Female moles are intersex — they have testicle-like tissue that helps them grow big and tough

Moles have a pretty tough life. They live underground, in the dark, burrowing through heavy dirt. And when faced with an enemy, there's nowhere to turn — they have to fight. 

In most mammals, females tend to be at a disadvantage when it comes to face-to-face combat, because they tend to be smaller and less aggressive than males. 

But female moles have evolved a secret weapon: a hybrid organ made up of both ovarian and testicular tissue. This effectively makes them intersex, giving them an extra dose of testosterone to make them just as muscular and aggressive as male moles.

"As a consequence, basically the whole body of the female, they get masculinized," geneticist Darío Lupiáñez told Quirks & Quarks host Bob McDonald. "They become the body builders of nature." 

Lupiáñez co-led a study to understand how the moles' genes facilitated this advantage, which was recently published in the journal Science. The research was part of a collaboration between the Max Planck Institute for Molecular Genetics and the Max Delbrück Center for Molecular Medicine in the Helmholtz Association in Germany.

Same genes, different instructions

The team worked with Iberian moles, commonly found in Spain and Portugal, however this intersex adaptation has been documented in at least six mole species.

"We know that intersexuality happens in species like humans, dogs or cats. But the difference actually in moles, it happens all the time, so all the females are intersexual. And this is really something unique among mammals," said Lupiáñez.

To understand why this happens, the researchers completely sequenced the genome of the Iberian mole and sifted through their billions of genetic "letters" or nucleotides, looking for those responsible for the change. They also compared the mole genome to several other animals, including humans. "I can really tell you that this is like looking for a needle in a haystack," said Lupiáñez. 

They found that it wasn't the genes that had changed, but rather the regulatory elements, or instructions, associated with those genes, which gave the female moles this advantage.

"In moles, you have all the components there, but the only thing is that there have been new connections established," he said. "These new genes get completely different patterns of expression, and they start to make new functions … which in this case is to help females develop as intersexual."

Female moles look and act just like the males

As a result of these different genetic instructions, the moles develop an organ made up of both ovarian and testicular tissue. This organ, called ovotestis, still functions as an ovary, but doesn't produce sperm. It does, however produce the same large amounts of testosterone as found in male moles.

This hormonal boost means that females and males are roughly the same size, equally muscular, and equally aggressive. Even their external genitalia looks alike, with the female's clitoris protruding much like the male's penis.

This boost of testosterone doesn't affect the moles fertility or reproductive abilities. 

"What normally happens within mammals is that the males have these famous Y chromosomes and females do not. So this is exactly the same for moles," Lupiáñez said. "So from this perspective, you have two completely separate sexes."

He adds that this natural doping can make it very challenging for researchers in the field to figure out whether a mole is a male or a female. 

"It took me quite a while of trying to really figure out when I had a female in front of me or a male," he said. "They are pretty tough ... they become pretty aggressive and pretty wild to handle. But you know, with time we learn how to handle them with care."

Produced and written by Amanda Buckiewicz.