What the large penises of tiny crustaceans tell us about evolution

This is because of significant differences in the size of the critters’ carapaces—males’ tend to be a good deal longer than females’. What also tends to stand out in male specimens is their outsized reproductive apparatus.

“The genitals are really big in the male,” Hunt says. “There’s an estimate from one species where basically a third of the internal volume of the animal is the male reproductive parts. So it’s not quite like humans.”

That many of these arthropods are so well endowed was a boon to the scientists involved, because it allowed them to more precisely frame their research question. Instead of just asking whether sexual dimorphism could have an impact on long-term species survival rates, Hunt and the other authors could be more specific, and look at whether allocating energy to male sex organs instead of other areas could be harmful for a species down the road.