Green anacondas show some of the most extreme size dimorphism of any terrestrial vertebrate. Females regularly exceed four metres and can pass five, while males are typically closer to two to three metres. The mass difference is greater still, since weight scales with the cube of length; a large female can outweigh a male many times over.
Breeding follows from that asymmetry. A receptive female attracts a cluster of males that wrap around her for days or weeks, competing by position and persistence rather than by fighting. Anacondas are ovoviviparous: eggs are retained internally and the female gives birth to live young after a gestation of roughly six to seven months.
Sexual cannibalism has been observed, with females occasionally constricting and consuming a courting male. How common this is in the wild remains unclear, as it has been documented in only a small number of cases. The frequently repeated figure of anacondas reaching nine or ten metres is not supported by verified measurements.