Having Big Genitals Sometimes Leads to Extinction, Say Paleobiologists

Bigger isn't always better, at least for some species.

For most animals, survival is all about being good at getting laid. That’s why sexual dimorphism exists: Males often look and act differently than females of their same species because it helps them attract and secure mates. Some examples are the bright colors the male bird of paradise flaunts while sexy-dancing for its more demure-looking mate, or the bulbously attractive nose of a male sea elephant. According to a Nature study released Wednesday, however, one highly coveted aspect of sexual dimorphism is sometimes a cock block to survival: very large genitalia.

The study, authored by scientists at the Smithsonian’s National Museum of Natural History, shows that the size and shape of at least one species’ genitals has spelled out disaster for longevity. Their analysis of the fossils of ostracods — tiny, shrimp-like crustaceans that are still around today — indicated that individuals with larger and more elongated sex organs had “estimated extinction rates that were ten times higher than those of the same species with the lowest investment.”

“Investment” here refers to genitalia — namely, the amount of energy any species invests into growing bigger sex organs. The idea in this paper is that the energy older ancestors of ostracods spent on having large sex organs left them unable to develop traits that would have left them better equipped for survival, albeit with smaller dicks.

“We showed that when males are larger and more elongated than the females, those species tend to not last as long in the fossil record,” explained co-author and paleobiologist Gene Hunt, Ph.D., in a statement released Wednesday. “They have a higher risk of extinction.”

Female (top) and male (below) of the ostracod *Cypideis salebrosa*. The genitalia is highlighted in blue.

M. Joāo Fernandes Martins/Smithsonian

Hunt and his team studied 6,000 ostracod specimens, some of which were part of the collections stored at the Smithsonian’s National Museum of Natural History, the University of Southern Mississippi, and Louisiana State University. Assessing the shapes and sizes of the specimens, they took note of the geological layer in which they were collected. This specimen collection included included 93 species of ostracods that lived between 85 and 65 million years ago.

While some of the specimens indicated that the species lived throughout the entire 20-million-year span, others species had only existed for a few thousand years. The difference between survival and annihilation was the size and shape of the male’s genitalia. While larger organs presumably could produce more sperm, hypothetically increasing the creature’s opportunity for reproductive success, it appears that they lacked adaptations necessary to cope with environmental threats.

Fossil ostracods on a slide from the collection at the Smithsonian.

Gene Hunt, Smithsonian

Male ostracods that are alive today are no longer hindered by penis size. Now, in a sexual dimorphism twist, they have two penises.

Hunt explains that, moving forward, this over-emphasizing of sexual dimorphism is important to keep in mind when considering how other animals may be threatened with extinction. It appears that to survive, animals need to strike a balance between getting busy and revving up all the other things you need to do to live. A male ostracod cannot live by its large genitalia alone.

“If devoting so much energy to reproduction made it harder for species in the past to adapt to changing circumstances,” says Hunt, “perhaps that same should apply to species we’re concerned about conserving in the present day.”

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