IV.

Reproduction

Female anacondas give off a special scent called a pheromone that males can detect with their flicking tongues. Like some other types of snakes, anacondas sometimes form breeding balls (also called mating balls) in which many males swarm over a single female to attempt to mate. Up to a dozen or more male anacondas may wrap themselves around a much larger female, trying to insert their sex organs into the female’s cloaca. The males scratch with their tiny hind leg spurs to stimulate the female to mate. A successful male leaves a waxy plug in the female's cloaca to block other males from mating. Anaconda breeding balls take place in shallow water and can last for weeks. Females may also breed with single males they encounter.

Anacondas give birth to live young, unlike pythons, which all lay eggs. However, anacondas are actually ovoviviparous, meaning they retain eggs internally until the young hatch. While they are carrying developing eggs inside, females typically do not eat. The gestation period is about six to seven months. The number of young born can range from 20 to 100.

Two forms of cannibalism are reported with female anacondas. Large females sometimes kill and eat male anacondas, more than often following mating. Like other live-bearing members of the boa family, female anacondas also eat stillborn offspring and undeveloped eggs expelled when giving birth. Since females do not feed while carrying developing young, both behaviors may be related to nutritional needs. The females may eat adult males as a ready protein source before they stop feeding. Consuming stillborn young and unhatched eggs likely allows the females to recover more quickly from a long period without food.