Green anaconda

The green anaconda (Eunectes murinus), also known as the giant anaconda, emerald anaconda, common anaconda, common water boa, or southern green anaconda, is a semi-aquatic boa species found in South America and the Caribbean island of Trinidad. It is the largest, heaviest, and second longest snake in the world, after the reticulated python. No subspecies are currently recognized. Like all boas, it is a non-venomous constrictor.

The term "anaconda" often refers to this species, though the term could also apply to other members of the genus Eunectes. Fossils of the snake date back to the Late Pleistocene in the Gruta do Urso locality.

Taxonomy
In the famous 10th edition of Systema Naturae of 1758, Carl Linnaeus cited descriptions by Albertus Seba and by Laurens Theodorus Gronovius to erect the distinct species murina of his new genus Boa, which contained eight other species, including Boa constrictor. The generic name Boa came from an ancient Latin word for a type of large snake. The first specimens of Boa murina were of immature individuals from 2.5 to 3.0 ft in length. In 1830, Johann Georg Wagler erected the separate genus Eunectes for Linnaeus's Boa murina after more and larger specimens were known and described. Because of the masculine gender of Eunectes, the feminine Latin specific name murina was changed to murinus and that stayed like ″"murinus" forever.

Linnaeus almost certainly chose the scientific name Boa murina based on the original Latin description given by Albertus Seba in 1735: "Serpens testudinea americana, murium insidiator" [tortoise-patterned (spotted) American snake, a predator that lies in wait for mice (and rats)]. The Latin adjective murinus (murina) in this case would mean "of mice" or "connected with mice", understood in context as "preying on mice", and not as "mouse-gray-colored", another possible meaning of Latin murinus, as now often wrongly indicated for E. murinus.

Early English-language sources, such as George Shaw, referred to the Boa murina as the "rat boa". The Penny Cyclopaedia (Vol. 5) entry for boa explained: "The trivial name murina was given to it from being said to lie in wait for mice." Linnaeus described the appearance of the Boa murina in Latin as rufus maculis supra rotundatis [reddish-brown with rounded spots on upper parts] and made no reference to a gray coloration. Early descriptions of the green anaconda by different authors variously referred to the general color like brown, glaucous, green, or gray.

Common names for E. murinus include green anaconda, anaconda, common anaconda, and water boa.

Description
The green anaconda is the world's heaviest and one of the world's longest snakes, reaching a length of up to 5.21 m long. More typical mature specimens reportedly can range up to 5 m, with adult females, with a mean length of about 4.6 m, being generally much larger than the males, which average around 3 m. Weights are less well studied, though reportedly range from 30 to 80 kg in a typical adult.

It is the largest snake native to the Americas. Although it is slightly shorter than the reticulated python, it is far bulkier. The bulk of a 5.2 m green anaconda is comparable to that of a 7.4 m reticulated python. Reports of anacondas 35 - 40 ft or even longer also exist, but such claims must be regarded with caution, as no specimens of such lengths have ever been deposited in a museum and hard evidence is lacking. The longest and heaviest verified specimen encountered by Dr. Jesús Antonio Rivas, who had examined more than 1,000 anacondas, was a female 5.21 m long and weighing 97.5 kg. In 1937, a specimen shot in Guyana was claimed to have measured 5.9 m long and weighed 163 kg.

The color pattern consists of an olive green background overlaid with black blotches along the length of the body. The head is narrow compared to the body, usually with distinctive orange-yellow striping on either side. The eyes are set high on the head, allowing the snake to see out of the water while swimming without exposing its body. The anaconda's jaw bones splay open at the front because they are loosely connected. This allows it to swallow prey larger than the size of its head. The windpipe in its mouth allows it to breathe while swallowing its prey. Its largest organ is the liver. The digestion process takes many days to complete. During this time the anaconda behaves very sluggishly.

Difficulties in determining maximum size of anacondas
The remote location of the snake's habitat has historically made locating, capturing, and returning specimens difficult. Transporting very large specimens to museums, especially before substantial decay, is difficult, though this has not prevented the return of much larger and more cumbersome crocodilian specimens. Skins can stretch substantially, increasing the snake's size by more than 50% if stretched during the tanning process.

Reports without physical proof are considered dubious if from non-scientists, as such individuals may at worst be more interested in promoting themselves or telling a good tale, or at the least may not be sufficiently trained in proper measurement methods. Observational reports of animals which were not captured are even more dubious, as even trained scientists often substantially overestimate the size of anacondas prior to capture. According to the Guinness Book of World Records, this species has been perhaps subject to the most extreme size exaggerations of any living animal.

Historical records
Numerous historical accounts of green anacondas are reported, often of improbable sizes. Several zoologists, notably Henry Walter Bates and Alfred Russel Wallace, among others, note rumors of snakes beyond 30 or long, but in each case, their direct observations were limited to snakes around 20 ft in length. Numerous estimates and second-hand accounts abound, but are generally considered unreliable. To prove the point of overestimating, in Guyana in 1937, zoologist Alpheus Hyatt Verrill asked the expedition team he was with to estimate the length of a large, curled-up anaconda on a rock. The team's guesses ran from 6.1 to 18.3 m. When measured, this specimen was found to be 5.9 m.

Almost all specimens in excess of 6 m, including a much-publicized specimen allegedly 11.36 m long, have no voucher specimens including skins or bones.

The skin of one specimen, stretched to 10 m, has been preserved in the Instituto Butantan in São Paulo and is reported to have come from an anaconda of 7.6 m in length. While in Colombia in 1978, herpetologist William W. Lamar had an encounter with a large female specimen 7.5 m long, estimated to weigh between 136 and 180 kg. In 1962, W. L. Schurz claimed to have measured a snake in Brazil of 8.46 m with a maximum girth of 112 cm.

One female, reportedly measuring 7.9 m in length, shot in 1963 in Nariva Swamp, Trinidad, contained a 1.5 m caiman. A specimen of 7.3 m, reportedly with a weight of 149 kg, was caught at the mouth of the Kassikaityu River in Guyana, having been restrained by 13 local men, and was later air-lifted for a zoo collection in the United States, but died in ill health shortly thereafter. The largest size verified for E. murinus in captivity was for a specimen kept in Pittsburgh Zoo and PPG Aquarium, which grew to a length of 6.27 m by the time she died on July 20, 1960. When this specimen was 5.94 m long, she weighed 91 kg.

The estimated weight for an anaconda in the range of 8 m would be at least 200 kg. National Geographic has published a weight up to 227 kg for E. murinus, but this is almost certainly a mere estimation. Weight can vary considerably in large specimens depending on environmental conditions and recent feedings, with Verrill's aforementioned specimen, having been extremely bulky, scaled at 163 kg, whereas another specimen considered large at 5.06 m, weighed only 54 kg.

Estimates of maximal size
Size presents challenges to attain breeding condition in larger female anacondas. While larger sizes provide the benefit of a larger number of offspring per clutch, the breeding frequency of the individuals reduces with size, indicating that a point exists at which the advantage of a larger clutch size is negated by the female no longer being able to breed. For the anaconda, this limit was estimated at 6.7 m in total length. This is consistent with the results of a revision of the size at maturity and maximum size of several snakes from North America, which found that the maximum size is between 1.5 and 2.5 times the size at maturity.

The minimum size of breeding anacondas in a survey of 780 individuals was 2.1 m in snout–vent length, indicating that maximum size attained by anacondas following this pattern would be 5.3 m in snout–vent length. However, most anacondas are captured from the llanos, which is more accessible to humans and has smaller prey available, while the rainforest, which is much less explored and has more plentiful large prey, may be home to larger snakes.

The green anaconda is an apex predator, positioning them at the top of the food chain.

Distribution and habitat
Eunectes murinus is found in South America east of the Andes, in countries including Colombia, Venezuela, the Guianas, Ecuador, Peru, Bolivia, Brazil, the island of Trinidad, and as far south as northern Paraguay. The type locality given is "America".

In April 2021, green anacondas were added to Florida's list of prohibited nonnative species. eleven green anacondas have been verifiably observed in the wild in Florida, including one juvenile found in the Florida Everglades. The range of these specimens, some of which were able to evade capture, spans from Gainesville to Homestead, Florida - a distance of over 350 mi. Florida is the only state in the continental United States with suitable habitat for the species.

While individual specimens have been observed or captured in Florida dating back to 2010, there is no known established population. However, the ecological, economic, and safety risks posed by their existence in the state are deemed 'very high' by the Florida Fish and Wildlife Conservation Commission, with the ecological threat reaching the extreme risk level, as introduced anacondas are apex predators, able to compete with Florida's native species.

Anacondas live in swamps, marshes, lagoons, and slow-moving streams and rivers, mainly in the tropical rainforests and seasonally flooded savannas of the Amazon and Orinoco basins. They are cumbersome on land, but stealthy and sleek in the water. Their eyes and nasal openings are on top of their heads, allowing them to lie in wait for prey while remaining nearly completely submerged.

Behavior
Anacondas are mostly nocturnal and aquatic. They float underwater and are known to swim quickly. Their eyes and nose are located on the top of the head, allowing the snake to breathe and watch for prey while the rest of the body is hidden underwater. When prey passes by or stops to drink, the anaconda strikes, without eating or swallowing it, and coils around it with its body, then constricts and suffocates the prey to death.

Feeding
Anacondas are apex predators with a wide variety of prey, almost anything they can overpower, including fish, amphibians, birds, various mammals and other reptiles. Particularly large anacondas may consume tapirs, deer, peccaries, capybaras, jaguars, and caimans, but such large meals are rare. Juvenile anacondas feed on small birds and young caiman typically 40–70 grams in weight. As they develop, their diet becomes increasingly complex. Prey availability varies more in grasslands than in river basins.

In both habitats green anacondas have been found to feed on large prey, usually 14% to 50% of their own mass. Examples of prey include broad-snouted caimans, spectacled caimans, yacare caimans, black caimans, smooth-fronted caimans, wattled jacanas, capybaras, red-rumped agoutis, collared peccaries, South American tapirs, boa constrictors, brown-banded water snakes, green iguanas, cryptic golden tegus, scorpion mud turtles, gibba turtles, Arrau turtles, savanna side-necked turtles, red side-necked turtles, and northern pudús. Capybaras are common prey for the green anaconda.

Large prey occasionally causes serious injuries and death. This risk is likely reduced when anacondas can drown the prey. Some feed on carrion and conspecifics, usually inside or around water. Large anacondas can go weeks to months without food after a large meal, because they have a low metabolism. Females have increased postpartum feeding to recover from their reproductive investment.

Many local stories and legends report anacondas feeding on humans, but there is little evidence of this. Cannibalism among green anacondas is known. Most recorded cases involving a larger female consuming a smaller male. Scientists hypothesize several explanations, including the great sexual dimorphism of the species, and a female's need for food after breeding to sustain the long gestation. The nearby male simply provides the opportunistic female a ready source of nutrition.

Reproduction
Individuals are solitary until the mating season, which occurs during the rainy season, and can last several months, usually from April to May. Typically, females leave pheromone trails for males to follow, but how males track it is unclear. It may be that the pheromone is airborne. This theory is supported by the observation of females that remain motionless, while many males move towards them from all directions. Male anacondas also often flick their tongues to sense chemicals that signal the presence of a female.

Often many males find the same female and wrap around her to copulate with her. Such a "breeding ball" of up to 12 males can stay in this position for two to four weeks as males wrestle in slow-motion to mate with the female.

During mating, males use their spurs to arouse the female. They aggressively press their cloacal regions hard against the female body, while continuously scratching her with their spurs. This can produce a scratching sound. Mating approaches its climax when the stimulus of the males' spurs induces the female to raise her cloacal region, allowing the cloacae of the two snakes to move together. The male then coils his tail around the female, and they copulate. Often, the strongest and largest male wins, but the females may choose another male as they are much larger and stronger. Courtship and mating occur almost exclusively in water.

Mating is followed by a gestation period of six to seven months. The species is ovoviviparous, with females giving birth to live young. Litters usually consist of 20 to 40 offspring, although as many as 100 may be produced. After giving birth, females may lose up to half their weight.

Neonates are around 70 - 80 cm long and receive no parental care. Because of their small size, they often fall prey to other animals. If they survive, they grow quickly until sexual maturity in a few years, then grow more slowly.

When no males are available, facultative parthenogenesis is possible, producing viable homozygous litter. In 2014, green anaconda in West Midland Safari Park gave birth to three young through parthenogenesis.

Longevity
Green anacondas in the wild live for about 10 years. In captivity, they can live 30 years or more. The 2023 Guinness Book of World Records for the oldest living snake in captivity is a green anaconda aged 37 years 317 days, verified on 14 May 2021 by Paul Swires, at Montecasino Bird & Reptile Park in Johannesburg, South Africa.

In popular culture
Anacondas have been portrayed in horror literature and films, often incredibly gigantic, agile and with the ability to swallow adult humans. These traits are occasionally attributed to other species, such as the Burmese python and the reticulated python, but to less extent than to the green anaconda. Despite having the capability to overpower a man, there is no verified evidence of this species consuming humans, unlike the reticulated python. This is possibly because large specimens inhabit remote areas deep inside the Amazon jungle, which is isolated from humans, unlike the python in Asia. Among the most popular of such films are the 1997 film Anaconda and its four sequels.