Eastern brown snake
|Eastern brown snake|
|Eastern brown snake, NSW, Australia|
The eastern brown snake (Pseudonaja textilis), often referred to as the common brown snake, is an extremely venomous snake of the family Elapidae, native to eastern and central Australia and southern New Guinea. Considered the world's second-most venomous land snake after the Inland Taipan and based on its LD50 value (subcutaneous) in mice, it is responsible for about 60% of snake-bite deaths in Australia. First described by André Marie Constant Duméril in 1854, the adult eastern brown snake is a slender snake up to 2 m (7 ft) long with variable upperparts that can be various shades of brown, ranging from pale brown to almost black. Its underside is pale cream-yellow, often with orange or grey splotches.
The eastern brown snake is found in most habitats except dense forests. It has become more common in farmland and on the outskirts of urban areas, benefitting from agriculture due to the increased numbers of its main prey, the introduced house mouse. The species is oviparous.
French zoologist André Marie Constant Duméril described the eastern brown snake in 1854 as Furina textilis, in French Furine tricotée (knitted furin), from a specimen collected in October 1846 by Jules Verreaux. He remarked that the fine-meshed pattern on the snake's body reminded him of fine stockings, which was the inspiration for the name. German herpetologist Johann Gustav Fischer described it as Pseudoelaps superciliosus in 1856, from a specimen collected from Sydney. German-British zoologist Albert Günther described Demansia annulata in 1858. Italian naturalist Giorgio Jan named Pseudoelaps sordellii and Pseudoelaps kubingii in 1859.
Gerard Krefft, curator of the Australian Museum, reclassified Duméril's species in the genus Pseudonaja in 1862 after collecting multiple specimens and establishing that the markings of young snakes faded as they grew into adult brown snakes. He concluded the original description was based on an immature specimen and sent an adult to Gunther, who did catalogue it under the new name the same year when cataloguing new species of snakes in the British Museum's collection. After examining all specimens, Günther concluded that Furina textilis and Diemansia annulata were named for young specimens and Pseudoelaps superciliosus, P. sordelli, and P. kubingii were named for adults, and all represented the same species, which he called Diemenia superciliosa. Belgian-British naturalist George Albert Boulenger gave it the name Diemenia textilis in 1896.
The brown snakes were moved from Diemenia to Pseudonaja by Australian naturalist Eric Worrell in the early 1960s on the basis of skull morphology, and reinforced by American herpetologist Samuel Booker McDowell in 1967 on the basis of the muscles of the venom glands. This classification has been followed by subsequent authors. Within the genus Pseudonaja, the eastern brown snake is unique in having a diploid pattern of 38 chromosomes.
A 2008 study of mitochondrial DNA across its range showed three broad lineages: a southeastern clade from South Australia, Victoria, and southeastern and coastal New South Wales; a northeastern clade from northern and western New South Wales and Queensland; and a central (and presumably northern) Australian clade from the Northern Territory. The central Australian clade had colonized the region around Merauke in southern West Papua, and the northeastern clade had colonized Milne Bay, Oro, and Central Provinces in eastern Papua New Guinea in the Pleistocene via landbridges between Australia and New Guinea.
P. textilis is monotypic. Raymond Hoser described all New Guinea populations as Pseudonaja textilis pughi based on a differing maxillary tooth count from Australian populations. However, this difference was inconsistent and as there is no single distinct New Guinea population genetically the taxon is not recognised. Australian herpetologists Richard W. Wells and C. Ross Wellington described Pseudonaja ohnoi in 1985 from a large specimen from Mount Gillen near Alice Springs, distinguishing it on the basis of scale numbers, however it is not regarded as distinct.
The eastern brown snake is of slender to average build with no demarcation between its head and neck. Its snout appears rounded when viewed from above. Most specimens have a total length (including tail) up to 1.5 m (4.9 ft), with some large specimens reaching 2 m (6.6 ft). The maximum recorded size for the species is 2.4 m (7.9 ft).
The adult eastern brown snake is variable in colour. Its upperparts range from pale to dark brown, or sometimes shades of orange or russet, with the pigment more richly coloured in the posterior part of the dorsal scales. Eastern brown snakes from Merauke have tan to olive upperparts, while those from eastern Papua New Guinea are very dark grey-brown to blackish. It has a dark tongue. The iris is blackish with a paler yellow-brown or orange ring around the pupil. The eastern brown snake's chin and underparts are cream or pale yellow, sometimes fading to brown or grey-brown towards the tail. There are often orange, brown or dark grey blotches on the underparts, more prominent anteriorly. The ventral scales are often edged with dark brown on their posterior edges.
Juveniles can vary in markings, but generally have a black head, with a lighter brown snout and band behind, and a black nuchal band. Their bodies can be uniform brown, or have many black bands, or a reticulated pattern, with all darker markings fading with age. Snake clutches in colder areas often have more banded than unbanded young snakes.
Its yellowish underparts serve to distinguish it from the dugite and Peninsula brown snakes, which are entirely brown or brown with grey underparts. The eastern brown snake has flesh-pink skin inside its mouth, whereas the northern brown snake and western brown snake have black skin. Large eastern brown snakes are often confused with mulga snakes (Pseudechis australis), whose habitat they share in many areas, but may be distinguished by their smaller heads. Juvenile eastern brown snakes have head markings similar to red-naped snakes (Furina diadema), grey snakes, (Hemiaspis damelii), Dwyer's snakes (Suta dwyeri) and the curl snake (Suta suta).
The eastern brown snake has 17 rows of dorsal scales at midbody, 192 to 231 ventral scales, 45 to 75 divided subcaudal scales (occasionally some of the anterior ones are undivided), and a divided anal scale. Its mouth is lined with six supralabial scales above and seven (rarely eight) sublabial scales below. Its nasal scale is almost always undivided, and rarely partly divided. Its eyes have two or rarely three postocular scales.
Distribution and habitat
The eastern brown snake is found along the east coast of Australia, from Malanda in far north Queensland, along the coasts and inland ranges of Queensland, New South Wales, Victoria, and to the Yorke Peninsula in South Australia. Disjunct populations occur on the Barkly Tableland and the MacDonnell Ranges in the Northern Territory. and the far east of the Kimberley in Western Australia, and discontinuously in parts of New Guinea, specifically northern Milne Bay Province and Central Province in Papua New Guinea, and the Merauke region of Papua Province, in the Indonesian part of New Guinea.
The eastern brown snake occupies a varied range of habitats from dry sclerophyll forests (eucalypt forests) and heaths of coastal ranges, through to savannah woodlands, inner grasslands, and arid scrublands and farmland, as well as more arid areas that are intermittently flooded. It is more common in open habitat and also farmland and the outskirts of urban areas. It is not found in rainforests or other wet areas. Because of their mainly rodent diet, they can often be found near houses and farms. Such areas also provide shelter in the form of rubbish and other cover, with snakes using sheets of corrugated iron or buildings as hiding spots, as well as large rocks, burrows and cracks in the ground.
The concrete slabs of houses have been used by eastern brown snakes hibernating in winter, with 13 recorded coiled up together under a 5 m by 3 m slab of a demolished house between Mount Druitt and Rooty Hill in western Sydney, and another 17 (in groups of one to four) under smaller slabs within 20 m in late autumn 1972. Groups of up to six hibernating eastern brown snakes have been recorded from under other slabs in the area. In July 1991 in Melton, 6 eastern brown snakes were uncovered in a nest in long grass.
The eastern brown snake is active during the day, though it may retire in the heat of hot days to come out again in the late afternoon. It is most active in spring, with males venturing out earlier in the season than females, and is sometimes active on warm winter days. Individuals have been recorded basking on days with temperatures as low as 14 °C (57 °F). Occasional nocturnal activity has been reported. Australian naturalist David Fleay reported it could outpace a person running at full speed. Many people mistake defensive displays for aggression.
The eastern brown snake reacts when confronted with one of two neck displays. During a partial display, the snake raises the front part of its body horizontally just off the ground, flattening its neck and sometimes opening its mouth. In a full display, the snake rises up vertically high off the ground, coiling its neck into an S shape, and opening its mouth. The snake is able to strike more accurately from a full display and more likely to deliver an envenomed bite. Due to the snake's height off the ground in full display, the resulting bites are often on the upper thigh.
A field study in farmland around Leeton that monitored 455 encounters between eastern brown snakes and people found that the snake withdrew around half the time and tried to hide for almost all remaining encounters. In only 12 encounters did the snake advance. They noted that snakes were more likely to notice dark clothing and move away early, reducing the chance of a close encounter. Close encounters were more likely if a person was walking slowly, but a snake was less likely to be aggressive in this situation. Encountering male snakes on windy days with cloud cover heightened risk, as the snake was less likely to see persons until they were close and hence more likely to be startled. Similarly, walking in undisturbed areas on cool days in September and October risked running into courting male snakes, that would not notice people until close as they were preoccupied with mating.
Eastern brown snakes mate during spring; they are oviparous. Males engage in 'ritual combat' with other males for control of territory. The most dominant male will mate with females in the area. The females produce a clutch of 10–35 eggs, with the eggs weighing 8.0 g each on average. in late spring or early summer. They do not guard the nest after the eggs are laid — the juvenile snakes are totally independent of the mother.
The eastern brown snake appears to hunt by sight more than other snakes, and a foraging snake raises its head like a periscope every so often to survey the landscape for prey. It generally finds its food sources in their refuges rather than chasing fleeing prey. The adult is generally diurnal, while juveniles sometimes hunt at night. Their diet is made up almost wholly of vertebrates, with mammals predominating. Introduced house mice are a common prey item, though they eat mammals as large as rabbits. Small birds, eggs, and even other snakes are also consumed. Small lizards such as skinks are more commonly eaten than frogs, as eastern brown snakes generally forage in areas over 100 m distant from water. Snakes larger than 7 cm (2.8 in) from snout to vent eat predominantly warm-blooded prey, while smaller snakes mainly eat ectothermic animals.
The eastern brown snake has been observed coiling around and constricting prey to immobilise and subdue it. Herpetologists Richard Shine and Terry Schwaner proposed that it might be resorting to constriction when attacking skinks as it might facilitate piercing the skink's thick scales with its small fangs.
Although the eastern brown snake is susceptible to cane toad toxins, young individuals avoid eating them, which suggests they have learnt to avoid them. Some evidence indicates they are immune to their own venom and that of the mulga snake, a potential predator.
The eastern brown snake rarely eats during winter, and females often do not eat while containing eggs.
The eastern brown snake is considered to be the second-most venomous terrestrial snake in the world, behind only the inland taipan of central east Australia. It is the most commonly encountered dangerous snake in Adelaide, and is also found in Melbourne, Canberra, Sydney, and Brisbane.
The venom has a subcutaneous LD50 range of 36.5 —53 μg/kg, and consists mostly of neurotoxins (pre- and post-synaptic neurotoxins) and blood coagulants. These snakes kept at venom supply laboratories yield an average of 2–10 mg of venom per milking. Engelmann and Obst (1981) give a venom yield of 2 mg (dry weight). As with most venomous snakes, the volume of venom produced is largely dependent on the size of the snake. Worrell (1963) reported a milking of 41.4 mg from a relatively large 2.1-m (6.9-ft) specimen. This record is atypical, as the eastern brown snake yields a low volume of venom, which is reported as not more than 10 mg, averaging only 2 mg (dry weight).
Analysis of venom in 2016 found—unlike most other snake species—that the venom of juvenile eastern brown snakes differed from that of adults; prothrombinases (found in adults) were absent and the venom did not affect clotting times. Snakes found with a similar profile generally preyed upon dormant animals such as skinks.
Clinically, the venom of the eastern brown snake is known to cause diarrhea, dizziness, collapse or convulsions, renal failure, paralysis, and cardiac arrest. Without medical treatment, bites can be fatal. As this species tends to initiate its defence with nonfatal bites, the untreated mortality rate in most snakebite cases reported is 10–20%, a relatively low figure.
The eastern brown snake is the second-most commonly reported species responsible for envenoming of dogs in New South Wales. Dogs and cats are much more likely than people to have neurotoxic symptoms such as weakness or paralysis. One dog bitten suffered a massive haemorrhage of the respiratory tract requiring euthanasia.
The venom is uniformly toxic to warm-blooded vertebrates, yet reptile species differ markedly in their susceptibility.
The neurotoxin of the eastern brown snake is textilotoxin, a presynaptic neurotoxin. It is a potent neurotoxin and represents 3% of the crude venom by weight. Southcott and Coulter (1979) reported that textilotoxin acted on the prejunctional nerve terminal by selectively blocking the release of acetylcholine. This blockage had no effect on the resting membrane potential of the muscle cells, nor was the nerve conduction altered.
Sutherland (1983) reported that textilotoxin had direct presynaptic actions and no appreciable effect on muscle or acetylcholine receptors. The presynaptic blockade was due to the phospholipase, a component of the textilotoxin acting on the axolemma.
Hamilton et al. (1980) showed that the crude venom produced "coated omega figures" in the axolemma of the rat nerve terminals. Those figures are probably due to the action of the textilotoxin.
Barnett et al. (1980) isolated a postsynaptic neurotoxin called pseudonajatoxin A. It has 117 amino acid residues and a high molecular weight of 12,280, meaning it is slow to act. It causes irreversible blockade by firm binding to the acetylcholine receptors.
Kellaway (1933), stated that P. textilis venom possessed a strong, highly diffusible coagulation factor. Denson (1969) concluded that the coagulation factor was a complete prothrombin activator. Masci et al. (1988) found the prothrombin activator to be a major component of the venom with a high molecular weight of larger than 200,000. They found it was related antigenically to the prothrombin activator of O. scutellatus venom, able to activate citrated blood plasma, warfarin plasma[clarification needed], factor V, and factor X-deficient plasmas and will hydrolyse peptide p-nitroanilide substrate S-2222.Ca++ and phospholipids have little effect on it. Doery and Pearson (1961) showed that P. textilis venom was low in direct haemolytic properties and phospholipase. Kaire (1964), reported it had the least amount of heat-stable anticoagulant of most Australian snakes.
Standard first aid treatment for any suspected bite from a venomous snake is for a pressure bandage to the bite site, the victim to move as little as possible, and to be conveyed to a hospital or clinic, where they should be monitored for at least 24 hours. Tetanus toxoid is given, though the mainstay of treatment is the administration of the appropriate antivenom.
Dogs and cats can be treated with a caprylic acid fractionated bivalent whole IgG equine antivenom.
Eastern brown snakes are readily available in Australia via breeding in captivity. They are regarded as challenging to keep and suitable only for experienced snake-keepers due to their speed and toxicity.
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