Green anacondas spend most of their time in shallow water, being most common in open wetlands. Their eyes and nostrils are positioned on top of their heads, so that they can lie hidden underwater with only their heads at the surface. Anacondas are ambush predators: they wait for prey to come to the water's edge to drink, then they strike with lightning speed. Their bodies are powerful enough to squeeze the life out of a horse or a fully grown black caiman. Anacondas can kill humans, but only occasionally do so.
Male anacondas have claw-like spurs on their lower bodies, which they use to stimulate females. A single female may be tangled up with several males during mating. Like other boas, anacondas do not lay eggs but give birth to live young.