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. A 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 use their tongues to capture abd sense these pheromones.
Many males often find the same female, wrapping her and other males in a ball called "breeding ball". The group could stay in this position for two to four weeks. This ball acts as a slow-motion wrestling match between the males. The strongest male may outclass others and copulate OR the much stronger female may choose one male to mate.
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 . Courtship and mating occur almost exclusively in water.
Mating is followed by a gestation period of six to seven months. The species is ovoviviparous,meaning it lays eggs in her womb, the eggs hatches and babies are then delivered outside. Litters usually consist of 20 to 40 offspring. The female can lose upto half of her former weight.
Neonates are around 70–80 cm (2.3–2.6 ft) long and receive no parental care. Should they survive, they grow rapidly until they reach sexual maturity in their first few years, after that their rate of growth diminishes.
However, facultative parthenogenesis is also possible in the absence of a male.
Really informative