How Big Is a Green Anaconda? The World's Heaviest Snake

Typical size of adult green anacondas

The green anaconda is not unusually long by giant snake standards, but it is extraordinarily thick and heavy. A big female has a body as wide as a grown man’s thigh, and her weight can match that of an adult person.

Weight swings a lot. A female that has just eaten a capybara or is carrying a litter can be far heavier than she is after giving birth, when she may lose up to half her body weight. That is one reason weight records are hard to compare.

Why are female anacondas so much bigger than males?

Few land animals show such a large size gap between the sexes. In field studies in Venezuela’s llanos, adult females were several times heavier than adult males. Researchers link this to reproduction.

A female carries her young inside her body for six to seven months, and litter size rises with her body size. A big female can produce 40 or more babies, while a small one produces far fewer. Larger females can also tackle bigger prey such as capybaras and caimans, building up the fat reserves needed to survive a long pregnancy with little or no food.

Males, by contrast, do not gain much from great size. In a breeding ball, where several males wrap around one female, the winners are not always the biggest, so males may gain more from maturing early than from growing huge. Females sometimes eat males after mating, which is another hint of how lopsided the size difference is.

The biggest anacondas ever measured

The best data come from scientists who catch and measure wild snakes. The largest wild anaconda measured by herpetologist Jesús Rivas, who studied hundreds of anacondas in Venezuela, was a female 5.21 m (17 ft) long that weighed 97.5 kg (215 lb). That is close to the practical limit for well-documented wild animals.

The longest verified anaconda was a captive female at the Pittsburgh Zoo that grew to about 6.27 m (20 ft 7 in) before she died in 1960. At 5.94 m (19 ft 6 in), she weighed about 91 kg (200 lb). A Guyana anaconda measured at 5.9 m (19 ft 4 in) in 1937 was reported to weigh 163 kg (359 lb), but that figure is often treated with caution.

Some biologists think the largest females may live in remote rainforest rivers, where prey is plentiful and few scientists work. Most measured anacondas come from the open llanos, which are easier to reach.

Why giant anaconda claims are rarely true

Stories of anacondas 9, 12 or even 18 m (30, 40 or 60 ft) long go back centuries. None has been supported by a preserved specimen. There are several reasons the numbers grow in the telling.

• People overestimate coiled snakes. In 1937, members of an expedition in Guyana guessed a resting anaconda’s length at between 6 and 18 m (20 and 60 ft). It measured 5.9 m (19 ft 4 in).
• Skins stretch. A snake skin can stretch by 20 to 50 percent or more during skinning and tanning. One skin kept at the Instituto Butantan in Brazil measures about 10 m (33 ft) but is said to come from a snake about 7.6 m (25 ft) long.
• Measuring a live giant is hard. A conscious anaconda will not lie straight, and estimates of kinks and curves add error.
• Photos mislead. Holding a snake close to the camera makes it look much bigger than the people behind it.

Some reported anacondas of 7 to 8 m (23 to 26 ft) came from experienced naturalists, but without a body, skeleton or careful scientific measurement they remain unconfirmed.

The unclaimed reward for a 30-foot snake

In the early 1900s, a reward was offered through the New York Zoological Society, now the Wildlife Conservation Society, for a live, healthy snake longer than 30 ft (9.14 m). The prize started at 1,000 US dollars, an offer often linked to Theodore Roosevelt, and rose over the decades to 5,000 dollars, 15,000 dollars in 1978 and 50,000 dollars in 1980.

No one ever claimed it. Given how many giant anaconda stories circulated during those decades, the empty prize is strong evidence that 30-foot anacondas are, at best, extremely rare.

How does the anaconda compare with other giant snakes?

The reticulated python of Southeast Asia is longer, regularly passing 6 m (20 ft), with a measured maximum just under 7 m (23 ft). But it is slender. Estimates published in the snake literature suggest that a 5.2 m (17 ft) green anaconda is about as heavy as a 7.4 m (24 ft) reticulated python. The Burmese python is the second heaviest snake, with wild females in Florida reaching about 98 kg (215 lb).

For the length record holder, see how long is a reticulated python?, and for a ranking of all the giants, see the largest snakes. The head-to-head details are in anaconda vs python.