Giant genitals were the downfall of some ancient crustaceans

I can’t beat Nature‘s headline, so I won’t even try. “Giant genitals were the downfall of some ancient crustaceans.” The creatures from the Late Cretaceous put so much junk in their junk, they got junked themselves:

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.

That’s a condensation of an article here, which has details like:

The authors analysed the shapes and sizes of fossil exoskeletons of 93 ostracod species. These ostracods inhabited what is now eastern Mississippi between 84 million and 66 million years ago, during the Late Cretaceous period, at a time when an interior sea split North America into eastern and western halves. The authors’ analysis of fossils, along with a statistical modelling approach, enabled them to uncover a curious pattern. When comparing species, it emerged that those in which males were very different from females had a poorer prognosis for continued existence. The authors’ models predict a tenfold increase in extinction risk per unit time when species in which males are larger than females, with large differences in shape between the sexes, are compared with species in which the males are smaller than the females, with small differences in shape between the sexes.

The importance of this finding for our understanding of evolution makes it of interest to more than just ostracod enthusiasts. Sexual reproduction opens the door for sexual selection, the selection of characteristics that promote successful mating. Therefore, the generation of offspring requires both survival skills and the ability to compete for opportunities to reproduce.

If males invest heavily in characteristics that aid different tasks from those undertaken by females, the population could benefit if strong selection weeds out suboptimally performing males and leads to the species’ genome becoming better adapted over time. However, there is also a danger that selection for male reproductive success could result in characteristics that are harmful to females, whose ability to reproduce is more valuable for population persistence than is male input. …Another simpler and perhaps under-studied effect is that, from a population-growth perspective, large, growing males consume resources that could have been put to better use if left for females.