"No one has ever described that before," study co-author Rolanda Lange, who now works as a postdoctoral researcher at Monash University in Melbourne, Australia, told LiveScience. "If you see two animals injecting fluids into each other's heads during mating, that's just weird. Even if you work in this area, it's just weird."
The slugs stab each other in the head several minutes after penetrating each other with their penile bulbs, and remain stabbed and pumping prostate fluid into each other's circulatory systems for the majority of their 40-minute mating period, the researchers report.
The team has not determined why the slugs target the region between their mate's eyes, but they speculate the creatures might use allohormonesto manipulate some aspect of their mate's nervous system, which is centered directly behind the eyes.
Joris Koene, a biologist at VU University Amsterdam who was not involved in this research but has studied similar stabbing behavior in other hermaphroditic animals, such as earthworms, thinks it's possible that the slugs are targeting their mates' nervous systems, but is not yet convinced this is the case.