It’s “the first-known instance of ‘cephalo-traumatic secretion transfer’,” we are told in the abstract. This sounds like a head fuck.

I jump to the introduction.

“Traumatic mating often serves to inject prostate fluids or other gland secretions with putative allohormonal functions into the female.”

For most people, I’m guessing this will be a new one. But I have a confession. I studied sperm competition at the University of Sheffield (which, in the 1990s at least, was like studying sperm competition at the University of Sperm Competition). Back then, traumatic insemination was threatening to go mainstream. A friend in the bird lab conducted a pioneering investigation into the sex lives of mallards (with their now famously cork-screwed penises), a chum in the insect lab down the corridor busied himself with the intromittent organ of the bed bug (think hyperdermic needle) and elsewhere another colleague had just discovered toxic proteins in Drosophila ejaculates that reduce the lifespan of females.

Those were the days.

Almost 20 years on, we now recognise that sexual conflict between males and females is pretty common in the animal kingdom; the mention of traumatic insemination just doesn't raise eyebrows like it used to. In nature, however, there’s always a species prepared to go that bit further, like this head-fucking hermaphrodite for instance, a sea slug from the genus Siphopteron whose bizarre mating ritual is described in a paper just out in Proceedings of the Royal Society B.

As I read up on the description of a “typical copulatory sequence”, my eyebrows raise ever so slightly. This is a good sign. The researchers act as matchmakers, putting pairs of sea slugs into 1.25 ml wells “for one hour or until ongoing copulations had finished”.

These creatures are small, I think. No wonder this hasn’t been observed before.

The slugs start by intertwining, inverting their penises as they do so (Fair dinkum). There is, I learn, “frequent pre-copulatory biting into the partner’s posterior visceral hump” (not so surprising, perhaps). This is followed by “reciprocal penile bulb insertion” into the female genital opening (So far, so normal). Then, I read this:

“Seconds thereafter, the penile stylet was inserted into the mating partner’s foreheads.”

I read it again. And again.

X marks the spot where sea slug A gently inserts its penile stylet into sea slug B's forehead. Photograph: Johanna Werminghausen Photograph: Johanna Werminghausen

I scan down the article a little further to discover that “stylet insertion always occurred in direct vicinity of the mating partner’s eyes [ouch!] with the stylet deeply inserted” (This stuff should come with a warning). “Both mating partners remained still during copulation.” (No shit Sherlock!)

I watch a video the researchers have kindly distributed with their paper. I turn up the volume but there is no sound. I am disappointed. Good Old Fashioned Lover Boy would have been a nice touch. At the end of love-making, when the penile stylet is withdrawn, I notice a nasty needle-like tip.

The slugs aren’t the only ones with a sore head. What on God’s gentle earth is going on here? I imagine the discussion section of the paper will give me some clues. By observing closely related species of sea slug (all belonging to the Siphopteron genus), the researchers find considerable variation in this process of stylet insertion. It’s possible that it doesn’t really matter where the injection is made; as long as it gets into the other slug’s circulation, it can do whatever it’s doing. But the researchers are intrigued (as am I) that this particular species should be so specific about where it injects.

We propose that the consistent site selection for penile stylet placement is adaptive and directed towards neural ganglia.

That’s a lovely sentence.

The implication is that these slugs are fucking with heads and minds. The paper doesn’t go much further. I know scientists have got to whet the appetite for future publications, but this is just too tantalizing. So I pick up the phone to call Rolanda Lange, an evolutionary biologist at the University of Tuebingen in Germany and the lead author of the study.

“The penis is very extendable,” she tells me, “so theoretically it could move anywhere on the body and it just doesn’t.” Before the stylet is inserted, the slugs also make a careful investigation of each other’s bodies, apparently searching for the right spot to penetrate. “The area around the stylet is heavily ciliated so it seems like it’s also a sensory organ,” she says.

I ask Lange how she plans to figure out the purpose of this bizarre behaviour. In collaboration with researchers in Munich, she and her colleagues plan to plunge pairs of copulating slugs into liquid nitrogen. By studying sections of tissue, it should be possible to locate precisely where the stylet is releasing its secretions. “Are they really going into the brain?”

Until then, these slugs will remain a complete head-fuck.

• This article was amended on November 2013 to remove a swearword from the headline.