Of all the bizarre forms of animal copulation, only one has earned the description “cephalo-traumatic.” Sea slugs in the genus Siphopteron are hermaphroditic mollusks smaller than a pea, bearing stretchy, multipronged penises. These organs have both a spiny bulb for releasing sperm into a partner’s genital opening and a pointy “stylet” for … various other kinds of stabbing. Before or during copulation, one slug will pierce the other’s flesh and inject its prostate fluid. (Sometimes, the wounding is mutual.) The purpose of this injection is not clear, though some have suggested that it adjusts the recipient’s hormones or prevents it from copulating with another mate immediately after the interaction. And while many Siphopteron seem to have a rather arbitrary stabbing style—piercing at various places along the body—a new species with the rather bland name Siphopteron sp. 1 is remarkable. It pokes unerringly into its partner’s forehead (see arrows in video above), where scientists believe its prostate juices may interact with a mate’s neurons. The researchers that unveil this bizarre ritual online today in the Proceedings of the Royal Society B remain baffled as to its evolutionary purpose.