SEM of Male ostracodes of the species Veenia ponderosa

This 78-million-year-old male ostracod — a small crustacean — had a shell longer than a female’s to accommodate male sex organs. Credit: Gene Hunt

Evolution

Giant genitals were the downfall of some ancient crustaceans

Overinvestment in sex organs can make species slow to meet new challenges.

Some ancient crustaceans put so much energy into enlarging their genitals that they became extinct.

Many animals evolve traits that appeal to potential mates — through a process called sexual selection — sometimes developing exaggerated or cumbersome features. Whether such adaptations boost or limit a species’ chance of survival is tricky to establish in studies of extinct species, because gender is not always evident from fossils.

Gene Hunt at the National Museum of Natural History in Washington DC and his colleagues studied the fossil record of small, chalky-shelled crustaceans called ostracods, some of which survive today. To accommodate their sex organs, the males’ shells are more elongated than the females’, making it easy to identify an individual’s sex.

The researchers found that ostracod species whose males evolved particularly large sex organs were much more likely to have become extinct than those with more-modest genitalia. Expending energy on physical flourishes for the sake of sexual selection could limit species’ ability to respond to change, making them more vulnerable to extinction, as well as to threats from invasive species, climate change and human activities, the authors say.

See related News & Views: When sex differences lead to extinction