The Eastern brown snake is a highly venomous snake native to eastern and central Australia and southern New Guinea. They are variable in color. Their upper parts range from pale to dark brown, or sometimes shades of orange or russet. Eastern brown snakes from Merauke have tan to olive upperparts, while those from eastern Papua New Guinea are very dark grey-brown to blackish. The tongue of these snakes is dark, and the irises are blackish with a paler yellow-brown or orange ring around the pupil. Their chin and underparts are cream or pale yellow, sometimes fading to brown or grey-brown towards the tail. Often, orange, brown, or dark grey blotches occur on the underparts. The ventral scales are often edged with dark brown on their posterior edges.
Eastern brown snakes are 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. They are common in southeastern Queensland between Ipswich and Beenleigh. These snakes live in different 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 drier areas that are intermittently flooded. They are more common in open habitat and also farmland and the outskirts of urban areas. They are 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 another cover; the snakes use sheets of corrugated iron or buildings as hiding spots, as well as large rocks, burrows, and cracks in the ground.
Eastern brown snakes generally solitary, with females and younger males avoiding adult males. They are active during the day, though they may retire in the heat of hot days to come out again in the late afternoon. Eastern brown snakes 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. These snakes generally find their food source in their refuges rather than chasing fleeing prey. Adults generally hunt during the day, while juveniles sometimes hunt at night. Eastern brown snakes rarely eat during winter, and females rarely eat while pregnant with eggs. These snakes are most active in spring, the males venturing out earlier in the season than females, and are sometimes active on warm winter days. The occasional nocturnal activity has been reported. At night, they retire to a crack in the soil or burrow that has been used by a House mouse. During winter, they hibernate, emerging on warm days to sunbathe. Many people mistake defensive displays of these snakes for aggression. When confronted, Eastern brown snakes react 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 can strike more accurately from a full display and more likely to deliver an envenomed bite.
Eastern brown snakes are carnivores. Their diet is made up almost wholly of vertebrates, with mammals predominating - particularly the introduced House mouse. Mammals as large as feral rabbits have been eaten. Small birds, eggs, and even other snakes are also consumed.
Eastern brown snakes are polygynous which means that one males mates with several females. They generally mate from early October onwards - during the Southern Hemisphere spring. These snakes are oviparous meaning that they lay eggs. During the mating season, males engage in ritual combat with other males for access to females. The appearance of two males wrestling has been likened to a pleated rope. The most dominant male will mate with females in the area. The females produce a clutch of 10 to 35 eggs, with the eggs typically weighing 8.0 g (0.28 oz) each. The eggs are laid in a sheltered spot, such as a burrow or hollow inside a tree stump or rotting log. Multiple females may even use the same location, such as a rabbit warren. Ambient temperature influences the rate at which eggs develop; eggs incubated at 25 °C (77 °F) hatch after 95 days, while those at 30 °C (86 °F) hatch after 36 days. Snakelets are born fully developed. They are independent at hatching and can reach reproductive maturity by 31 months of age.
There are no major threats facing Eastern brown snakes at present.
According to IUCN, the Eastern brown snake is locally common and widespread throughout its range but no overall population estimate is available. Currently, this species is classified as Least Concern (LC) on the IUCN Red List and its numbers today are stable.