Green anaconda (Eunectes murinus)
|Also known as:||Anaconda, water boa|
|Size||Maximum length: 6 – 9 m (2)|
|Weight||up to 227 kg (2)|
Listed on Appendix II of CITES (1).
While the anaconda may not hold the title of the worlds’ longest snake—coming second to the reticulated python—its huge girth means that it is undoubtedly the largest (2). During the 19th and 20th century, many accounts of giant specimens of anaconda were reported. While almost certainly exaggerations, it is possible that in remote, deep rainforest rivers, under optimum conditions of prey availability, the green anaconda may grow to record proportions (3). The green anaconda is well-adapted for its semi-aquatic lifestyle, with its eyes and nostrils positioned on top of the head, enabling the snake to see and breathe while the rest of the body is submerged (4). The colouring of the body provides excellent camouflage, with olive green upperparts boldly marked with pairs of dark ovals on either side of the spine, which are sometimes fused. The flanks are also marked with smaller, light-centred dark spots, while the head, which is relatively small compared to the thick body, bears a distinctive stripe, which runs from the rear edge of the eye, diagonally downwards to the back of the head. The stripe is edged with black and varies in colouration, from greenish to orange (3). In addition to its gigantic proportions, another remarkable feature of the green anaconda is that it exhibits the greatest size difference between the sexes of any terrestrial vertebrate. The female dwarfs the male and is almost five times heavier (5).
The green anaconda is found in northern South America, occurring in Venezuela, Colombia, Brazil, Ecuador, eastern Paraguay, northern Bolivia, north-east Peru, Guyana and French Guiana. It can also be found on the island of Trinidad off the north-east coast of Venezuela (4) (6).
A semi-aquatic species, the green anaconda is typically found in shallow water, either in seasonally flooded savanna, such as Llanos grasslands of Colombia and Venezuela, or in the rivers of the vast Amazon Basin (3). When on land, this species is most commonly found amongst thick vegetation, although it may also climb trees (4) (7).
Although capable of moving over land, the bulky body of the adult green anaconda is best suited for life in the water, where it gracefully and stealthily seeks out prey (2) (3) (4). A variety of prey is taken according to the size of the anaconda, with smaller individuals taking fish and other small vertebrates, while larger specimens prey upon deer, capybara, and even full-grown caimans (4) (7). In addition, in rare cases this species has also been known to attack humans (8). Prey is taken by surprise, with a swift strike from the jaws, after which coils of the muscular body are quickly thrown around the animal, often dragging it underwater (4) (7). Lacking venom, the green anaconda relies on constriction to kill its prey, asphyxiating the animal, before swallowing it whole. Elastic ligaments allow the jaws to stretch widely, accommodating prey many times the width of the head and body. Large meals provide enough nourishment that the snake can go weeks or months without food (2).
Studies of the green anaconda in seasonally-flooded habitats show that it mates during the dry season, from around mid-February to late May. Males seek out a mate by following scent trails, and due to the greater numbers of males relative to females, several often converge on a single female. This leads to a phenomena known a “breeding ball”, in which the smaller males and the single, large female form a mass of writhing bodies, in which the males attempt push one another out the way in order to access the female’s cloaca and mate (5). This association can last for up to month, during which the female may mate multiple times. This mating system may help to explain the pronounced size difference between the sexes; as larger male specimens can be mistaken for females, it is disadvantageous for males to grow beyond a certain size (9). Interestingly, after mating the female may eat one or more of its mating partners. This behaviour may help it to survive pregnancy, during which food is not taken for up to seven months (10). After mating, in order to survive the dry season, green anacondas shelter underground in mud or caves formed in river banks, or they seek out deeper water (3) (11). The female typically gives birth to litters of 20 to 40 live young, though a record of 82 has been reported. The young measure around 60 centimetres in length, and take roughly six years to reach sexual maturity (2) (3) (4). From birth to adulthood, the green anaconda undergoes a dramatic 500-fold increase in mass, a greater increase than any other snake species (12). While the average lifespan is 10 years in the wild (2), individuals have been known to live for over 25 years (4).
Although not yet assessed by the IUCN, the green anaconda faces a number of threats, which could have a significant impact on its population. It is hunted, both legally and illegally, in many parts of its range for its skins and for sale in the growing, illegal pet trade. Local people also frequently kill anacondas under the pretext of protecting livestock, pets or even people. However, in many cases, a widespread fear and dislike of snakes results in individuals of this species being persecuted, even when found in remote areas. Habitat loss and degradation are also threatening this species, and even in areas where habitat protection exists, a lack of enforcement is allowing illegal deforestation to occur (11).
The green anaconda is listed on Appendix II of the Convention on Trade in International Species (CITES), thereby regulating all commercial trade in this species through the use of permits and annual export quotas (1). In addition, in some parts of its range, such as Venezuela, the green anaconda is protected by national laws, making all use of green anaconda products illegal. Unfortunately, despite these regulations, illegal hunting remains problematic. In Bolivia, where hunting is permitted, efforts have been made to establish a sustainable use program, but at the current time political unrest has halted its development (11).
Conservation initiatives which can provide a balance between maintaining livelihoods for local people, while maintaining the green anaconda’s population are urgently required to prevent this species’ decline. The development of sustainable use programs is, however, hampered by several factors such as difficulties in obtaining accurate population estimates. Therefore, ecotourism may be one of the most effective ways to conserve this spectacular snake (11).
To learn more about anaconda conservation see:
Rivas, J.A. (2007) Conservation of green anacondas: how Tylenol conservation and macroeconomics threaten the survival of the world's largest snake. Iguana, 14: 74-85. Available at:
To learn more about reptile conservation visit:
International Reptile Conservation Foundation:
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- Cloaca: a common cavity into which the reproductive, alimentary and urinary systems open.
CITES (June, 2009)
National Geographic (September, 2009)
- O'Shea, M. (2007) Boas and Pythons of the World. New Holland Publishers, London.
- Burnie, D. (2001) Animal. Dorling Kindersley, London.
- Rivas, J.A. (2000) Life history of the green anaconda (Eunectes murinus) with emphasis on its reproductive biology. Unpublished Ph. D. dissertation at the University of Tennessee, 0: 1 - 287.
J. Craig Venter Institute (September, 2009)
- Boos, H.E.A. (2001) The snakes of Trinidad and Tobago. Texas A and M University Press, College Station, Texas.
Rivas, J.A. (1999) Predatory attacks of green anacondas (
Eunectes murinus) on adult human beings. Herptological Natural History, 6: 158 - 160.
- Rivas, J.A. and Burghardt, G.M. (2001) Sexual size dimorphism in snakes: wearing the snake’s shoes. Animal Behaviour, 62: 1 - 6.
- Rivas, J.A. and Owens, R.Y. (2000) Eunectes murinus (green anaconda): cannibalism. Herpetological Review, 31: 44 - 45.
- Rivas, J.A. (2007) Conservation of green anacondas: how Tylenol conservation and macroeconomics threaten the survival of the world's largest snake. Iguana, 14: 74 - 85.
Jesus Rivas (September, 2009)