The greater the difference between sexes, the greater the chance of extinction

Researchers collected their evidence from millions of years of ostracode fossils.

When it’s easy to tell the males and females of a species apart, they may be at a greater risk of extinction, a new study suggests. Researchers looked at thousands of ostracode fossils from the Late Cretaceous epoch (85-66 million years ago) to quantify the extent to which there are observable difference between the sexes. They found that the greater the difference, the faster the species went extinct.

We spoke to one of the authors, Gene Hunt of the Smithsonian Institution, about the work.

ResearchGate: What does it mean for a species to be sexually dimorphic?

Gene Hunt: A sexual dimorphic species is one in which the sexes differ in observable characteristics. Absent sexual dimorphism, one cannot tell males and females apart without looking at the sex organs or genetics.

RG: What motivated you to look into the relationship between sexual dimorphism and extinction?

Hunt: One of the collaborators, Rowan Lockwood, is a friend and fellow paleontologist. She is married to another of the co-authors, John Swaddle, who is mostly a bird ecologist and evolutionary biologist. He knew that folks in the sexual selection community were interested in how that process might affect extinction. There were reasonable arguments that sexual selection might either increase or decrease extinction risk. Studies had tried to test this, but it can be hard to test ideas about extinction from living species alone; indirect proxies for extinction have to be used. John asked Rowan about fossil taxa that might be sexually dimorphic enough to address this problem using real extinctions. Dimorphism is useful here because it is an indicator of sexual selection. That led to John and Rowan talking to me, because they knew that I worked on ostracodes and that ostracodes were known to have sexually dimorphic shells. That’s a long answer, I know, but it shows the benefits of when people from different disciplines talk to each other.

RG: What did you discover in this study?

Hunt: We found that ostracode species with strong sexual dimorphism – males that were much larger and more elongated than females – had higher extinction risk than species with more modest dimorphism. Information from extant ostracode species suggests that the more dimorphic males are investing more energy in reproduction because their larger and more elongate shells are correlated to the size of the male sex organs. Thus, it seems that greater investment in reproduction can hinder the survival of the species overall.

SEM of Female ostracodes of the species Veenia ponderosa, Specimens ~78 million years old, collected from Arkansas. Female is ~750 microns long Credit: Gene Hunt.

SEM of Male ostracodes of the species Veenia ponderosa, Specimens ~78 million years old, collected from Arkansas. Male is ~810 microns long Credit: Gene Hunt.

RG: How did you conduct the research?

Hunt: We measured thousands of ostracodes to quantify sexual dimorphism in ostracode species from the Late Cretaceous (85-66 million years ago) of the US Coastal Plain. We then tracked the survival of species through time in the fossil record and tested if strongly dimorphic species went extinct faster than less dimorphic species. They did.

RG: Does this have implications for any other species living today?

Hunt: We don’t know how general this result is yet. There is some mixed evidence that sexual selection increases extinction risk in other groups (birds and mammals) from studies using indirect proxies for extinction. The mechanism is pretty general – males in many species invest a lot in competing for mates – so it is possible that it broadly operates.

RG: What does this mean for animal conservation efforts?

Hunt: If the pattern we find turns out to be general, it would suggest taking into account sexual selection when making judgments about extinction risk in present day species. This would be in addition to factors that are presently considered, such as species’ geographic range, populations size, population growth trajectory, etc.

RG: What’s next for your research?

Hunt: For this general area, we have been moving forward in time to look at sexual dimorphism in ostracodes in the next geological interval, the Paleocene. Our present study ends right at the end of the Cretaceous, at the mass extinction that killed the dinosaurs (and much more). We are interested in testing if this mass extinction induced a shift in dimorphism – perhaps species with high sexual dimorphism preferentially went extinct also during and after this mass extinction.

Featured image courtesy of Gene Hunt.