If you’ve ever complained about having bad sex, you really have no idea. Human women may have to complain about poor stamina or incompetent technique but the female seed beetle (or bean weevil; Callosobruchus maculatus) has to contend with her partner’s nightmarish penis – an organ covered in hard, sharp spikes. Just see if you can look at the picture on the right without wincing.
It’s no surprise then that females sustain heavy injuries during sex. But why have male beetles evolved such hellish genitals? What benefits do they gain by physically harming their partners?
It’s possible that the injuries directly benefit the males, either because they stop the females from mating again or spend more efforts in raising their fertilised eggs to avoid the strain of future liaisons.
The alternative is that the spikes could give the males an edge in “sperm competitions“, where they compete with rivals not through direct combat, but through fertilising as many eggs as possible. In this theory, the spines are important for winning these competitions, and the wounds they inflict are simply a nasty side-effect.
Cosima Hotzy and Goran Arnqvist from Uppsala University think that the latter theory is right. They have found that the penile spines are vital to a male’s success – those with the longest spikes fertilise the most eggs and father the most young. Size, it seems, really does matter.