I.

Introduction

Anaconda (snake), common name for large South American snakes that kill by squeezing prey. Anacondas are among the largest and most powerful snakes in the world. Female anacondas grow bigger than males and can reach enormous size, ranking as the heaviest of all snakes. Anacondas spend much of their time in water. They hunt reptiles and mammals, as well as fish and birds. The biggest anacondas can swallow prey as large as caimans or deer. They rarely attack humans.

The name anaconda is said to come from a Tamil word meaning “elephant killer,” originally used for pythons in Sri Lanka. Local names for the snakes in South America include the Spanish term matatoro, meaning “bull killer,” and the Native American terms sucuri, yakumama, and jibóia. Anacondas are members of the boa family and are sometimes called water boas.

II.

Habitat and Range

Anacondas live mainly in wet tropical regions with marshes, lakes, or rivers. They will also move into areas that flood during part of the year such as the llanos (treeless prairies) found in Venezuela and Colombia. In some regions, they use caves under tree roots along river banks as a shelter during the dry season or droughts.

Anacondas are only found in South America east of the Andes. The four species have combined ranges from Venezuela south to Argentina. The green anaconda is found through much of the Amazon River basin, as well as in the Orinoco River basin and parts of the Guiana region, and on the island of Trinidad. The yellow anaconda is found further south, through the Paraná and Uruguay river basins from Bolivia and southern Brazil into Paraguay and Argentina. The dark-spotted or de Schauensee’s anaconda lives in parts of northeastern Brazil and French Guiana. The Bolivian anaconda is mainly restricted to Bolivia.

III.

Physical Description and Behavior

Like boas and pythons, anacondas retain primitive features that indicate ancient lizard ancestors. The snakes have traces of a pelvis and hind limbs. The tiny hind limbs in anacondas are reduced to clawed spurs that are larger in males and used to stimulate the females during mating. Anacondas also have two functioning lungs, unlike more advanced snakes, which have only one functional lung.

Anacondas show the greatest difference in size between adult males and females of any kind of vertebrate (animals with a backbone) except for certain fishes. The average female green anaconda, the largest species, is almost 5 times as heavy as an average male. Fully grown female green anacondas may reportedly reach 10 m (32 ft) in length and weigh up to 250 kg (about 550 lb), although around 6 m (20 ft) and 107 kg (235 lb) is average. The yellow anaconda is smaller, up to 4 m (13 ft) in length and 30 kg (66 lb) in weight. Anacondas are sometimes called the largest snakes in the world. Reticulated pythons are longer on average, reaching 11 m (33 ft), but female green anacondas are heavier with a bigger girth.

Anacondas have loosely constructed skulls and jaws, with some bones held together by ligaments or skin that allow the jaws to spread apart to many times their normal size. The lower jaws can open almost 180 degrees and are not fused together in front, permitting a huge gape. Massive jaw muscles give anacondas a powerful bite. The jaws have more than 100 sharp, backward-curving teeth that prevent prey from pulling away. Anacondas, along with boas and pythons, have two extra rows of teeth on the roof of the mouth, called pterygoid teeth. These additional rows of teeth aid in holding and ingesting prey, and can be moved independently, helping the snake “walk” its jaws and head over prey to swallow the victim.

An important feature found in anacondas, boas, and pythons are heat-sensing pits along the lips. These organs can detect the body heat given off by warm-blooded animals even in dark conditions. A special smelling organ, called Jacobson’s organ, on the roof of the mouth reads the chemical clues collected by the snake’s flicking tongue. An anaconda’s hearing is limited, but it may detect sounds and vibrations underwater, as well as through the ground. Its skin is also sensitive.

The anaconda’s scientific name Eunectes means “good swimmer” in Greek and reflects the snake’s mainly aquatic habits. Its eyes and nostrils are set high on its head, allowing it to see and breathe with the rest of its body under water. An anaconda can reportedly hold its breath for 45 minutes or more, but in normal circumstances the snake may stay submerged for around 10 minutes.

Anacondas have skin colors and patterns that provide camouflage in swampy or jungle environments. Shades of green, brown, and yellow are common, patterned with dark spots and blotches. The underbelly scales are generally lighter in color.

It takes male anacondas around 18 months to reach sexual maturity. Females need three years. Anacondas can live 30 or more years, but the average life span in the wild is about 10 years for snakes that reach adulthood.

IV.

Reproduction

Female anacondas give off a special scent called a pheromone that males can detect with their flicking tongues. Like some other types of snakes, anacondas sometimes form breeding balls (also called mating balls) in which many males swarm over a single female to attempt to mate. Up to a dozen or more male anacondas may wrap themselves around a much larger female, trying to insert their sex organs into the female’s cloaca. The males scratch with their tiny hind leg spurs to stimulate the female to mate. A successful male leaves a waxy plug in the female's cloaca to block other males from mating. Anaconda breeding balls take place in shallow water and can last for weeks. Females may also breed with single males they encounter.

Anacondas give birth to live young, unlike pythons, which all lay eggs. However, anacondas are actually ovoviviparous, meaning they retain eggs internally until the young hatch. While they are carrying developing eggs inside, females typically do not eat. The gestation period is about six to seven months. The number of young born can range from 20 to 100.

Two forms of cannibalism are reported with female anacondas. Large females sometimes kill and eat male anacondas, more than often following mating. Like other live-bearing members of the boa family, female anacondas also eat stillborn offspring and undeveloped eggs expelled when giving birth. Since females do not feed while carrying developing young, both behaviors may be related to nutritional needs. The females may eat adult males as a ready protein source before they stop feeding. Consuming stillborn young and unhatched eggs likely allows the females to recover more quickly from a long period without food.

V.

Hunting and Diet

Anacondas are mainly ambush predators, similar to other constrictors. However, they are more specialized for aquatic hunting. Smell and body heat help the snakes find prey. Sounds or vibrations carried through water may also help. Eyesight is probably used only for the final up-close attack. Anacondas often swim just below the surface, hidden by vegetation before they strike. They first bite to hold the prey then throw massive coils around its body.

Anacondas use constriction to squeeze prey to death. They gradually tighten their powerful coils, preventing an animal from breathing. Recent research indicates that the enormous pressure of the coils may also stop the heart from beating, quickly cutting off circulation and the blood supply to the brain. Constriction may also snap the animal's neck or spine, and break other bones. In many cases, anacondas simply drown victims by holding them underwater. Once dead, the prey is generally swallowed headfirst.

Adult anacondas feed on a range of animals, usually caught in or near water. Mammals hunted include capybaras, deer, tapirs, peccaries, and on occasion jaguars or other cats. Domestic dogs or sheep may also fall victim to anacondas. Aquatic birds and reptiles such as caimans are also commonly preyed on. Young anacondas eat fish, frogs, and small rodents. After a large meal, anacondas may not eat again for months.

Humans are the main threat to adult anacondas, which have few predators in the wild. However, female anacondas sometimes eat male anacondas. Large caimans may eat anacondas they can overpower. Young anacondas are prey for many animals, including caimans and members of the cat family.

VI.

Humans and Anacondas

Anacondas figure prominently in the religions of native peoples of the Amazon region, often as spirits that protect forests or rivers. In some legends, giant anacondas traced out the winding courses that rivers follow.

Anacondas are not generally hunted as food, but some tribal groups use anaconda fat as a folk medicine for respiratory ailments. In some regions, anacondas are routinely killed out of fear or as potential threats to livestock. Illegal trade in skins is now the biggest threat to anaconda populations. Habitat loss is also a problem when jungle areas in parts of South America are cleared for ranching. International trade in live anacondas is possible with a permit under current Convention on International Trade in Endangered Species of Wild Fauna and Flora (CITES) regulations.

Anacondas are often displayed in zoos and have been bred in captivity. However, they are rarely kept as pets. Unlike the generally docile species of pythons and boas available in the pet trade, anacondas can be aggressive and difficult to handle. When agitated, they can also give off an offensive smell from musk glands in the cloaca.

Anacondas are large enough and strong enough to overpower and kill a human with their coils. In the wild such attacks appear to be rare—more rare at least than attacks by pythons on humans. There are no authenticated cases of anacondas eating humans, although tribal peoples consider the snakes a threat to children. As with pythons, anacondas would probably have difficulty swallowing an adult human victim headfirst because of the width of the shoulders.

In the popular imagination, particular fascination centers on the maximum size anacondas can reach. The largest anaconda reported by a source considered reliable was a specimen about 11 m (37 ft) long. The snake was shot and measured by scientists, but it allegedly revived and escaped before it could be preserved for further research. The British adventurer Percy H. Fawcett claimed to have shot an anaconda over 18 m (60 ft) long, a size debated by experts. Tales of even more gigantic anacondas lack scientific evidence.

Scientific understanding of anacondas has increased considerably in recent years. Field studies of the giant snakes provide important new information. Researchers working in Venezuela have captured green anacondas and force-fed them radio transmitters, allowing the snakes to be regularly tracked for the first time to fill in many details about their behavior and diet.

Scientific classification: Anacondas belong to the family Boidae. There are four species, all in the genus Eunectes. The common anaconda is classified as Eunectes murinus, and the yellow anaconda as Eunectes notaeus. The dark-spotted or de Schauensee’s anaconda is Eunectes deschauenseei. The Bolivian anaconda is Eunectes beniensis. A species once considered separate and classified as “Eunectes barbouri” is now considered the same as Eunectes murinus.