Eastern green mamba
|Eastern green mamba|
|Eastern green mamba geographic range|
The eastern green mamba (Dendroaspis angusticeps) is a highly venomous snake species of the mamba genus Dendroaspis native to the coastal regions of southern East Africa. First described by a Scottish surgeon and zoologist Andrew Smith in 1849, it has a slender build with bright green upperparts and yellow-green underparts. The adult female averages around 2.0 metres (6.6 ft) in length, and the male is slightly smaller.
A shy and elusive species, the eastern green mamba is rarely seen. This elusiveness is usually attributed to the species' green colouration which blends with its environment, and its arboreal lifestyle. It has also been observed to use "sit-and-wait" or ambush predation like many vipers, unlike the active foraging style typical of other elapid snakes. The eastern green mamba preys on birds, eggs, bats, and rodents such as mice, rats, and gerbils.
The eastern green mamba's venom consists of both neurotoxins and cardiotoxins. Symptoms of envenomation include swelling of the bite site, dizziness and nausea, accompanied by difficulty breathing and swallowing, irregular heartbeat and convulsions progressing to respiratory paralysis. Bites that produce severe envenomation can be rapidly fatal.
The eastern green mamba was first described as Naja angusticeps by a Scottish surgeon and zoologist, Andrew Smith in 1849, who reported it from Natal and east to Maputo Bay. The specific name angusticeps is derived from the Latin word angustus, "narrow", and ceps, an abbreviated form of caput ("head") when used in a compound word. The German-British zoologist Albert Günther described Dendroaspis intermedius from the Zambezi River in northern Mozambique in 1865. This was subsequently synonymised with D. angusticeps.
In 1896, the Belgian-British zoologist George Albert Boulenger combined the species Dendroaspis angusticeps with the black mamba (Dendroaspis polylepis), a lumping diagnosis that remained in force until 1946, when the South African herpetologist Vivian FitzSimons published a paper after examining approximately 50 eastern green mamba and 85 black mamba specimens. He concluded that the differences in build, scalation, coloration and behaviour warranted splitting them into separate species. The British biologist Arthur Loveridge augmented FitzSimons' work with material from outside South Africa, noting some overlap in scalation but supporting the separation. A 2016 genetic analysis showed the eastern green and black mambas are each others' closest relatives, their common ancestor diverging from a lineage that gave rise to Jameson's mamba (Dendroaspis jamesoni) and the western green mamba (Dendroaspis viridis).
In addition to being called the eastern green mamba, the species is also commonly known as the common green mamba, East African green mamba, white-mouthed mamba, or simply the green mamba.
The eastern green mamba is a large snake, with a slightly compressed and very slender body with a medium to long tapering tail. Adult males average around 1.8 metres (5 ft 11 in) in total length, while females average 2.0 metres (6 ft 7 in). This species rarely exceeds lengths of 2.5 metres (8 ft 2 in). In general, the total length is 4–4.3 times the length of the tail. The adult eastern green mamba has bright green upperparts—occasionally with isolated yellow scales—and a pale yellow-green belly. Sometimes they are duller-coloured before moulting. Juveniles are blue-green, becoming bright green when they are around 75 centimetres (2 ft 6 in) long. The coffin-shaped head is long and slender, with a prominent canthus which is slightly demarcated from the neck. When threatened or otherwise aroused, the eastern green mamba is capable of flattening its neck area, though no real hood is formed. The medium-sized eyes have round pupils, the borders of which have a narrow golden or ochre edge; the irises are olive green, becoming bright green posteriorly. The inside of the mouth may be white or bluish white.
Any green snake is often called a green mamba in southern Africa; other green species include green forms of the boomslang (Dispholidus typus), which can be distinguished by its larger eyes and short head. It is also venomous. Small eastern green mambas could be confused with green bushsnakes of the genus Philothamnus.
The number and pattern of scales on a snake's body are a key element of identification to species level. The eastern green mamba has between 17 and 21 rows of dorsal scales at midbody, 201 to 232 ventral scales, 99 to 126 divided subcaudal scales, and a divided anal scale.[a] Its mouth is lined with 7–9 supralabial scales above, with the fourth one located under the eye, and 9-11 sublabial scales below. Its eyes have 3 preocular and 3–5 postocular scales.
Distribution and habitat
This eastern green mamba is native to regions near the coastlines of southern Africa and East Africa. The eastern green mamba's range extends from Kenya south through Tanzania, Malawi, eastern Zimbabwe, and eastern Zambia. It can also be found in Zanzibar and northern Mozambique. An isolated and genetically distinct population is found in South Africa from the extreme northeastern part of Eastern Cape along the KwaZulu-Natal coastline and into southern Mozambique.
An elusive species, the eastern green mamba is primarily arboreal (living in trees), where it usually well camouflaged in the foliage. It is believed by some herpetologists that its habitat is limited to tropical rainforests in coastal lowlands, however, according to other experts, the eastern green mamba can also be found in coastal bush, and dune and montane forest. Unlike its close relative the black mamba, the eastern green mamba is rarely found in open terrain and prefers relatively dense, well-shaded vegetation. In addition to wild forest habitats, it is also commonly found in thickets and farm trees (such as citrus, mango, coconut, and cashew). In coastal East Africa it is known to enter houses and may even shelter in thatched roof dwellings. Specimens have been found at elevations up to 1,500 metres (4,900 ft) above sea level.
The eastern green mamba's conservation status has not been assessed by the International Union for Conservation of Nature (IUCN). It is, however, a fairly common species of snake throughout its range, and populations are believed to be stable. Large concentrations of two to three individuals per hectare have been documented in coastal Kenya and southern Tanzania, and in one instance a group of five eastern green mambas were seen in a single tree. Although populations of this species are stable overall, habitat destruction and deforestation pose a possible threat. In South Africa, it is rated as "vulnerable" as its habitat is highly fragmented and being transformed into coastal housing developments.
A diurnal species, the eastern green mamba is active by day and sleeps at night coiled up in foliage or rarely a hollow within the trunk or branch. An agile snake and an adept climber, it is not commonly found on the ground though may come down to bask in the morning sun (thermoregulation). In a study of the movement patterns of two adult specimens over a 27-day period, the researcher found their activity range areas to be very low, comparable to other predators who ambush prey rather than actively hunt. This is in contrast to most elapid species, including other mambas, who tend to actively hunt or forage for prey. The study's preliminary evidence sheds some light on this species' method of hunting prey and suggests that it may be an ambush predator due to the sit-and-wait behaviour displayed. However, this evidence does not preclude active foraging. A specimen systematically hunting a sleeping bat was observed.
There is no evidence that the eastern green mamba migrates. Thought to be relatively sedentary, it can remain in the same location for days at a time, apparently moving most commonly to find food or mates. On average, individuals move only about 5.4 metres (18 ft) per day. The eastern green mamba generally avoid confrontation with humans or other predators when possible, only biting if pressed.
Reproduction and lifespan
The eastern green mamba is solitary, except during breeding season. Gravid females tend to be sedentary, but males actively search out and court prospective mates during the rainy season—between April and June. Males have been observed engaging in agonistic behaviour and may fight each other over potential mating opportunities, or possibly to establish a dominance hierarchy. Typically, a male initiates a fight by moving on top of the other’s body and tongue-flicking, after which the two snakes twine together and push in an attempt to pin each other's head to the ground. Male-male combat can last for several hours, though is less aggressive than commonly seen in the black mamba and combatants do not bite each other.
Males locate females by following a scent trail. The male courts the female by aligning his body along the female’s while rapidly tongue-flicking. If the female is receptive to mating, she will lift her tail and cloacal juxtaposition will follow shortly. Courtship and mating take place in the trees, after which the female lays 4–17 eggs (an average of 10 to 15 eggs are laid), which occurs in October and November. The eggs are small and elongated, usually measuring 47–58 x 25–28 mm. The eggs are usually laid in a hollow tree, in leaf litter. The incubation period is around 3 months. When the young emerge from the eggs, they are approximately 30 to 45 cm (12 to 18 in); those born in captivity averaging 44 cm (17 in) in length. Individuals usually reach adult coloration at a length of 75 cm (30 in) Hatchlings tend to grow 50 to 80 cm (20 to 31 in) in length in the first year of life. As the hatchlings age, their growth rates decrease but they never completely stop growing.
The eastern green mamba preys primarily on birds and their eggs as well as small mammals including bats. It is believed to eat arboreal lizards as well. It uses a sit-and-wait strategy of foraging, though an eastern green mamba has been recorded actively hunting sleeping bats. The species has also been known to raid the nests of young birds. Sit-and-wait tactics may be successful with highly mobile prey, such as adult birds or rodents. Documented prey include the sombre greenbul, which occur in dense areas of natural and cultivated vegetation along Kenya's coastline. Ionides and Pitman (1965) reported a large bushveld gerbil in the stomach of a green mamba in Tanzania. Although the bushveld gerbil does not occur in Kenya, green mambas prey on the seven species of gerbil that inhabit various portions of its range.
The eastern green mamba is the most commonly encountered of the three species of green mamba, though has the least toxic venom. Although it generally avoids people, the it is a highly venomous snake. The peak period for bites is the species' breeding season from September to February, during which mambas are most irritable. A survey in southern Africa from 1957 to 1979 recorded 2553 venomous snakebites, 17 of which were confirmed as being from eastern green mambas. Of these 17 cases, 10 had symptoms of systemic envenomation, though none died. The snake tends to bite repeatedly, so there can be multiple puncture wounds. A bite can contain 60–95 mg of venom by dry weight. The subcutaneous murine median lethal dose (LD50) is 1.3 mg/kg. The LD50 in mice through the IV route is 0.45 mg/kg.
Symptoms of envenomation by this species include pain and swelling of the bite site, which can progress to local necrosis or gangrene. Systemic effects include dizziness, and nausea, accompanied by difficulty breathing and swallowing, irregular heartbeat, and convulsions. Neurotoxic symptoms such as paralysis may be mild or absent. Bites that produce severe envenomation can be rapidly fatal. Case reports of rapidly fatal outcomes, in as little as 30 minutes, have been recorded for this species.
In 2015, the proteome (complete protein profile) of eastern green mamba venom was assessed and published, revealing 42 distinct proteins and the nucleoside adenosine. The predominant agents are those of the three-finger toxin family, including aminergic toxins, which act on muscarinic and adrenergic receptors, and fasciculins, which are anticholinesterase inhibitors that cause muscle fasciculation. Another prominent component are a group of proteins known as dendrotoxins; although structurally homologous to kunitz-type protease inhibitors, they block voltage-dependent potassium channels stimulating the release of acetylcholine and causing an excitatory effect. Another kunitz-type protein present is calcicludine, which blocks high-voltage-activated calcium channels. Individually, most of these components do not exhibit potent toxicity in vitro, but are thought to have a synergistic effect with each other in nature.
The composition of eastern green mamba venom is similar to that of other mambas, all of which contain predominantly three-finger toxin agents, apart from the black mamba. The potent neurotoxic element—alpha-neurotoxin—is also absent. It is thought this may reflect the preferred prey items—small mammals for the mainly land-dwelling black mamba versus birds for the other predominantly arboreal mambas. Unlike many snake species, the venom of mambas has little phospholipase A2 content.
Standard first aid treatment for any suspected bite from a venomous snake is the application of a pressure bandage to the bite site, minimisation of movement of the victim and conveyance to a hospital or clinic as quickly as possible. The neurotoxic nature of green mamba venom means an arterial tourniquet may be of benefit. Tetanus toxoid is sometimes administered, though the main treatment is the administration of the appropriate antivenom. A polyvalent antivenom produced by the South African Institute for Medical Research is used to treat eastern green mamba bites.
- A divided scale is one split down the midline into two scales.
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