The Red-bellied black snake is a venomous snake endemic to Australia. It is one of eastern Australia's most commonly encountered snakes. It is not an aggressive species and generally retreats from encounters with people, but can attack if provoked. These snakes have a glossy black top body with a light-grey snout and brown mouth, and a completely black tail. They lack a well-defined neck; their head merges seamlessly into the body. Their flanks are bright red or orange, fading to pink or dull red on the belly. All these scales have black margins. Snakes from northern populations tend to have lighter, more cream or pink bellies. Males in this species are generally slightly larger than females.
Red-bellied black snakes are native to the east coast of Australia. They can be found in the Blue Mountains, Canberra, Sydney, Brisbane, Melbourne, Cairns, and Adelaide. The Macquarie Marshes mark a western border to its distribution in New South Wales, and Gladstone in central Queensland marks the northern limit to the main population. To the south, they occur across eastern and central Victoria and extend along the Murray River into South Australia. Disjunct populations occur in the southern Mount Lofty Ranges in South Australia and in North Queensland. Red-bellied black snakes are most commonly seen close to rivers, dams, streams, billabongs, and other bodies of water, although they can venture up to 100 m (350 ft) away, including into nearby backyards. In particular, they prefer areas of shallow water with tangles of water plants, logs, or debris. They can also be found in the urban forest, woodland, plains, grasslands and bushland areas.
Red-bellied black snakes are generally solitary and diurnal reptiles. When not hunting or basking, they may be found beneath timber, rocks, and rubbish or down holes and burrows. They like to bask in warm, sunny spots in the cool, early morning and rest in shade in the middle of hot days and may reduce their activity in hot, dry weather in late summer and autumn. Rather than entering true hibernation, Red-bellied black snakes become relatively inactive over winter, retreating to cover and at times emerging on warm, sunny days. They are known to hibernate in groups of up to six individuals. Males are more active in the Southern Hemisphere spring (early October to November) as they roam looking for mates; one reportedly traveled 1,220 m (0.76 mi) in a day. In summer, both sexes are less active generally. Red-bellied black snakes can hide in many places in their habitat, including logs, old mammal burrows, and grass tussocks. They can flee into the water and hide there; they may stay submerged for 23 minutes. When swimming, they may hold their full head or the nostrils above the water's surface. At times, these snakes may float without moving on the water surface, thus looking like a stick. Within their habitat, they have territories within which they may have some preferred places to reside. Red-bellied black snakes are generally not aggressive, typically withdrawing when approached. If provoked, they recoil into a striking stance as a threat, holding their head and front part of their body horizontally above the ground and widening and flattening their neck. They may bite as a last resort.
Red-bellied black snakes are carnivores. Their diet primarily consists of frogs, but they also prey on reptiles and small mammals. They also eat other snakes, including those of their own species.
Red-bellied black snakes have a polygynandrous (promiscuous) mating system; this means that both males and females have multiple partners. In spring (October-November), males often engage in ritualized combat for 2-30 minutes. They wrestle vigorously, but rarely bite, and engage in head-pushing contests, where each snake tries to push his opponent's head downward with his chin. The male seeks out a female and rubs his chin on her body, and may twitch, hiss, and rarely bite. Pregnancy takes place any time from early spring to late summer. Females become much less active and band together in small groups in late pregnancy. They share the same retreat and bask in the sun together. Red-bellied black snakes are ovoviviparous; they give birth to live young after 14 weeks' gestation, usually in February or March. The young, numbering between 8 and 40 have an average length of around 12.2 cm (4.8 in). Females leave them right after giving birth. Young snakes become reproductively mature when they reach SVL (snout-vent length) of 78 cm (31 in) for males or 88 cm (35 in) for females. Females can breed at around 31 months of age, while males can slightly earlier.
Main threats to Red-bellied black snakes include habitat fragmentation and decline of frog populations which are their preferred prey. The habitat of these snakes has been particularly vulnerable to urban development and it is also highly fragmented. Red-bellied black snakes also suffer from the introduced Cane toad. These snakes have a low tolerance to Cane toad toxins and it is thought that they have impacted heavily on snakes' numbers in northern parts of their range.
According to IUCN, the Red-bellied black 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.
Due to their diet habits, Red-bellied black snakes help control populations of pests such as rats and mice.