Some think that species with extreme sex differences become more vulnerable to extinction because extravagant male displays require lots of resources and make it harder to evade predators. The counterargument is that because it is harder for such males to survive, only those with the best genes get to pass them on and that sexual selection can therefore speed up adaptation and make species more resilient.
So who’s right? It’s a tricky question to answer by looking at living species, because they obviously haven’t gone extinct. So Gene Hunt and colleagues at the Smithsonian Institution turned to the fossil record. They looked at 93 species of tiny crustaceans called ostracods, or seed shrimp, that lived between about 84 million and 66 million years ago.
The males in this family of ostracods have larger, more elongated shells than the females because of their bigger genitals (a trait still present in living relatives of the extinct species). So the team could determine which species invested the most in male sexual displays by comparing the size and shape of male and female shells.
The species with the biggest differences between the sexes were found to have tenfold higher extinction rates than those with the least. This suggests that large genitals may be useful for attracting females and pumping out more sperm in the short term but that they take resources away from other functions needed for long-term survival, Hunt says.
This fits with studies of several other animals. For example, a survey of North American birds found that species with brightly colored males had 23 percent higher rates of local extinction than those with duller males.
The findings may have implications for conservation efforts. Species with flashier males may be more vulnerable to climate change and habitat destruction, Hunt says.