In ideal environmental conditions, a slug may live up to six years; however, most slugs live two years. Most slugs start off as eggs, hatch into immature adults and develop into adults in just under a year, though there are a few species of slugs that have live births.
Slugs are hermaphrodites, meaning they posses both male and female reproductive organs. In difficult times, a slug can fertilize itself and lay its own eggs, but typically it attempts to mate with another slug by releasing a chemical in its slime while it's in heat. Another slug crosses this slime path and smells the chemical with its body. Eventually, the two slugs find each other and copulate for several hours. The slugs may take turns fertilizing each other's eggs with their sperm. Occasionally, one or both of the slugs' penises get tangled up during mating, and they are forced to chew their way out of the situation. If a slug's penis is chewed off, it can no longer fertilize eggs, but it can continue to reproduce as a female. After this process, a slug waits for the right conditions to lay its fertilized eggs. If its environment is moist, it lays around 30 eggs within a few days in the soil of a protected area, such as beneath a log or a rock. These eggs don't hatch until conditions are right. Slug eggs can remain dormant for years until environmental conditions are moist enough for them to hatch.