Earthworms don’t appear to be the most astute observers—they have no eyes, they wander foolishly onto heavily trafficked sidewalks whenever it rains, and they have simple pairs of cell clusters for brains. But when it comes to sex, these hermaphroditic critters can detect something that many humans can’t: whether or not their partner is a virgin. And they compensate the sperm load accordingly.
Earthworm sex is fascinatingly complex and slimy. It begins when two worms rub against each other, causing the clitellum (that swollen thing that looks like an armband) to ooze a tremendous amount of mucus and enclosing themselves in a slimy tube of lovemaking. In a process that can last up to an hour, the worms then ejaculate sperm into the slime, which makes it way into the other worm’s sperm repositories. The earthworms then separate and slither their separate ways. Meanwhile, eggs are produced in a separate segment and released into egg sacs. The clitellum secretes a slimy ring that slides over the egg sacs, collecting the eggs and bringing them to the sperm repositories. After the sperm slides into the slime to fertilize the eggs, the ring slips off the worm and seal itself up into a cocoon. Ah, life.
But worms aren’t exactly monogamous—they can mate with more than one worm before sliming the eggs over to the sperm, making their slime a competitive environment for the sperm of each mate. New research in the Proceedings of The Royal Society B-Biological Sciences shows that while worms are in their slimy embrace, they judge the quality of their mates—and adjust the volume of their ejaculate based on that judgement. Most notably, if the mate is not a virgin, the worm will triple the amount of sperm transferred to drown out the previous mate’s sperm. Worms can also refine the volume of sperm based on the mate’s size, using less with smaller (less fertile) partners. This saves the rest of their load for a better mate, allowing them to transfer about five times more sperm to larger non-virgins than to virgins.