Green anaconda

• 1. Introduction

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  The green anaconda, Eunectes murinus, once fooled an entire expedition team. In Guyana in 1937, zoologist Alpheus Hyatt Verrill pointed to a large snake curled on a rock and asked his companions to guess its length. Their answers ran from 6.1 to 18.3 metres. When the snake was measured, it came to 5.9 metres. The green anaconda is a non-venomous boa from South America, and it ranks among the longest and heaviest snakes alive. It spends most of its time in the water, and it kills by constriction. Yet few animals have attracted so much exaggeration. Where did its odd names come from? How big does it truly grow? And how does a swamp ambusher hunt, breed and stay alive?

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• 2. Names and Mice

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  In 1758, in the tenth edition of his Systema Naturae, Carl Linnaeus placed this snake in his new genus Boa as the species murina. That genus already held eight other species, including the boa constrictor. Linnaeus drew on descriptions by Albertus Seba and Laurens Theodorus Gronovius. The first specimens were immature animals only 2.5 to 3.0 feet long.

  The species name has a widely repeated misreading. Seba's Latin description of 1735 called it a spotted American snake that lies in wait for mice and rats. So murinus means preying on mice, and not mouse-gray, as many now claim. Linnaeus himself described the animal as reddish-brown with rounded spots on its upper parts. He never mentioned gray. Early English writers such as George Shaw called it the rat boa, and the Penny Cyclopaedia explained that the name came from its supposed habit of waiting for mice.

  In 1830, Johann Georg Wagler created the genus Eunectes for the species, once larger specimens had been described. Because Eunectes is masculine, the Latin species name changed from murina to murinus.

  The Carib people had their own word for it for centuries before any of this. Akayima, with the variants okoyimo and okoimo, comes from Cariban languages, where akayi means snake. The ending -ima marks largeness so strongly that it lifts the word into a category of its own. Literally, the name means The Great Snake.

  Scientists have proposed splitting off new species more than once. Eunectes gigas dates to Latreille in 1801, and Eunectes barbouri to Dunn and Conant in 1936. Eunectes akayima was proposed by Rivas and colleagues in 2024. All three are now treated as synonyms of Eunectes murinus. The question that has dogged the animal longest is simply how large it gets.

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• 3. Measuring a Giant

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  At up to 6.32 metres long, the green anaconda is the heaviest snake in the world and one of the longest. Mature adults reportedly reach about 5 metres. Females are much larger than males, averaging around 4.6 metres against roughly 3 metres for males. Weights are less well studied, but typical adults reportedly weigh between 30 and 80 kilograms. No other snake native to the Americas is bigger.

  The reticulated python is slightly longer, yet the anaconda is far bulkier. A green anaconda of 5.2 metres has about the same bulk as a reticulated python of 7.4 metres.

  Jesús Antonio Rivas has examined more than 1,000 anacondas. The longest and heaviest one he verified was a female of 5.21 metres that weighed 97.5 kilograms.

  Part of the difficulty is geography. The snake lives in remote country, which makes finding, catching and hauling out specimens hard. Museums have still received far larger and clumsier crocodilians, so transport alone does not explain the gap. Skins add another error, because stretching during tanning can add more than 50% to the apparent length.

  People add the rest. Reports without physical proof are doubtful when they come from non-scientists, who may want to boast, tell a good tale, or simply lack training in measuring. Even trained scientists often overestimate an anaconda before it is caught. The Guinness Book of World Records says this species has perhaps suffered the most extreme size exaggeration of any living animal.

  Weight is as slippery as length. Verrill's rock-top snake in Guyana scaled 163 kilograms, while another specimen considered large, at 5.06 metres, weighed only 54 kilograms. Recent meals and local conditions swing the number. A snake of around 8 metres, if one existed, would be estimated at no less than 200 kilograms.

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• 4. Tall Tales and Records

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  Henry Walter Bates and Alfred Russel Wallace both heard rumours of anacondas beyond 30 or 40 feet. What each of them actually saw was closer to 20 feet. Many estimates and second-hand stories exist, but they are generally considered unreliable. Almost every specimen claimed beyond 6 metres has no skin or bones to back it up. That includes a much-publicised snake allegedly 11.36 metres long.

  Some claims come with slightly more to examine. At the Instituto Butantan in São Paulo, a skin stretched to 10 metres is preserved, though it is reported to have come from a snake of 7.6 metres. In 1962, W. L. Schurz claimed to have measured a Brazilian snake at 8.46 metres, with a maximum girth of 112 centimetres. In Colombia in 1978, herpetologist William W. Lamar met a female of 7.5 metres, estimated at 136 to 180 kilograms.

  A female reportedly 7.9 metres long, shot in 1963 in Nariva Swamp, Trinidad, held a caiman of 1.5 metres. At the mouth of the Kassikaityu River in Guyana, 13 local men restrained a snake reported at 7.3 metres and 149 kilograms. It was flown to a zoo collection in the United States and died in poor health soon afterward.

  The largest verified captive lived at the Pittsburgh Zoo and PPG Aquarium. When she was 5.94 metres long, she weighed 91 kilograms. She reached 6.27 metres before she died on the 20th of July 1960. National Geographic has published a weight of up to 227 kilograms, but that is almost certainly only an estimate.

  Biology suggests a ceiling. Bigger females produce larger litters, but they breed less often as they grow. At some size the gain in litter is cancelled by the loss of breeding, and for the anaconda that limit was estimated at 6.7 metres. It fits a review of North American snakes, which found that maximum size runs 1.5 to 2.5 times the size at maturity.

  In a survey of 780 anacondas, the smallest breeder measured 2.1 metres from snout to vent. That would put the maximum near 5.3 metres by the same measure. Most anacondas are caught in the llanos, which are easier to reach and hold smaller prey. The less explored rainforest, with plentiful large prey, may shelter bigger snakes.

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• 5. Built for Water

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  Olive green skin overlaid with black blotches gives the green anaconda its name. The head is narrow compared with the body, usually with orange-yellow striping down each side. The eyes and nostrils sit on top of the head. That lets the snake breathe and watch for prey while the rest of its body stays hidden underwater.

  The jaw is built for enormous meals. Flexible ligaments, not bone, connect the front of the jaw bones, so they splay open. The snake can swallow prey larger than its own head. A windpipe in the mouth lets it keep breathing during the swallow. Its largest internal organ is the liver. Digestion takes many days, and through that time the anaconda behaves very sluggishly.

  In Colombia, Venezuela, the Guianas, Ecuador, Peru, Bolivia and Brazil, it lives east of the Andes. It also lives on the island of Trinidad, and as far south as northern Paraguay. The core of its range is the Amazon and Orinoco basins. There it occupies swamps, marshes, lagoons and slow-moving streams, in tropical rainforest and seasonally flooded savanna. On land it is cumbersome, but in water it is stealthy and sleek.

  Anacondas are mostly nocturnal. They float below the surface and can swim quickly. When prey passes by or stops to drink, the anaconda strikes. It does not bite to eat. It coils around the animal, squeezes, and suffocates it.

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• 6. What Anacondas Eat

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  Capybaras are a common meal for the green anaconda, which will take almost anything it can overpower. Prey includes fish, amphibians, birds, mammals and other reptiles. Juveniles feed on small birds and young caimans. Particularly large adults may take tapirs, deer, peccaries, capybaras, jaguars and caimans, though such meals are rare. In both grassland and river basin habitats, anacondas have been found eating large prey, usually 14-50% of their own mass. Prey availability varies more in grasslands than in river basins.

  The recorded menu is long. It includes broad-snouted, spectacled, yacare, black and smooth-fronted caimans, along with wattled jacanas, red-rumped agoutis and collared peccaries. South American tapirs, boa constrictors, green iguanas, several kinds of turtle and northern pudús also appear on it.

  In southeastern Peru, potential prey also includes lowland pacas, brocket deer, cocoi herons, neotropical otters and giant otters. Observers there have seen anacondas climb to the top of trees after green iguanas and brown-throated sloths. Anacondas may also take domestic animals such as dogs, chickens, cattle, water buffalo, sheep, cats and wild boar.

  Large prey sometimes injures or kills the snake, a risk that likely shrinks when it can drown the victim. Because its metabolism is low, a large anaconda can go weeks to months without food after a big meal. Some feed on carrion. Even as an apex predator, a large adult can end up as a meal for Orinoco crocodiles, black caimans and jaguars.

  Cannibalism is also known. Most recorded cases involve a larger female eating a smaller male. Scientists point to the great size difference between the sexes. They also suggest that a female needs food after breeding to sustain her long gestation, and a nearby male is an easy source.

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• 7. Breeding and Lifespan

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  The rainy season brings the mating season, which usually runs from April to May, though it can last several months. Until then, individuals are solitary. Females leave pheromone trails for males to follow. How the males track them is unclear, but the trail may be airborne, since males converge from all directions on females that sit motionless. Males also flick their tongues to sense chemical signals of a female.

  Often many males find the same female and wrap around her. This breeding ball can include up to 12 males and last two to four weeks, as the males wrestle in slow motion. The males scratch the female with their spurs, sometimes producing a scratching sound, and press against her. When she raises her cloacal region, the male coils his tail around her. The largest, strongest male often wins, but the much larger female may choose another. Courtship and mating happen almost entirely in water.

  Gestation lasts six to seven months. The species is ovoviviparous, so females give birth to live young. Litters usually hold 20 to 40 offspring, though as many as 100 may be produced. A mother may lose up to half her weight afterward. Newborns measure 70 to 80 centimetres and get no parental care. Their small size makes them prey for other animals. Survivors grow quickly until sexual maturity in a few years, then more slowly.

  When no males are around, facultative parthenogenesis is possible, producing viable homozygous litters. In 2014, a green anaconda at West Midland Safari Park gave birth to three young this way.

  In the wild, green anacondas live about 10 years. In captivity, they can reach 30 years or more. The 2023 Guinness Book of World Records lists a green anaconda at Montecasino Bird & Reptile Park in Johannesburg, South Africa, as the oldest living snake in captivity. Paul Swires verified its age on the 14th of May 2021 at 37 years and 317 days.

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• 8. Fiction and Florida

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  Horror novels and films often show anacondas as incredibly gigantic and agile, able to swallow adult humans. Those traits are sometimes given to Burmese and reticulated pythons too, though less often than to the green anaconda. Despite being able to overpower a man, the green anaconda has no verified record of eating one. The reticulated python does. One possible reason is that large anacondas live in remote parts of the Amazon, far from people, unlike the python in Asia.

  The real-world story has moved to Florida. Individual green anacondas have been seen or captured there since 2010. In April 2021, the state added the species to its list of prohibited nonnative species. By August 2023, eleven had been verifiably observed in the wild, including a juvenile in the Florida Everglades. Their locations span from Gainesville to Homestead, a distance of more than 350 miles, and some evaded capture.

  The Florida Fish and Wildlife Conservation Commission rates the ecological, economic and safety risks as very high, with the ecological threat at the extreme level. Introduced anacondas could compete with native species and prey on white-tailed deer, American alligators, American crocodiles, wading birds and other wildlife. Florida is the only state in the continental United States with suitable habitat. As of 2023, no established population is known.

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