Worms secrete pheromones to attract males when they are old and running out of self-sperm.

“What happens naturally, normally, if you don't stress the ancestors, is that the worms start secreting the pheromone only when they are old. This is also people will-- Exactly. Because they only make the sperm at a particular time and then they run out of self sperm. They can't self-fertilize. So they have to call the males if they want to continue to mate.”