Green anaconda

The remote location of the snake's habitat has historically made locating, capturing, and returning specimens difficult. Transporting very large specimens to museums, especially before substantial decay, is difficult (though this has not prevented the return of much larger and more cumbersome crocodilian specimens).[10] Skins can stretch substantially, increasing the snake's size by more than 50% if stretched during the tanning process. Reports without physical proof are considered dubious if from non-scientists, as such individuals may at worst be more interested in promoting themselves or telling a good tale, or at the least may not be sufficiently trained in proper measurement methods. Observational reports of animals which were not captured are even more dubious, as even trained scientists often substantially overestimate the size of anacondas prior to capture.[10] According to the Guinness Book of World Records, this species has been perhaps subject to the most extreme size exaggerations of any living animal.[9] At the same time, determining a maximum possible or plausible size is difficult, because anacondas are known to continue to grow throughout their lives. Older reports, in particular, could include individuals that in times of less pressure from humans lived longer and reached greater sizes.

Numerous historical accounts of green anacondas are reported, often of improbable sizes. Several zoologists (notably Henry Walter Bates and Alfred Russel Wallace, among others) note rumors of snakes beyond 30 or 40 ft long, but in each case, their direct observations were limited to snakes around 20 ft in length. Numerous estimates and second-hand accounts abound, but are generally considered unreliable. To prove the point of overestimating, in Guyana in 1937, zoologist Alpheus Hyatt Verrill asked the expedition team he was with to estimate the length of a large, curled-up anaconda on a rock. The team's guesses ran from 6.1 to 18.3 m (20.0 to 60.0 ft); when measured, this specimen was found to be 5.9 m (19.4 ft).[9]

Almost all specimens in excess of 6 m (19.7 ft), including a much publicized specimen of 11.36 m (37.3 ft) long, have no voucher specimens including skins or bones.[9]

The skin of one specimen, stretched to 10 m (32.8 ft), has been preserved in the Instituto Butantan in São Paulo and is reported to have come from an anaconda of 7.6 m (24.9 ft) in length.[9] While in Colombia in 1978, herpetologist William W. Lamar had an encounter with a large female specimen 7.5 m (24.6 ft) long, estimated to weigh between 136 and 180 kg (300 and 397 lb).[10] In 1962, W.L. Schurz claimed to have measured a snake in Brazil of 8.46 m (27.8 ft) with a maximum girth of 112 cm (3.67 ft).[9] One female, reportedly measuring 7.9 m (25.9 ft) in length, shot in 1963 in Nariva Swamp, Trinidad, contained a 1.5 m caiman.[9] A specimen of 7.3 m (24.0 ft), reportedly with a weight of 149 kg (328 lb), was caught at the mouth of the Kassikaityu River in Guyana, having been restrained by 13 local men, and was later air-lifted for a zoo collection in the United States, but died in ill health shortly thereafter.[9] The largest size verified for E. murinus in captivity was for a specimen kept in Pittsburgh Zoo and PPG Aquarium, which grew to a length of 6.27 m (20.6 ft) by the time she died on July 20, 1960. When this specimen was 5.94 m (19.5 ft) long, she weighed 91 kg (201 lb).[9] The estimated weight for an anaconda in the range of 8 m (26.2 ft) is at least 200 kg (440 lb).[9] National Geographic has published a weight up to 227 kg (500 lb) for E. murinus, but this is almost certainly a mere estimation.[13] Weight can vary considerably in large specimens depending on environmental conditions and recent feedings, with Verrill's aforementioned specimen, having been extremely bulky, scaled at 163 kg (359 lb), whereas another specimen considered large at 5.06 m (16.6 ft), weighed only 54 kg (119 lb).[9][14]

Size presents challenges to attain breeding condition in larger female anacondas. While larger sizes provide the benefit of a larger number of offspring per clutch, the breeding frequency of the individuals reduces with size, indicating that a point exists at which the advantage of a larger clutch size is negated by the female no longer being able to breed.[2] For the anaconda, this limit was estimated at about 6.7 m (22.0 ft) in total length.[2] This is consistent with the results of a revision of the size at maturity and maximum size of several snakes from North America, which found that the maximum size is between 1.5 and 2.5 the size at maturity.[15] The minimum size of breeding anacondas in a survey of 780 individuals was 2.1 m (6.9 ft) in snout-vent length, indicating that maximum size attained by anacondas following this pattern would be 5.3 m (17.4 ft) in snout-vent length.[2] However, most anacondas are captured from the llanos, which is more accessible to humans and has smaller prey available, while the rainforest, which is much less explored and has more plentiful large prey, may be home to larger snakes.[2][16]

Senckenberg Museum exhibit of a capybara (Hydrochoerus hydrochaeris) being swallowed by an anaconda

Primarily aquatic, they eat a wide variety of prey, almost anything they can manage to overpower, including fish, birds, a variety of mammals, and other reptiles. Particularly large anacondas may consume large prey such as tapirs, deer, capybaras, and caimans, but such large meals are not regularly consumed. The green anaconda's eyes and nose are located on the top of the head, allowing the snake to breathe and watch for prey while the vast majority of the body is hidden underwater.[26] Many local stories and legends report the anaconda as a man-eater, but little evidence supports any such activity. They employ constriction to subdue their prey. Cannibalism among green anacondas is also known, most recorded cases involving a larger female consuming a smaller male. While the exact reason for this is not understood, scientists cite several possibilities, including the dramatic sexual dimorphism in the species, and the possibility that a female anaconda requires additional food intake after breeding to sustain the long period of gestation. The nearby male simply provides the opportunistic female a ready source of nutrition.[27]

Skeleton of E. murinus, exhibit in the National Museum of Nature and Science, Tokyo, Japan

This species is solitary until the mating season, which occurs during the rainy season, and can last for several months, usually from April to May. During this time, males must find females. Typically, female snakes lay down a trail of pheromones for the males to follow, but how the males of this species track a female's scent is still unclear. Another possibility is that the female releases an airborne stimulant. This theory is supported by the observation of females that remain motionless, while many males move towards them from all directions. Male anacondas also frequently flick their tongues to sense chemicals that signal the presence of a female.[28]

Many males can often find the same female. Although more than one male may not be necessary, this results in odd clusters referred to as "breeding balls", in which up to 12 males wrap around the same female and attempt to copulate. The group could stay in this position for two to four weeks. This ball acts as a slow-motion wrestling match between the males, each one fighting for the opportunity to mate with the female.

During mating, males make use of their spurs to arouse the female. They aggressively press their cloacal regions hard against the female body, while continuously scratching her with their spurs. This can produce a scratching sound. Mating approaches its climax when the stimulus of the males' spurs induces the female snake to raise her cloacal region, allowing the cloacae of the two snakes to move together. The male then coils his tail, surrounding the female and they copulate.[29] The strongest and largest male is often the victor. However, females are physically much larger and stronger and may decide to choose from among the males. Courtship and mating occur almost exclusively in water.

Mating is followed by a gestation period of six to seven months. The species is ovoviviparous, with females giving birth to live young. Litters usually consist of 20 to 40 offspring, although as many as 100 may be produced. After giving birth, females may lose up to half their weight.

Neonates are around 70–80 cm long and receive no parental care. Because of their small size, they often fall prey to other animals. Should they survive, they grow rapidly until they reach sexual maturity in their first few years, after which their rate of growth continues at a slower pace.[25]

However, when no male anacondas are available to provide offspring, facultative parthenogenesis is possible. In August 2014, West Midlands Safari Park announced that on 12 August 2014 a female green anaconda, which was being kept with another female anaconda, through parthenogenesis had given birth to three young.[30][31]