The greater the difference between the sexes, the more likely a species may be to go extinct, reports a paper published online this week in Nature.

Sexual selection sees certain members of a species have greater success reproducing because they have characteristics that better help them attract or compete for mates. This can lead to marked physical differences between the sexes, called ‘sexual dimorphism’. How this affects species development, however, is much debated. Some studies suggest sexual selection increases adaptation rates, making species more resistant to extinction. Others indicate that the cost of exaggerated, sex-specific characteristics increases extinction risk. These studies are limited, however, as they only consider living species - relying on extinction proxies rather than the real thing.

To address this, Gene Hunt and colleagues turn to the abundant fossil record of ostracods, which spans from their appearance 450 million years ago to the present day. Also called seed shrimp, ostracods are tiny, shelled crustaceans whose species are sexually dimorphic to varying degrees. Male ostracods typically grow more elongated shells to house their larger sex organs, with investment in a bigger muscular sperm pump probably improving ejaculate quality.

The authors studied 93 ostracod species across the Late Cretaceous (around 66-84 million years ago) in eastern Mississippi and found that the more sexually dimorphic species had higher extinction rates, up to ten times that of the least dimorphic species. The male ostracods that invest more towards reproduction may, as a result, have fewer resources available for other survival functions. If this trend holds for other animals, the authors conclude, intense sexual selection should be considered in conservation efforts for at-risk species.