The boomslang ( or ; Dispholidus typus) is a highly venomous snake in the family Colubridae. The species is native to Sub-Saharan Africa.
Etymology
Its common name means "tree snake" in Dutch and Afrikaans – boom meaning "tree", and slang meaning "snake". In Afrikaans, the name is pronounced .
Taxonomy
The boomslang is a colubrid snake within the subfamily Colubrinae. It belongs to the genus Dispholidus, which contains two other species, D. pembae and D. punctatus.
The boomslang is thought to be closely related to members of the genera Thelotornis, Thrasops, Rhamnophis, and Xyelodontophis, with which it forms the taxonomic tribe Dispholidini.
Close relationships can be shown in the cladogram below:
Subspecies
Two subspecies are recognised, including the nominotypical subspecies.
- D. t. kivuensis
- D. t. typus
The trinomial authority in parentheses for D. t. typus indicates that the subspecies was originally described in a genus other than Dispholidus.
Description
thumb|250px|Raiding the communal nest of [[sociable weavers]]
The average adult boomslang is in total length (including tail). Some exceed . The eyes are exceptionally large, and the head has a characteristic egg-like shape. Colouration is highly variable. Males are light green with black or blue scale edges, but adult females may be brown demonstrating sexual dimorphism.
Weight varies from , with an average weight of .
In this species, the head is distinct from the neck and the canthus rostralis is distinct. The pupil of the very large eye is round. The boomslang has excellent eyesight and often moves its head from side to side to get a better view of objects directly in front. The maxillary teeth are small anteriorly, seven or eight in number, followed by three very large, grooved fangs situated below each eye. The mandibular teeth are subequal. The body is slightly compressed. The dorsal scales, which are arranged in 19 or 21 rows, are very narrow, oblique, strongly keeled, with apical pits. The tail is long, and the subcaudal scales are paired. Ventral scales are 164–201; the anal plate is divided; and the subcaudals are 91–131.
Habitat
The boomslang is an excellent climber and is highly arboreal, living mainly in forested areas. D. typus lives in karoo shrubs, savannahs, lowland forests, and in grasslands. The boomslang is not restricted to trees and can often be found on the ground hunting, feeding, or taking shelter. It will occasionally hide underground when the weather is harsh.
Reproduction
The boomslang is oviparous, and an adult female can produce on average, 8 to 14 leathery eggs and up to 27 have been observed, which are deposited in a hollow tree trunk or rotting log. The eggs have a relatively long (3 months on average) incubation period. Male hatchlings are grey with blue speckles, and female hatchlings are pale brown. They attain their adult colouration after several years. Hatchlings are about to in length all of which it swallows whole. The boomslang will also feed on other snakes, including cannibalising members of its own species. The venom of the boomslang is primarily a hemotoxin; it works via a process in which many small clots form in the blood, causing the victim's circulatory system to improperly coagulate, resulting in excessive bleeding and death. The venom has been observed to cause bleeding in tissues such as muscle and the brain (among other organs), while, at the same time, clogging capillaries with tiny blood clots. Other signs and symptoms include headache, nausea, sleepiness, and confusion, leading to cardiac arrest and unconsciousness.
Because boomslang venom is slow-acting, symptoms may not become apparent until many hours after a bite. Its median lethal dose (LD<sub>50</sub>) in mice is 0.1 mg/kg (intravenously). 0.071 mg/kg (IV) has also been reported. 12.5 mg/kg (subcutaneously) and 1.3–1.8 mg/kg (intraperitoneal). Based on the very low venom quantities produced by D. typus, and the very serious effects found in a good part of the reported cases in humans, it has been suggested that the venom's LD<sub>50</sub> is lower in humans than in mice, with only 2 to 3 mg being enough to potentially kill a healthy adult.
In 1957, herpetologist Karl Schmidt died after having been bitten by a juvenile boomslang, which he had doubted could produce a fatal dose. He made notes on the symptoms he experienced almost to the end. D. S. Chapman reported eight serious envenomations by boomslangs between 1919 and 1962, two of which were lethal. Up unto this point, many herpetologists believed that rear-fanged colubroid snakes were incapable of producing a lethal bite.
Boomslang monovalent antivenom was developed during the 1940s. The South African Vaccine Producers manufactures a monovalent antivenom for use in boomslang envenomations. Treatment of bites may also require complete blood transfusions, especially if over 24-48 hours have passed without antivenom.
The boomslang is a timid snake, and bites generally occur only when people attempt to handle, catch, pursue or kill the animal. When confronted and cornered, it inflates its neck and assumes an S-shaped striking pose, a key indicator of any snake species feeling threatened.