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The Watery World of the Monster Anaconda

A spectacled caiman (Caiman crocodilus) catches a lift on the back of a huge green anaconda (Eunectes murinus) in the Amazon river basin in Brazil. Sylvain Cordier/Getty Images

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All snakes can swim, though some are better than others. One of the most powerful species alive today is the green anaconda (Eunectes murinus). Perfectly at home in the rivers and swamplands of Amazonia, it's got eyes that face upward like a crocodile's, allowing the creature to scan muddy banks for prey — concealing its muscular coils below the water's surface.

That's why anacondas are also called "water boas." The shoe fits, or it would if they had any feet. Apart from the green anaconda, science recognizes three other species: the yellow anaconda (Eunectes notaeus), the darkly spotted anaconda (Eunectes deschauenseei) and the Bolivian anaconda (Eunectes beniensis). All four belong to the boa family, and they're all South American natives.

By every metric, the green anaconda stands out. Not only is it the biggest member of the Eunectes quartet, but it's arguably the biggest snake in the world. Yet trying to pin down the animal's maximum size is fraught with challenges.

The Long and Short of It

Amazon folklore is packed with stories about giant monster snakes extending 60 to 100 feet (18.2 to 30.4 meters) long. The fossil record tells us that a colossal serpent really did slither across the continent 60 million years ago. Named Titanoboa, it's thought to have reached 50 feet (15.2 meters) in overall length and weighed approximately 2.5 tons (2.26 metric tons).

Well, green anacondas are nowhere near that size. A common length for this species is about 19.7 feet (6 meters) — although males, being the smaller of the two sexes, rarely exceed 13.1 feet (4 meters). The biggest anaconda ever reliably documented was 27.2 feet (8.3 meters) long.

Yet rumors of anacondas growing two or three times larger persist. Some claims hinge on enormous skins cut from dead snakes. The problem is, those are really easy to distort. Even if you're not trying to.

Herpetologist William H. Lamar proved this point in 1978, when he killed a wild anaconda of ample size. The freshly slain corpse was a respectable 24.58 feet (or 7.49 meters) long. After he skinned the reptile, Lamar measured its disembodied hide. Despite his best efforts, he couldn't avoid stretching out the skin as he worked—which gave it a post-mortem length of 34.58 feet (10.54 meters).

The biggest and bulkiest Eunectes murinus can weigh 440 pounds (200 kilograms) or more. So, as a species, the green anaconda is considered the world's heaviest snake — but not necessarily the longest.

Lengthwise, the Asian reticulated python probably has it beat. According to "The New Encyclopedia of Snakes" by Chris Mattison, there've been "several authenticated reports" of large "retics" measuring about 28 feet (8.5 meters) from end to end.

Hunting Habits

Anacondas have an affinity for slow-moving rivers, muddy swamps and seasonally flooded plains. They seldom venture far away from flowing or standing water — although some species may choose to hunt in forests on occasion.

Not only do the snakes have eyeballs situated on the tops of their heads, but the nostrils are also located in this region. Thus, a swimming or soaking anaconda can easily see and smell what's going on above the water. It makes prey capture a whole lot easier for these semiaquatic reptiles.

Anacondas kill by constriction, using their jaws to seize the victim before immobilizing it with tightly wrapped coils. Naturalists used to think that snakes who used this technique were, in effect, strangling their prey. However, according to new research involving the red-tailed boa (Boa constrictor), the real cause of death is cardiac arrest.

Wild anacondas feed on fish, lizards, birds, bird eggs, other snakes, carrion and a variety of mammals. Adults have been known to swallow up caimans: feisty crocodilians related to alligators. Another notable beast often taken by large anacondas is the capybara. Earth's biggest rodents, capybaras are shaggy web-footed herbivores which stand about 1.6 feet (0.48 meters) tall.

Being amphibious, they regularly cross paths with anacondas. And the mammals know how to put up a fight: Older anacondas sometimes display bite wounds left by capybaras they've attacked.

At this point, you're probably wondering if people are on the menu. There's little doubt that a sufficiently big green anaconda could kill and eat a human being. Yet no such incident has ever been confirmed.

That said, anacondas — like most creatures — will defend themselves if cornered. While none of these snakes are venomous, they can inflict deep bite wounds. The snakes may also secrete a foul-smelling liquid when distressed.

When Snakes Have a Ball

Among reptile-keepers, anacondas aren't nearly as popular as red-tailed boas or big pythons. They need huge enclosures, high humidity levels and water dishes large enough for the snakes to use as soaking pools. Captive-bred individuals who've been handled all their lives tend to be more docile than anacondas captured in the wild. Even so, you shouldn't get an anaconda of any species unless you're an experienced reptile hobbyist who understands large constrictors.

In 2012, the U.S. Fish and Wildlife Service banned the importation — and interstate transport — of the yellow anaconda. Growing up to 15 feet (4.6 meters) long, this snake could potentially threaten all sorts of native species. And as the U.S. Geological Survey reports, free-ranging yellow anacondas have been sighted in Florida and Arkansas. Those snakes were most likely former pets.

Anacondas don't belong in the everglades or the Arkansas delta. But in their natural habitat, these reptiles are fascinating to watch. During the breeding season, several males may attempt to mate with a single female — at the same time. The result is a "breeding ball," a dog-pile which sees as many as 13 lovelorn males all squirming around a mother-to-be.

Like most boas, anacondas give birth to live young, with litters containing anywhere from four to 82 baby snakes. And we now know green anacondas don't need mates to get pregnant. In 2014, one female kept at a British reptile park birthed three live babies even though she'd never been kept with a male of her species. A near-identical situation recently unfolded at the New England Aquarium.

Known as "parthenogenesis," this style of asexual reproduction has also been observed in Komodo dragons and Burmese pythons.

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