We normally associate high testosterone levels with male dominance and aggression. Among ruffs – a shore bird that breeds across Europe and Asia – some males suffer from too much of it. Together with international colleagues, researchers at the Max Planck Institute for Biological Intelligence have now shown how the birds eliminate the excess: they produce a super enzyme that rapidly breaks down the hormone. The study has revealed that a single gene drives this process and that it happens in the blood – a tissue that plays a previously overlooked role in the regulation of sex hormones.

Testosterone is the male hormone par excellence: it contributes to the male phenotype and influences sexual development and aggressive behavior, among other things. In the animal kingdom, we usually connect high testosterone levels with greater assertiveness and reproductive success. Most testosterone is produced in the testes, and the blood carries it to other parts of the body.

A recent study led by Clemens Küpper has now shown how male ruffs get rid of testosterone. This begs the question, however, of how the males of this bird species, who compete fiercely to attract females, could benefit from discarding this important sex hormone.

There are three types of males, known as morphs, that differ from one another in appearance and behavior. “Independents”, which account for the vast majority, have darker plumage and energetically defend a small territory in mating arenas called leks in order to impress females. The slightly smaller “Satellites”, with lighter-colored plumage, court peacefully in alliance with an Independent. The rarer “Faeder” males take an ingenious approach: they are disguised as females, which lets them sneak unnoticed into the mating arenas.

For the Satellites and Faeders, whose strategies aren’t based on aggressive behavior, too much testosterone is counterproductive. Earlier studies had shown that these types of males have significantly lower testosterone levels in their blood than Independents.

Until recently, very little was known about the genetic factors involved in regulating testosterone. Ruffs can shed light on this, since the three morphs only differ from one another in a “supergene,” a DNA region comprising about 100 genes. It originated around four million years ago, when a chromosome fragment broke out and then reinserted itself in the opposite direction.

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While performing gene expression analyses, the researchers noticed a gene within the supergene that codes for an enzyme that breaks down testosterone. They found that it was being produced in large quantities in Faeders and Satellites – but not in the testes, which secrete testosterone. When the team measured the testosterone levels in the testes, however, they were surprised to find that Faeders and Satellites produced far more testosterone there than the Independents. This begged the question of how these two morphs could maintain low testosterone levels in their blood.

The researchers found that the Faeders and Satellites have evolved a very powerful enzyme version, a “super enzyme”, that degrades testosterone very efficiently. The super enzyme was present in large quantities in the blood of Satellites and Faeders but was completely absent in the blood of Independents. “Based on previous research in other animals, we had assumed that this enzyme plays no role in the blood,” explains Alex Zemella, one of the first authors. “But our results revealed a previously unknown site there that the enzyme acts on. This means that the amount of circulating testosterone can be regulated directly in the blood – something that had previously been overlooked.”

This highlights the sophisticated strategies that males have evolved to increase their reproductive success. “All males need testosterone in their testes for producing sperm,” explains Jasmine Loveland, one of the lead authors. “But the hormone also has other effects, for example in the brain, where it can promote aggressive and courtship behavior. It is interesting that the non-aggressive ruffs seem to have increased levels of the super enzyme throughout the brain too, especially in the hypothalamus. These results, along with the low testosterone levels in the blood, likely play an important role in shaping their mating strategies.”

The discovery that changes in a single gene can affect testosterone levels is opening up new lines of research. Going forward, the team wants to look more closely at how complex social behaviors are regulated in ruffs and continue exploring the diversity within the sexes.