Snail Sex Tape

What It Covers

Radiolab host Molly Webster and executive editor Soren Wheeler explore the surprisingly complex reproductive biology of snails with evolutionary biologist Menno Schilthuizen from Leiden University, covering hermaphroditic anatomy, multi-hour mating sessions, three-foot slug penises, and calcium-tipped "love darts" that chemically manipulate partners to increase fertilization success.

Key Questions Answered

• •Snail dispersal and evolution: Snails travel only 1–5 meters between birth and first reproduction, making them uniquely observable subjects for evolutionary study. Researchers can walk a few kilometers through limestone habitats and watch shell morphology shift from smooth to ribbed, large to small — compressing continental-scale evolutionary patterns into a human-walkable transect.
• •Hermaphroditic mating mechanics: All snails carry both male and female reproductive organs, with both openings located on the right cheek, just behind the eye tentacle. During mating, both partners simultaneously evert their penises — inverting like a glove finger — and insert into each other, maintaining this dual connection for two to seven hours, sometimes an entire night.
• •Sperm digestion and storage strategy: Receiving snails digest the majority of incoming sperm — roughly 999 out of every 1,000 cells — as a nutritional resource, storing only select sperm for later fertilization. This creates an evolutionary arms race where the sperm-donating partner must find mechanisms to bypass the receiver's digestion and storage control systems.
• •Love dart hormone delivery: Snails fire a 1-centimeter calcium dart — shaped like a needle, knife, or arrow depending on species — into their partner's skin near the genital opening. The dart carries hormone-laced mucus that causes involuntary muscle contractions, closes the sperm-digesting organ's entrance, and redirects sperm toward eggs, measurably increasing the shooter's reproductive success.
• •Dart variation as evolutionary evidence: Love darts have evolved independently multiple times across snail species, producing extreme variation: some species carry one dart, others four; some are disposable, others reusable; one Japanese species stabs a partner up to 3,300 consecutive times in a single mating session, recharging the dart with fresh hormone between each insertion.