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 (yes, blood is a tissue!) that plays a previously overlooked role in the regulation of sex hormones.