Eunectes murinusAnaconda, Green Anaconda

Geographic Range

Green anacondas are found throughout the tropical lowlands of South America. This species is particularly common in the Orinoco basin of eastern Columbia, Amazon River basin of Brazil, and the seasonally flooded Llanos grasslands of Venezuela. Additional countries where they may be found include Ecuador, Paraguay, Bolivia, the Bolivarian Republic of Argentina, the Guianas, Guyana, Peru, Suriname, and Trinidad. Small populations of green anacondas have also been introduced into Florida. (Duellman, 2005; Grzimek, 2003; O'Shea, 2007; Rivas, 2000; Schmidt and Inger, 1957; Shine, 1992; Strimple, 1993)


Green anacondas are semi-aquatic snakes that inhabit shallow, slow-moving freshwater habitats, as well as tropical savannas, grasslands, and rainforests. (Duellman, 2005; Grzimek, 2003; O'Shea, 2007; Rivas and Owens, 2000; Rivas, 2000; Shine, 1992)

  • Aquatic Biomes
  • lakes and ponds
  • rivers and streams

Physical Description

Green anacondas are one of four closely related species of constrictors, the other species being Eunectes notaeus (yellow anaconda), E. deschauenseei (dark-spotted anaconda), and E. beniensis (Bolivian anaconda). These boas can be distinguished from other constricting snakes by the absence of the supraorbital bone in the roof of the skull. Boas have an external horny claw, a hind limb remnant that is more evident in males than in females. Like all snakes, anacondas have a forked tongue thay helps them locate prey and mates and to navigate their environment, in conjunction with the tubular Jacobson’s organ in the roof of the snake’s mouth. (Duellman, 2005; Dunn and Conant, 1936; Grzimek, 2003; O'Shea, 2007; Schmidt and Inger, 1957)

Green anaconda coloration is characteristically dark olive-green dorsally, gradually changing to yellow ventrally. They have round dorsal blotches that are brown with diffused black borders, and are dispersed over the mid to posterior length of their body. Like other Eunectes species, they have narrow ventral scales and small, smooth dorsal scales. The scale plates at the anterior portion of their body are much larger than those at the posterior end. Their skin is soft, loose, and can endure extended periods of water absorption. Anacondas have dorsal nostrils and small eyes that are positioned on the top of the head. They also have a prominent black post-ocular stripe that runs from the eye to the angle of the jaw. (Hsiou and Albino, 2009; O'Shea, 2007)

Green anacondas are the largest snakes in the world. There are records of 10 to 12 meter anacondas weighing up to 250 kg, though the actual maximal size of an anaconda is the subject of much dispute. Females typically have a much larger mass and length than males, with males reaching an average of 3 meters in length and females 6 meters. The sex of an anaconda is also reflected by the size of the spurs located in the cloacal region. Males have larger spurs (7.5 millimeters) than females, regardless of the female’s size. (Duellman, 2005; Grzimek, 2003; Miller, et al., 2004; O'Shea, 2007; Rivas and Burghardt, 2001; Rivas, 2000; Schmidt and Inger, 1957; Shine, 1992)

  • Sexual Dimorphism
  • female larger
  • Range mass
    250 (high) kg
    550.66 (high) lb
  • Range length
    3 to 12 m
    9.84 to 39.37 ft
  • Average length
    6 m
    19.69 ft


Green anaconda neonates are larger than most snakes at birth. At a mean of 200 grams and 68 centimeters, they are on average 1% of the mass of the female that gave birth to them. They then undergo a 500 fold increase in biomass from birth to adulthood. Anacondas start to show evidence of sexual dimorphism after the first year of life. (Grzimek, 2003; Lamonica, et al., 2007; O'Shea, 2007; Rivas, 2000; Shine, 1992)

There are differences in rates of development between savanna and river-dwelling snakes. Strong seasonality of prey availability may be present in savanna environments, with the result that river-dwelling individuals often are larger and heavier than savanna-dwelling snakes. (Grzimek, 2003; O'Shea, 2007; Rivas, 2000)


Green anacondas are polyandrous. Studies have reported that green anacondas breed in multiple-male aggregations of up to 13 males. Mating can last for several weeks. During this time, a female can mate several times with the courting males. Males surround the female to make a breeding ball, in which the snakes form a mass of writhing bodies. Males compete to gain access to the female by coiling around her, searching for her cloaca with their tails. Visual or chemical cues do not seem to be involved. (Grzimek, 2003; Rivas and Burghardt, 2001; Rivas and Owens, 2000; Rivas, 2000; Shine, 1992)

Females are selective in mating aggregations. In conditions of high density or when females are easy to track, males can encounter each other, which may lead to combat. However, male-to-male combat is rare. If one male is exceptionally large, it can be mistaken for a female by other males and may be courted. Large males typically mate with the most fertile and largest females. (Rivas and Burghardt, 2001; Rivas, 2000)

Green anacondas reach sexual maturity at approximately 3–4 years of age. Mating occurs during the dry season, between March and May, with males searching for females to mate with. Males have short term sperm storage, using up their reservoir after mating is completed. After mating, the female may eat one or more of her mating partners, as she does not take in food for up to seven months. This behavior may be beneficial in helping her to survive through the gestation period. Afterwards, males usually leave the impregnated female and return to their home ranges; the female does not migrate. (Duellman, 2005; Miller, et al., 2004; O'Shea, 2007; Rivas and Owens, 2000; Rivas, 2000; Shine, 1992)

Females are ovoviviparous and incubate their eggs for 7 months until they give birth to live young. Their movements and foraging behaviors are limited to avoid compromising the health and success of their clutch. Females give birth in shallow water during the evening or late afternoon, at the end of the wet season. Females may give birth to as many as 82 young, averaging 20-40 offspring. Scientists report an association between clutch size and the size of the female, with large females typically having larger clutch sizes than smaller females. This association may be due to greater fat reserves in larger individuals. On average, these snakes breed every other year, allowing recuperation from the loss of energy required for reproduction. (Duellman, 2005; Miller, et al., 2004; Rivas, 2000; Shine, 1992)

  • Breeding interval
    Green anacondas breed every other year
  • Breeding season
    Breeding takes place from March-May. Birth occurs approximately 7 months later, at the end of the wet season.
  • Average number of offspring
  • Average gestation period
    7 months
  • Average age at sexual or reproductive maturity (female)
    3-4 years
  • Average age at sexual or reproductive maturity (male)
    3-4 years

Maternal provisioning and protection occurs throughout the gestation period. After birth, offspring are independent and receive no parental care. (Duellman, 2005; Grzimek, 2003)

  • Parental Investment
  • no parental involvement
  • pre-hatching/birth
    • provisioning
      • female
    • protecting
      • female


The lifespan of green anacondas in the wild averages 10 years. In captivity, they may live for over 30 years. (Miller, et al., 2004; Shine, 1992)


Anacondas are readily adaptable to changes in their environment. Such adaptability is useful during the annual dry season of seasonally flooded savannas. In order to survive, the anacondas that live in these areas must either find water or bury themselves in mud. In the latter case, they will undergo a state of dormancy for the duration of the dry period. Anacondas that reside near river basins are active throughout the year. (Alves and Pereira, 2007; O'Shea, 2007)

Green anacondas are most active in the early evening. They can move large distances over short time periods, especially during annual dry seasons and when males are seeking females. They preferentially migrate during cooler hours after the peak heat of the day. Anacondas are poikilotherms, but are able to regulate their body temperatures by changing the amount of surface area that is exposed to the sun. Grassland-dwelling females remain in areas near river banks, basking in the sun in elevated areas regularly during the wet season. River-dwelling females have been seen to bask on top of dense vegetation. Non-breeding snakes are rarely seen basking. After the breeding season, they move back to their foraging grounds. (Grzimek, 2003; O'Shea, 2007; Rivas, 2000)

  • Range territory size
    0.252 to 0.347 km^2

Home Range

Green anacondas have well-defined home ranges. During the dry season, they have small home ranges about 0.25 km^2. During the wet season, non-breeding females and males migrate to higher elevations and maintain a larger home range estimated to be about 0.35 km^2. They return to their original home range during the dry season. (Grzimek, 2003; O'Shea, 2007; Rivas, 2000)

Communication and Perception

Green anacondas are able to detect approaching animals using vibrations. They are also able to detect chemical cues of nearby animals in the air using their forked tongues and Jacobson's organs. Male anacondas also use these structures to detect the pheromones of nearby females during the mating season. In addition to their chemosensory abilities, anacondas have pit organs along the upper lip, which are able to sense heat signatures of prey organisms. They are able to perceive visual and auditory stimuli as well, although these senses are more poorly developed than the above sensory modalities. (O'Shea, 2007; Pough, et al., 2003; Rivas, 2000; Shine, 1992; Strimple, 1993)

Food Habits

Green anacondas are opportunistic apex predators, feeding on any prey that they can kill and swallow. Their diet includes various aquatic and terrestrial vertebrates such as fish, reptiles, amphibians, birds, and mammals. (Duellman, 2005; Grzimek, 2003; O'Shea, 2007; Schmidt and Inger, 1957)

Juvenile anacondas feed on prey such as small birds and juvenile caiman that are typically 40-70 grams in size. As they develop, their diet becomes increasingly complex. Prey availability varies more in grasslands than in river basins. Green anacondas in both habitats have been found to feed on large prey, usually ranging from 14% to 50% of its own mass. A few examples of their prey include broad-snouted caimans (Caiman latirostris), wattled jacanas (Jacana jacana), capybaras (Hydrochoerus hydrochaeris), red-rumped agoutis (Dasyprocta leporine), collared peccaries (Pecari tajacu), South American tapirs (Tapirus terrestris), red side-necked turtles (Rhinemys rufipes), and northern pudús Pudu mephistophiles. Green anacondas take a high risk by feeding on larger prey, which occasionally lead to serious injuries or even death. Some also feed on carrion and conspecifics, usually inside or around water. Occasionally, female green anacondas will feed on males. Large anacondas can go weeks to months without food after eating a large meal, because of their low metabolism. However, females show increased postpartum feeding rates to recover from their reproductive investment. (Duellman, 2005; Elvey and Newlon, 1998; Grzimek, 2003; Jácomo and Silveira, 1998; Martins and Oliveira, 1999; O'Shea, 2007; Pizzato, et al., 2009; Rivas and Owens, 2000; Schmidt and Inger, 1957; Strimple, 1993; Valderrama and Thorbjanarson, 2001)

Green anacondas rely on stealth and ambush techniques as they hunt. Their body pattern provides effective camouflage, allowing a submerged anaconda to be virtually invisible from a short distance away. They attack at any time of day, restraining their prey using their needle-sharp, curved teeth for a secure grip while killing it by constriction. The more the prey struggles, the tighter the coiling will become, until the victim becomes unconscious. Death occurs through respiratory arrest and circulatory failure. Since feeding usually takes place near the water, prey are as likely to die from drowning as from constriction. The snake then slowly releases its coils and ingests its victim headfirst. This technique allows it to reduce obstruction of the limbs as it swallows its meal whole. (Jácomo and Silveira, 1998; O'Shea, 2007; Rivas and Owens, 2000; Schmidt and Inger, 1957; Shine, 1992)

  • Animal Foods
  • birds
  • mammals
  • amphibians
  • reptiles
  • fish
  • carrion


Levels of predation pressure are typically based upon the size and health of an individual snake. Juveniles and small adult anacondas experience high mortality rates, as they are subject to predation by larger predators. There have been accounts of caiman and jaguars preying on young male snakes. As a result, small anacondas are extremely aggressive and bite frequently. Environmental pressures can also influence levels of predation, as savanna-dwelling anacondas experience increased predation during the dry season. Large anacondas, especially those that are females, experience lower rates of predation than their smaller counterparts. Female green anacondas will frequently cannibalize males, usually during breeding aggregations. (Elvey and Newlon, 1998; Rivas and Owens, 2000; Rivas, 2000; Strimple, 1993)

To avoid attackers, green anacondas will hide by burrowing into mud or fleeing into nearby water. However, when directly attacked or threatened, anacondas coil up into a ball. This posture allows them to protect their head, and also enables them to strike at the attacker. They also defend themselves by emitting an odor from their cloacal glands. (Grzimek, 2003; O'Shea, 2007; Rivas, 2000)

  • Anti-predator Adaptations
  • cryptic

Ecosystem Roles

Green anacondas act as predators to a wide array of vertebrate species, with young snakes also serving as prey to a number of large predators. Health assessments of green anacondas in captivity have found that they can harbor a number of internal parasites. Captive snakes are also predisposed to diseases. Opportunistic parasites are most likely due to suboptimal husbandry and captivity. For the most part, wild anacondas seem to withstand parasitic loads well and are seemingly healthy. (Calle, et al., 1994; Calle, et al., 2001; Ferreira, et al., 2002; Keirans, 1972; Miller, et al., 2004; Moravec and Santos, 2009; Rivas, 2000)

Commensal/Parasitic Species
  • Elizabethkingia meningoseptica (Class Flavobacteria, Phylum Bacteriodetes)
  • Stenotrophomonas maltophilia (Class Gammaproteobacteria, Phylum Proteobacteria)
  • Aeromonas hydrophila (Class Gammaproteobacteria, Phylum Proteobacteria)
  • Providencia rettgeri (Class Gammaproteobacteria, Phylum Proteobacteria)
  • Haemoproteus sp. (Phylum Apicomplexa, Kingdom Chromalveolata)
  • iguana ticks (Amblyomma dissimile)
  • Amblyomma fulvum (Family Ixodidae, Class Arachnida)
  • Phaeotabanus nigriflavus (Order Diptera, Class Insecta)
  • Stenotabanus bequaerti (Order Diptera, Class Insecta)
  • Stenotabanus cretatus (Order Diptera, Class Insecta)
  • Tabanus occidentalis (Order Diptera, Class Insecta)
  • Dracunculus brasiliensis (Family Dracunculidae, Phylum Nematoda)
  • Crepidobothrium sp. (Class Cestoda, Phylum Platyhelminthes)

Economic Importance for Humans: Positive

The indigenous peoples of Brazil and Peru practice the commercialization of anacondas. The folklore of these regions considers green anacondas to have magical and spiritual properties, and their body parts are sold for ritualistic purposes. Their fat is used as medicine against rheumatism, inflammation, infection, asthma, and thrombosis in these areas. (Alves and Pereira, 2007; Alves, et al., 2007; Grzimek, 2003; O'Shea, 2007)

Economic Importance for Humans: Negative

Green anacondas are among the only snakes that can reach the proportions necessary to possibly kill and consume a human being. However, attacks by green anacondas are rare due to low human population densities where the snakes are normally found. (O'Shea, 2007)

  • Negative Impacts
  • injures humans
    • bites or stings

Conservation Status

Possible threats to this species include habitat loss and the exotic pet trade. Anacondas are listed as a CITES Appendix II species, but information on them is relatively scarce. The Profauna (the Venezuelan Fish and Wildlife Service), the Wildlife Conservation Society, and the Convention for the International Trade of Endangered Species (CITES) has funded the Green Anaconda Project to further understand potential threats to this species. ("UNEP-WCMC Species Database: CITES-Listed Species", 2012; Rivas and Burghardt, 2001; Rivas and Owens, 2000)

Other Comments

The genus Eunectes is proposed to have originated in the Miocene Epoch. In the early 20th century, President Theodore Roosevelt offered a $5000 reward for the capture of a green anaconda and its transportation to the New York Zoological Society (now known as the Wildlife Conservation Society). This prize has since been withdrawn. (Hsiou and Albino, 2009; O'Shea, 2007; Schmidt and Inger, 1957)


Luckele Milord (author), Radford University, Joel Hagen (editor), Radford University, Jeremy Wright (editor), University of Michigan-Ann Arbor.



living in the Nearctic biogeographic province, the northern part of the New World. This includes Greenland, the Canadian Arctic islands, and all of the North American as far south as the highlands of central Mexico.

World Map


living in the southern part of the New World. In other words, Central and South America.

World Map


uses sound to communicate


an animal that mainly eats meat


flesh of dead animals.


uses smells or other chemicals to communicate


active at dawn and dusk


having markings, coloration, shapes, or other features that cause an animal to be camouflaged in its natural environment; being difficult to see or otherwise detect.


a substance used for the diagnosis, cure, mitigation, treatment, or prevention of disease


union of egg and spermatozoan


mainly lives in water that is not salty.


having a body temperature that fluctuates with that of the immediate environment; having no mechanism or a poorly developed mechanism for regulating internal body temperature.

indeterminate growth

Animals with indeterminate growth continue to grow throughout their lives.


(as keyword in perception channel section) This animal has a special ability to detect heat from other organisms in its environment.


referring to animal species that have been transported to and established populations in regions outside of their natural range, usually through human action.


offspring are produced in more than one group (litters, clutches, etc.) and across multiple seasons (or other periods hospitable to reproduction). Iteroparous animals must, by definition, survive over multiple seasons (or periodic condition changes).


makes seasonal movements between breeding and wintering grounds


specialized for swimming

native range

the area in which the animal is naturally found, the region in which it is endemic.


reproduction in which eggs develop within the maternal body without additional nourishment from the parent and hatch within the parent or immediately after laying.


chemicals released into air or water that are detected by and responded to by other animals of the same species


an animal that mainly eats fish


Referring to a mating system in which a female mates with several males during one breeding season (compare polygynous).


rainforests, both temperate and tropical, are dominated by trees often forming a closed canopy with little light reaching the ground. Epiphytes and climbing plants are also abundant. Precipitation is typically not limiting, but may be somewhat seasonal.

seasonal breeding

breeding is confined to a particular season


reproduction that includes combining the genetic contribution of two individuals, a male and a female


lives alone


a wetland area that may be permanently or intermittently covered in water, often dominated by woody vegetation.


uses touch to communicate


that region of the Earth between 23.5 degrees North and 60 degrees North (between the Tropic of Cancer and the Arctic Circle) and between 23.5 degrees South and 60 degrees South (between the Tropic of Capricorn and the Antarctic Circle).


Living on the ground.


the region of the earth that surrounds the equator, from 23.5 degrees north to 23.5 degrees south.

tropical savanna and grassland

A terrestrial biome. Savannas are grasslands with scattered individual trees that do not form a closed canopy. Extensive savannas are found in parts of subtropical and tropical Africa and South America, and in Australia.


A grassland with scattered trees or scattered clumps of trees, a type of community intermediate between grassland and forest. See also Tropical savanna and grassland biome.

temperate grassland

A terrestrial biome found in temperate latitudes (>23.5° N or S latitude). Vegetation is made up mostly of grasses, the height and species diversity of which depend largely on the amount of moisture available. Fire and grazing are important in the long-term maintenance of grasslands.


movements of a hard surface that are produced by animals as signals to others


uses sight to communicate


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