Insects with nightmarish spiky penises that can harm their mates are really nothing new. But a new study in the Journal of Evolutionary Biology has determined that given the right conditions, wielding a nightmarish spiky penis could also harm a different species.
Image credit: Kyogoku, D. and Sota, T. 2015
The study, led by Daisuke Kyogoku at Kyoto University in Japan, used the spiky penises of two closely related species of seed beetle to test a biological concept called reproductive interference. The idea suggests that when two closely related species live in the same place, courtship and sex can sometimes get a little confusing. If males from one species court and mate with females from the other species, the mistake may keep either one of the pair from having babies. The effect of those missing offspring, over the long haul, may help drive the two species apart. It could make them even more different in appearance, it could push apart the calls they use for courtship, or it could let one species outcompete the other one.
Kyoguku’s experiment tested whether genital differences could affect competition. The seed beetles they used don’t actually live in the same place: Callosobruchus maculatus infests cowpeas in Africa, Callosobruchus chinensis infests adzuki beans in Asia, so the experiment was something of a theoretical exercise. Cowpea weevils seem to be better at grabbing food and egg laying sites than adzuki bean weevils, but when both species are housed together in the lab, the cowpea weevils die out. Kyoguku determined that the die-out was the result of reproductive interference between the two species. The interference, in this case, was the adzuki bean weevil’s elongated penile spines.
All male Callosobruchus beetles have spiny penises, and eagerly mate with any female they run across, regardless of species. Female beetles typically evolve defences to the spikes on males of their own species. But when the researchers examined the effect of cross-species copulation, they found that female cowpea weevils that mated with adzuki bean males had visibly damaged genitalia and were unable to lay eggs even when they were already full of cowpea weevil sperm. Penile spines, the authors conclude, could help drive species apart.
[Source: Kyogoku and Sota, 2015]