What Makes A Species Go Extinct? Sex Differences Could Be Crucial

Many animals go to extreme lengths to reproduce, developing sex differences — great and small — to woo members of the opposite sex and compete with rivals. This "sexual selection" is thought to be how peacocks got their tails and even how triceratops developed their horns.

Researchers studying the fossils of tiny shelled crustaceans have honed in on a link between sex differences and extinction risk. The larger the difference between the sexes, they argue, the more likely a species is on its way out. The research is published this week in Nature.

4_11_Male Ostracod
An example male ostracod of the species Veenia ponderosa, is depicted. Gene Hunt

The success of sexual dimorphism for survival is a matter of great debate in the world of evolutionary science. Some research suggests it's good for a species' survival — improving their adaptation rate and making them less likely to become extinct. Exaggerated traits, however, might make a species more likely to die out. Impressive physical displays can attract predators, not just partners.

Comparing minute crustaceans called ostracods offers a great way to study sexual dimorphism, or the physical differences between the sexes. So far, studies have been limited by a focus on living species, quantifying extinction risk rather than extinction itself. The vast fossil record of ostracods stretches back some 500 million years and contains thousands of different species, many of which are now extinct.

It's impossible to work out the sex of many individual fossils. Ostracod species, on the other hand, exhibit noticeable sex-related features to varying degrees. Different sexes sometimes have different shaped shells of different sizes. Some females even hold "bulbous brood pouches," Giles Miller, principal curator of micropalaeontology at the Natural History Museum in London, told Newsweek.

4_11_Female ostracod
An example female ostracod of the species Veenia ponderosa, is depicted. Gene Hunt

In this study, researchers analysed 93 ostracod species from throughout the Late Cretaceous period, roughly 66 to 84 million years ago.

The team found that species with greater sex differences went extinct up to ten times more than the most sexually similar species. Males that invested too much effort in looking good for their mates, they suggested, may not have enough resources left for other important survival functions. Some extinct ostracod species, it seems, were putting too many evolutionary eggs in one basket.

The research "provides a strong indication, and the first good documentation from the fossil record, that sexual dimorphism tends to increase extinction risk," Jonathan Payne, a professor of geological sciences at Stanford University, told Newsweek. Neither Payne nor Miller were involved in the study.

Unfortunately, these tiny fossils can't tell us all that much about animals like humans. Not only are ostracods much, much smaller than us, but they live in the sea and they regulate their body temperature in a completely different way. The results, however, set scientists up for "exciting future research," Payne said. Analysing the fossil record of primates, for example, might help scientists understand more about sexual dimorphism in our own species.

Confirming a similar link in non-ostracod species would suggest "deeply-rooted" links between sexual dimorphism and extinction risk, he added.

The results could help conservationists understand more about living species at risk of extinction. If sexual dimorphism is a factor for non-ostracod species, it might help shed light on the way different animals respond to changing conditions. This, Miller said, "can help us identify populations at risk and help us manage that risk better."