Anaconda (snake)

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.

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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.

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