The boomslang (, , or ; Dispholidus typus) is a large, highly venomous snake in the family Colubridae.
Taxonomy and etymology
Its common name means "tree snake" in Dutch and Afrikaans – boom meaning "tree", and slang meaning "snake". In Afrikaans, the name is pronounced . 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.
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
The average adult boomslang is in total length. 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 are very narrow, oblique, strongly keeled, with apical pits, arranged in 19 or 21 rows. 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.
Geographic range
The boomslang is endemic to sub-Saharan Africa. It is found in South Africa, Eswatini, Mozambique, Botswana, Namibia and north through sub-Sahara Africa.
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. Boomslangs are not restricted to trees and can often be found on the ground to hunt, feed, or take shelter. They will occasionally hide underground when the weather is harsh.
Reproduction
The boomslang is oviparous, and an adult female can produce up to 30 eggs, 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 in length and pose no threat to humans, but are dangerously venomous by the time they reach a length around and a girth as thick as an adult's smallest finger.
Behaviour and diet
D. typus is diurnal and almost exclusively arboreal. It is reclusive, and moves from branch to branch when pursued by anything too large to eat. Its diet includes chameleons and other arboreal lizards, frogs, and occasionally small mammals, birds, and eggs from nesting birds and reptiles, all of which it swallows whole. Boomslangs will also feed on other snakes, including cannibalising members of their own species. During cool weather, boomslangs brumate for short periods, often curling up inside the enclosed nest of a weaverbird.
Venom
Many venomous members of the family Colubridae are harmless to humans because of small venom glands and inefficient fangs. However, the boomslang is a notable exception in that it has a highly potent venom, which it delivers through large fangs located in the back of the jaw. The boomslang is able to open its jaws up to 170° when biting. The venom of the boomslang is primarily a hemotoxin; it works by a process in which so many small clots form in the blood that the victim loses the ability to clot further and the victim may die as a result of internal and external bleeding. The venom has been observed to cause haemorrhaging into tissues such as muscle and the brain while at the same time clogging capillaries with tiny blood clots. Other signs and symptoms include headache, nausea, sleepiness, and confusion.