Guppies are small freshwater fishes that are popular in aquariums around the world. Unlike many fishes, where males and females squirt sperm and eggs into the surrounding water, guppy males fertilise females by delivering sperm into their bodies. They don’t have a penis, as such. Instead, they have modified a fin into a penetrating organ called a gonopodium.
These penis-ish organs are tipped with an unpleasant set of claws, hooks ridges and spines. These features evolve very quickly and they’re sometimes the only way of telling one guppy species from another. What are they for? It’s possible that the claws help males to latch onto females, even those that do not want to mate with them. Alternatively, they might help to hold sperm at the tips of the gonopodia, so they can be more easily implanted into the females.
To test these ideas, Lucia Kwan from the University of Toronto used a scalpel to slice off the claws of several males, leaving the rest of their gonopodia intact. She calls it “phenotypic engineering”. That’s a long way of saying: “a shave”.
She then released individual males into tanks with single females, and watched. It was a straightforward experiment with a clear result. Compared to shaved males, clawed ones transferred three times more sperm into unreceptive females, but the same amount into those that willingly mated.
This strongly suggests that the claws are a “sexually antagonistic trait”—one that benefits one sex over the other. In this case, they help the males to grasp resistant females. If they were simply for anchoring sperm, the de-clawed males should suffer when mating with all females, rather than just the unreceptive ones.
This is the latest in a small but growing line of phenotypic engineering genital-shaving studies, which aim to work out just why animal sex organs are so bizarrely adorned. One group laser-shaved the spikes from a fly’s penis to show that they’re like biological Velcro, allowing males to latch onto females. Another team did the same thing with a seed beetle’s penis to show that its terrifying spikes aren’t anchors—their role seems to be to puncture the female’s genital tract for reasons best known to the seed beetle.
The penises of these insects are just as varied in shape, size and spikiness as those of the guppies, so these experiments suggest that the battle of the sexes has fuelled the evolution of these groups, helping them to diversify into the many species we see today. To understand that process, scientists would need to compare the organs of many different species, but these shaving experiments are certainly a good first step.
Reference: Kwan, Cheng, Rodd & Rowe. 2013. Sexual conflict and the function of genitalic claws in guppies (Poecilia reticulata). Biology Letters http://dx.doi.org/10.1098/rsbl.2013.0267