Female moles grow testicles to fight through their brutal underground existence

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If any animal understands the horrors of trench warfare, it has to be the mole. Faced with an enemy, there’s no time for pleasantries. No place to hide. Aggression is all that matters.

To help them fight in this brutal world, evolution has granted the female mole a generous dose of ‘roid rage’ by tacking some testicles onto her ovaries – resulting in a unique bit of anatomy called an ovotestis.

Now, researchers have a better understanding of how this fascinating biological change came about.

“The sexual development of mammals is complex, although we have a reasonably good idea on how this process takes place,” says geneticist Darío Lupiáñez from the Max Planck Institute for Molecular Genetics.

“At a certain point, sexual development usually progresses in one direction or another, male or female. We wanted to know how evolution modulates this sequence of developmental events, enabling the intersexual features that we see in moles.”

Just like a more typical mammalian ovary, ovotestes nurture and release eggs for fertilization. They also happen to have a lump of testicular tissue stuck to one side.

While it’s not capable of making sperm cells, it does have what are known as Leydig cells for churning out a masculine-sized serving of androgens, or male sex hormones.

Usually the development of testicular tissue in mammals relies on the presence of a gene on the Y chromosome to ramp up testosterone production early in development.

Lacking a Y chromosome makes it a lot harder for an embryo to kick off the chain of events that produce testes. So just how it happens in female moles, which have two X chromosomes instead of an X and Y, has long been a mystery.

An in-depth analysis of their genomes now reveals just how this quirk of nature came about