What the Large Penises of Tiny Crustaceans Tell Us About Evolution

It’s no secret that male and female animals tend to differ in their appearance. Human males are larger on average than human females, for instance, to a degree consistent with what’s observed in other primates. Sometimes, as with peahens and their strutting peacock counterparts, the divergence can be more striking. According to a new study in Nature, though, less might be more in the long run when it comes to this kind of variation.

In the discipline of biology, intra-species sex differences are encompassed tidily by the term sexual dimorphism. Species with a high degree of dimorphism (e.g. pheasants) differ markedly, while those with low dimorphism (e.g. rosy-faced lovebirds) can require expert analysis to tell apart. The researchers behind the Nature paper, titled “High Male Sexual Investment as a Driver of Extinction in Fossil Ostracods,” looked to the fossil record to see if severity of sexual dimorphism could be meaningfully correlated with species endurance across generations.

“We wanted to look at sexual dimorphism and sexual selection and its role in extinction,” says co-author Gene Hunt, a paleobiologist at the Smithsonian’s National Museum of Natural History. “And we wanted to do extinction in the fossil record, where you have the true termination of lineages.”

This approach stands in contrast to that of previous studies on sexual dimorphism and extinction rates, which have relied on analyses of still-extant species that have experienced localized extinctions or notably moved onto the endangered species list. “It’s valid work,” says Hunt, “and it’s good, but our research gives us a complement to that, where you know that these really are extinctions, and that they really are non-human driven.”

The trick is finding extinct creatures in the fossil record with enough dimorphism to reliably distinguish between specimens of opposite sex—no mean feat, considering these fossils are often hundreds of millions of years old, and consist of little more than degraded skeletal remains.

Enter ostracods, the minute crustacean lifeforms (on the order of a millimeter in size) at the heart of the just-released paper. While finding the little guys can be a process (scientists’ method of choice is scraping up sediment in bulk and hoping to detect them on closer inspection back at the lab), once they are found, separating them by sex is surprisingly simple.

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.

Prioritizing penis and gonad development has its uses for individual males in a rivalrous reproductive environment. “It reflects the competition among males, sometimes called sperm competition, to fertilize the eggs of the females that they’re copulating with,” says Hunt. But bulking up sexually definitely has drawbacks in the zero-sum game of biology.

“The animal only eats a certain amount,” Hunt notes. “If you spend that energy growing your genitals to make lots of sperm, that’s energy you can’t keep in reserve to survive if there’s a food shortage or something like that.”

Indeed, the team found a definite inverse correlation between genital size and long-term species viability among ostracods. “We show that species with more pronounced sexual dimorphism, indicating the highest levels of male investment of reproduction, had estimated extinction rates that were ten times higher than those of the species with the lowest investment,” the paper summarizes. While sperm competition can be valuable for keeping a species’ genome strong, if taken to extremes, it seems pretty clear it’s an inadvisable strategy.

Where does the research go from here? Hunt is optimistic that the study he worked on will soon be verified by others tackling dimorphic fossils of their own—though only a handful of other obvious case studies exist. “I’d like to see people apply it to those groups where you possibly could, and maybe get creative in thinking about how to get to other groups.”

Hunt believes knowledge of the adverse effects of dimorphism could help us better predict and prepare for species endangerment down the road. “It might help us figure out which species might be more at risk,” he says. “If you have a species with strong dimorphism, maybe that’s a little extra thing to consider.”

Ryan P. Smith | READ MORE

Ryan graduated from Stanford University with a degree in Science, Technology & Society and now writes for both Smithsonian Magazine and the World Bank's Connect4Climate division. He is also a published crossword constructor and a voracious consumer of movies and video games.