San Francisco State University
Department of Geography
Geography 316:  Biogeography       Revised 5/20/00 bh

The Biogeography of the Green Anaconda   (Eunectus murinus )

by Joshua Lail, student in Geography 316

Kingdom: Animalia
Phylum: Chordata
Sub Phylum - Vertebrata
Class: Reptilia
Order: Saguamata
Family: Boidae
Genus: Eunectus
Species: Eunectus murinus

 Anaconda's are seldom found far from water and inhabit a very large variety of aquatic environments including rivers, large and small streams, lakes, ponds, swamps, ditches, temporary pools, and flooded forests. In addition, they can be found in areas, such as, dry forests, and the llanos (a seasonally inundated tropical savannah) primarily because of the abundance of surface water in these areas for six to eight months during the wet season (Strimple 1993).

Bacon (1978) reported that Trinidad anacondas have adapted to human modified environments, and it has become common in abandoned gravel pits that are rich in waterfowl, frogs, and fish. Anacondas are also found in sheltered waterways close to settlements in the country where poultry is reared (Quelch 1898).

  Natural History:
Opinions on the size attained by anacondas have been offered by many herpetologists and zoologists, some of whom actually had field experience and personally measured them. They range in size from 10.5 feet to about twenty five feet (Murphy 1997). The issue of how large snakes get, has been clouded by authors suggesting that the supersnakes (snakes reported to be in the 50 to 100 foot range) are species not currently known to science, but instead represent undiscovered species (Murphy 1997). Perry (1970) devoted an entire chapter to Sucuriji gigante, a supersnake of the amazon apparently distinct from the common or green anaconda. Dinardo (1993) wrote: Perhaps another (undescribed and truly huge) species of anaconda was responsible for the early reports - a Pleistocene relict already on the way to extinction. It is conceivable that the early South American explorers were viewing and already rare and disappearing species. Snake skins are not acceptable as proof a giant snake size because they are easily stretched to at least twice of its actual length (Murphy 1997). Perhaps the most tell-tale evidence incriminating snake skins as valid records of length comes from William H. Lamar, a respected herpetologist with years of experience in South America. Lamar was forced to kill a huge anaconda in Columbia in 1978. He carefully measured the snakes intact carcass and found it to be 24 feet seven inches in total length(Murphy 1997). Already aware that snake skins were susceptible to stretching, William was extremely careful as he skinned the snake in order that stretching be kept to a minimum. Upon measuring the skin, Lamar found that it now had a total length of 34 feet 7 inches (Murphy 1997).

The ages of wild snakes cannot be reasonably estimated beyond their third or fourth year of life (Pinney 1981). Generally, a snake that is large for its species is old, but how old cannot be determined. Herpetologist do not know enough about natural death rates from predation, disease, and old age to predict how long a species will live in the wild. Our only estimates of life spans of anaconda's are data on captive snakes. It is not known for sure whether any wild snakes would exceed our maximum known ages for captives. In the case of anacondas which have virtually no natural enemies, it is possible that they might survive longer in the wild than our zoo records indicate (Murphy 1997).The oldest individual anaconda on record is a female anaconda who lived for more than 31 years at the Basle Zoo, Switzerland (Pinney 1981).

Anacondas constrict their prey in order to hold on to it so that it does not escape, and to kill it. This usually occurs by the snake grabbing its prey with its mouth, if the prey is small enough, rolling the prey into a coil or two of the snakes body (Murphy 1997). They exact cause of death from constriction is controversial. Some say that constricting snakes kill by suffocation, while others that suffocation combines with a circulatory impairment that results in death (Hardy 1994). Anacondas swallow their prey head first, which aids in the time of swallowing. Anacondas are also known to swallow food while submerged (Wehekind 1955). Wehekind also stated: ... that the waters buoyancy facilitated the snakes movements in maneuvering food into position for swallowing. The size of prey is determined by a relation to the diameter of the prey to the diameter to the snakes head (Murphy 1997).

Eunectus murinus diet consists of fish, reptiles including- turtles. crocodiles, lizards, other snakes even the same species, birds mammals including- rodents, primates, pigs, anteaters, and sometimes small children (Murphy 1997).

There is no evidence that any snake has the ability to charm prey, but what most observers are actually seeing is the innate defense behavior of many animals to "freeze" when confronted by a predator. These animals have evolved a behavior that relies on their cryptic to conceal them from a visually oriented predator (Murphy 1997). However, this may not work well with snakes because they are also using chemo-sensory, heat sensing and vision to locate their prey, the prey animals may use the behavior at inappropriate times (e.g. when they are not well concealed by their coloration) (Lange 1912). Thus, "freezing" in the presence of anaconda, may very well work to the preys disadvantage when confronted by a snake. The body form and other characteristics of anacondas do not fit the model for active foragers. However, there is ample evidence that anacondas do sometimes actively forage for food, and many of these accounts involve the snakes taking domesticated or captive animals from cages (Murphy 1997).

Anaconda’s are often sit and wait predators, they often may have to go for extended periods of time between meals. Droughts, cold spells, declines in prey populations, and other changes in the local ecosystem may create a shortage of food for snakes and other animals. The ability for anacondas to survive these unpredictable circumstances has been honed by natural selection over the millennia (Murphy 1997). Today, anaconda’s have the ability to survive long periods without eating. Snakes have a low metabolic rate and can therefore go without food for long periods of time (Piney 1981). Fasting seems to have evolved from an adaptation to obtaining food from ambush (Murphy 1997). One of the longest fasts reported for any anaconda is that from Wucherer (1861) who discussed a fasting anaconda: Eunectus Murinus seems to be possess an extraordinary capability of fasting; a friend of mine kept the largest specimen I ever saw in close confinement for three years. and it was never known to swallow anything during this period. It died much emaciated.

Anacondas are very vulnerable to predators during and after feeding. These snakes if disturbed during feeding will coil around its prey in an attempt to drag its spoil away (Murphy 1997). The snake will not let go, even if it is attacked (Stimple 1993). After feeding the snake is even more vulnerable to attack, due to the large prey that anacondas eat. The anaconda cannot move very quickly on land due to the extra weight it is carrying (Murphy 1997).

Courtship is an important part of an anacondas mating process. The female anaconda secretes a pheromone (scent) that attracts many males looking to mate (Pinney 1981). The males wrap themselves around the female in a tangle (Murphy 1997). It has been recorded by Lopez (1984) that there has been up to eleven males wrapped around one female. It is not known why the males do this. Once a male is chosen, the courtship continues on a one on one basis (Pinney 1981).

During the mating process the male anaconda will wave his tail back and forth signifying that he is ready to mate. The male generally approaches the female from behind and rubs his chin over the females body while flicking his tongue to pick up her scent. The chin rubbing is thought to stimulate the receptors in the chin scales of the male which ready him for copulation. Most female snakes try to avoid the male at first, the male will often wind his tail around the female so that he may secure himself on top of her and bring their cloacal regions close together. Once the male has positioned himself properly, waves of muscle contractions pass from his tail to his head. These contractions are believed to stimulate the female. The ultimate result of these various contractions is permission of the female for the male to insert one of his hemipenes. Copulation of anaconda's may take several hours (Pinney 1981).

Once the female is impregnated the incubation of the embryos occurs. Temperature affects embryo growth rates (Murphy 1997). Warmer temperatures cause reptile embryos to develop faster, while cooler temperatures result in slower embryonic development. Gestation times vary greatly among anacondas. It can be as long as 280 days or as little as 182 days (Holmstrom 1980). These gestation times are relatively long for snakes, but in general, Boidae have long gestation periods (Murphy 1997). Anaconda young are viviparity or live birth (Pinney 1981). A major advantage viviparous snakes have over oviparous (egg laying) species is the precise control of embryo temperature (Murphy 1997). A female can orient her body during basking to warm the developing young, or she can place herself in shade or submerge them in water. The major disadvantage to viviparity is that the female must carry the weight and volume of the embryos wherever she goes, often making her more vulnerable to predators (Murphy 1997). Pregnant females will often fast frequently until birth due to the reduced room in the body cavity for processing food. The litter size varies between female to female regardless of size. The litter size can vary from 4 to about 82 from reproductive data (Murphy 1997). The new born sizes vary in length from about 20 inches to about 3 feet (Murphy 1997). After birth the new born anaconda's are left on their own (Pinney 1981).

 The evolution of anaconda's is thought to have occurred in the early Pleistocene, most likely from early lizards. Anaconda's most likely split from ancient boa constrictors, which are in the same family (Boidae). They probably evolved due to competition among boa constrictors due to shortages in food source. Anaconda's are much larger and feed on larger prey than do boa's, so there must have been a niche in the ecosystem for a new species to fill in South America during the Pleistocene (Murphy 1997).


The anaconda, Eunectus murinus, inhabits many river systems in tropical South America. It is known from Columbia, Venezuela, Guyana, Suriname, French Guiana, Ecuador, Peru, Brazil, and Bolivia. It also occurs on the island of Trinidad which lies just north of the Rio Orinoco, off the coast of Venezuela (Murphy 1997). The continuous distribution of this giant occurs between approximately 10 north latitude and 26 south latitude (see map).

Map of Distribution:


Other interesting issues:
Attacks on humans by anaconda's is very rare and can be usually attributed to some sort of self defense (Strimple 1993). Many attacks occur because of mistaken identity. One example of this is a boy being attacked while washing rice by a stream. The boy was bitten and held on the hand, until it realized what it had. It was obvious that the snake mistook the boy for an animal drinking water (Quelch 1898). Anaconda's do attack because of self defense, and have sometimes captured and eaten small children who were playing to closely, but this is very rare (Pinney 1981). Human sacrifices to large anaconda's seem improbable. Anaconda's are finicky eaters and none of them are "programmed" to recognize humans as food. Thus, a person placed in a cage or temple with an anaconda, is most likely to die of dehydration, stress, or starvation than of being constricted and eaten (Murphy 1997). On the other hand, a sacrifice scented with fish, bird, or other mammal odor may potentially be consumed (Murphy 1997).

While many of us would grimace at the thought of eating snakes, they are eaten by people in many cultures, and they provide these people with an excellent source of protein (Murphy 1997). Anaconda's may be favored food mainly due to their size.

Human exploitation of anaconda’s as a source of entertainment or as pets is not a very good idea. With their enormous size, bad temper, they are definitely unsuitable as house pets. It is also cruel and inhumane to bring in an enormous snake from the wild to use it as entertainment. These snakes are also used in zoos as educational tools, which gives the public a better understanding of these animals (Pinney 1981). Unfortunately anaconda skins are sometimes used as a source of leather, but in relatively small quantity, in relation to more popular snake skins (Murphy 1997).

Anaconda's will continue to flourish in the Amazon and Orinoco basins as long as humans does not exploit and destroy the snakes food source. The jungle habitat is currently being destroyed, but they are very adapted to a savannah environment, so they will not be very affected by destruction of a certain types of habitat. The largest problem that I see at this time is hunting for skins to make into leather, which is on the rise (Murphy 1997). The anaconda species has been living for thousands of years, so I expect that they will continue inhabiting the earth for many millennia to come

Bacon, P.R. 1978. Flora and Fauna of The Carribean. Port of Spain: Key publications.

Dinardo, J.R. 1993 Letter to the Editors: The Tympanum. Bull. Chicago Herpetology.

Hardy, D.L., Sr. 1994. A re-evaluation of Suffocation as the Cause of Death During Constriction by Snakes. Herpetology.

Holmstrom, W.F. 1981 . Post-parturient behavior of the Common Anaconda, Eunectus Murinus. Zool. Garten.

Lopez, C.G. 1984. Fauna Legendaria. Caracas. Editorial Arte.

Murphy, John C. 1997. Tales of Giant Snakes. Malabar, FL. Krieger Publishing Company.

Perry, R. 1970. The World of the Jaguar. New York. Taplinger Publishing Company.

Pinney, Roy 1981. The Snake Book. New York. Doubleday Company Inc.

Quelch, J.J. 1898. The Boa Constrictors of British Guiana. Ann. Mag. Natural History. Series.

Strimple, P.D. 1993. Overview of the Natural History of the Green Anaconda. Herpetology.

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