Female Brazilian Cave Insects Have Penises, And Now We Know Why

The Brazilian cave insect Neotrogla has reversed genital structures. Dr Kazunori Yoshizawa/Hokkaido University

Creatures are born, creatures reproduce, creatures die. In this three-step program of life, that middle portion – to reproduce – is especially important. So, just like us humans, many creatures great and small have developed elaborate, fascinating, and oddly complex ways to do just this.

Few come stranger than a handful of cave-dwelling insects from the tribe Sensitibillini, including the species Afrotrogla and the Brazilian Neotrogla. Japanese scientists discovered back in 2014 that the Neotrogla cave insect appeared to have reversed genital structures: the female has a long penis-like organ while the male has a vagina-like pouch.

After winning an Ig Nobel Prize in 2017, the researchers are now back with some big insights into why these bat-poop eating beasts have developed this “evolutionary novelty”.

Neotrogla indulges in 40- to 70-hour-long sex sessions whereby the female mounts the male, inserts her penis-like structure (known as a gynosome) into his vagina-like genitalia. The female’s gynosome also curls round inside the male and locks them together. Researchers have actually tried to pull mating pairs apart, only to snap the male’s body in two pieces because the connection is strong. Youch.

A hard capsule will form around the semen after the male injects sperm up the female’s sperm-storage organ. Since resources are so scarce here, the female can either use the content of the capsule to fertilize her eggs or she can also eat it for nutrition. Yum.

Neotrogla curvata female penis. Dr Kazunori Yoshizawa/Hokkaido University

But what’s the advantage of this unorthodox method of mating? As discussed in their new study in the journal Biology Letters, it’s something to do with the harsh life of living in a dark and dry cave, as the female penis has evolved at least twice independently in this environment.

“Evolution of a female penis is an extraordinarily rare event,” the study reads. “Nevertheless, our results show that it has arisen twice in this small group of insects.”

“Because they inhabit dry and oligotrophic caves, severe competition for seminal gifts has likely reversed their propensity for multiple mating (reversed direction of sexual selection).”

The females feature a tiny switching valve 0.3-millimeters wide at the entrance of the semen-storage organ, which can help them take in more semen. Writing in journal eLife last month, the same team of researchers speculates that "the ability to obtain greater amounts of semen thanks to the valve has led to fierce competition over semen among females, facilitating the evolution of the female penis."

In other words, any resources – including sperm – is much sought after in the barren environment of the cave. Therefore, the females can’t just sit around and wait for the males to make the move, they have adapted to become more aggressive, pro-active, and effective sperm-hunters.

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