity of these type of snakebites, further investigation is
needed.
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. Blistering, bruising and necrosis may be extensive. There may
be sudden hypotension, heart damage and dyspnoea. The blood may become
incoagulable with internal bleeding that may lead to haematuria and
haematemesis. Local tissue damage may require surgical excision and possibly
amputation. Healing may be slow and fatalities during the recovery period are
not uncommon.
Subspecies Taxon: West African Gaboon viper West Africa
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.
