There are a lot of interesting ways of getting down in the animal kingdom - but a sea slug found off the coast of Australia might just take the cake for the most bizarre mating rituals. Siphopteron quadrispinosum are hermaphroditic and mate by stabbing their sharp penis into the partner’s forehead. This announcement comes from a collaboration of German and Australian scientists and was published today in Proceedings of the Royal Society B.
While traumatic insemination certainly isn't unheard of in the sea slug world, the location that S. quadrispinosum chooses makes it a bit more unusual. These sea slugs have branched penises, with each side nearly as long as its entire body. One end is inserted into the partner’s female reproductive organ while the other side is shoved right into the partner’s forehead where the seminal fluid enters the circulatory system. The process takes about 40 minutes and the researchers have coined a new term to describe it: ‘cephalo-traumatic secretion transfer.’
After studying 32 sea slugs in 144 mating sessions, the team was surprised to see that the slugs routinely targeted the forehead. This is the first known instance of traumatic insemination where a creature with a flexible penis actually targets the injection location. Injecting seminal fluid in this location might be done because it affects the central nervous system, which may increase the likelihood of successful conception. Some seminal fluid that is inserted into the female reproductive tract is digested, not fertilized, so attempting conception from two different approaches may also boost success rates.