That ornament may not be worth the trouble Getty

The peacock’s tail may dazzle females, but can such extreme ornaments reduce males’ chances of surviving to such an extent that the whole species is far more likely to die out? Yes, says the most comprehensive study yet done.

It was Darwin who realised that sexual selection by females can lead to the evolution of extreme traits in males – from colourful plumage to extravagant dances and displays – that reduce their overall fitness. Biologists have been arguing about how this affects the long-term survival of species ever since.

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 counter argument 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 at the Smithsonian Institution and his colleagues instead turned to the fossil record. They looked at 93 species of tiny crustaceans called ostracods, or seed shrimp, that lived around between 84 and 66 million years ago.

Bigger genitals

The males in this family of ostrocods 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 ten-fold 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 take resources away from other functions needed for long-term survival, says Hunt.

This fits with studies of several other animals. For example, a survey of North American birds found that species with brightly-coloured males had 23 per cent higher local extinction rates 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.

Journal reference: Nature, DOI: 10.1038/s41586-018-0020-7