If you looked at the penis of a Drosophila fly under a microscope (for reasons best known only to yourself), you’d see an array of wince-inducing hooks and spines. These spines are present in all Drosophila and they’re so varied that a trained biologist could use them to identify the species of the owner.
What’s the purpose of these spines? Are they intended to actually wound the female during mating? Do they help the male fly to scrape out the sperm of his rivals? Do they actually pierce the walls of the female’s genital tract, allowing the male to bypass any barriers to his sperm, as other insects do? All of these explanations have been put forward, and it seems that all of them are wrong.
The spines are nothing more than biological Velcro. During sex, they help the male fly to clasp onto his mate from the inside so he can’t be easily dislodged. We know this thanks to Michal Polak and Arash Rashed, who shaved male flies with a laser to see if their sexual performance would be affected.
Nature is full of bizarre genitals that make those of humans look boring and pleasant by comparison. I’ve blogged before about the ballistic penises of ducks, the horrific, mace-like appendages of seed beetles and the stabbing weapons of bedbugs and some spiders. These penises have been shaped by millennia of sexual conflict, with males evolving new ways of spreading their sperm more effectively than their rivals. In response, females have evolved ever more extreme ways of exerting choice over who fertilises them.
The function of these genital armaments isn’t always clear. They could have any number of roles and sorting between them takes cunning and often eyebrow-raising techniques, including fluffing beetles with a tiny pump, and getting ducks to mate with glass tubes. So it is with Polak and Rashed’s flies. Their spines are too small to cut off by hand. So the duo used a laser instead, wielding the light with such surgical precision that they could cut off a third of each millimetre-long spine, or the entire structure.
They found that a partial shave did nothing, but the full treatment significantly reduced the odds of the males mating with females. With the spines, they were virtually guaranteed to mate if a female was around; without them, their chances fell to around 20%. It wasn’t for lack of trying either – all of the shorn males tried to woo a female and almost all tried to mate. They simply failed. They did all the right things – mounting, placing their genitals in the right place – but it was for nought. And if the spineless males were placed in direct competition with a normal one over a female, they almost always lost.
On the other hand, removing the spines didn’t actually affect the sperm injected by the few males who did manage to have successful sex. They injected just as much sperm, which fertilised just as many eggs.
These results clearly show that the main role of the penile spines is to allow males to latch onto their mates long enough to actually inject sperm (Drosophila mating takes around 10 minutes). In fact, the spines themselves are surprisingly mobile, capable of folding and unfolding over each other in a rhythmic way, presumably to draw the male and female genitals closer together.
Darwin himself suggested this way back in 1871, writing that “appendages at the apex of the abdomen in male insects” could function as devices “for holding her securely”. The male’s penis spines would help him to stay the course in the face of attempts to remove him. In many Drosophila species, females will try to kick their suitors off if they don’t want to mate, and rival males may try to inveigle their way into a mating embrace. Polak and Rashed say that the different types and intensities of sexual conflict in different species would have contributed to the diverse nature of their genital weapons.
Reference: Polak & Rashed. 2009. Microscale laser surgery reveals adaptive function of male intromittent genitalia. Proc Roy Soc B http://dx.doi.org/10.1098/rspb.2009.1720
More on sexual conflict:
- Ballistic penises and corkscrew vaginas – the sexual battles of ducks
- South African wildlife – Wait, that’s not a trunk…
- Traumatic insemination – male spider pierces female’s underside with needle-sharp penis
- Horrific beetle sex – why the most successful males have the spikiest penises
- Holy fellatio, Batman! Fruit bats use oral sex to prolong actual sex
- Male chimps trade meat for sex