A tiny beetle that has conquered almost every continent is locked in an intense sexual arms race. While the males have evolved spiked penises that look like something from a medieval war chest, the females have parried this advance by developing thicker vaginal linings and faster healing.
The seed beetle is well known for its penis. As a ball covered in many spikes, much has been done to investigate how such an extreme member could evolve. Findings, for example, have shown that the males with the prickliest pecker are also the most successful. But little (rather unsurprisingly, perhaps) has been said about how the unfortunate females cope with such a tricky companion.
The penis clearly damages the female’s vagina, raising the question of whether or not there is some form of arms race going on, and if so, how it's manifesting itself. The researchers looked at 13 separate populations of seed beetle and peered inside their vaginas to see what effect the spikes were having on their genitals.
Interestingly, at first they found no correlation between the amount of scarring observed and the thickness of the vaginal tract. Yet when they took into account the length of the spikes on the males’ penis, they found that in populations in which the males had the longest penis spines, the females correspondingly had the thickest tissue, presumably in an attempt to prevent the spines from piercing the vagina.
But the adaptations of the females did not stop there. The females also developed new immune responses: one that's thought to help limit the chance of infection and another that likely helps the female’s tissue regenerate and heal faster. Published in the Proceedings of the Royal Society B, the study provides a rare example of how “traumatic mating” can lead to an evolutionary arms race between the sexes.
What, though, is leading the males to evolve such a dangerously weaponized penis? Many members of the animal kingdom have spiked penises, which usually evolved to help the males keep it inside the vagina, particularly if there is a lot of competition from other males. But the seed beetle has taken this to the extreme.
It is thought that the damage inflicted by the penis, in the form of cuts and tears, might also allow chemicals ejaculated by the male to influence the female’s behavior, perhaps inducing her to produce more eggs. There is yet another theory that suggests the wounds may even lead the female to die once she has laid the fertilized eggs, and thus prevent her from mating with another male.