From Discover magazine:

“Goran Arnqvist from Uppsala University has been studying the seed beetle’s nightmarish penis for years, using it as a model for understanding the more general evolutionary pressures behind diverse animal genitals. In 2009, he and colleage Cosima Hotzy found that males with the longest spines fertilise the most eggs and father the most young. It wasn’t clear why. Maybe they help him to anchor himself to the female, or scrape out rival sperm. Or perhaps the fact that the spines actually puncture the female is important. Regardless, long spines seemed to give males the edge in their sperm competitions.

But Hotzy and Arnqvist had only found a correlation, by comparing the penises of male seed beetles from around the world. To really test their ideas, they wanted to deliberately change the length of the spikes to see what effect that would have.

They did that in two ways: they artificially bred males for several generations to have either longer or short spines; and they shaved them with a laser….Both techniques produced males with differently sized spines, but similarly sized bodies.

The duo found that males with the shorter spines were indeed less likely to successfully fertilise the females. They also found clues as to why the males benefit from their long spines.”