Scientists Just Solved a 100-Million-Year-Old Mystery About Platypus Mating

The platypus has always been one of nature’s biggest weirdos. It lays eggs, sweats milk, glows under UV light, and has venomous ankle spurs. But for over a century, scientists have been stumped by one fundamental question: how the hell does this animal determine biological sex?

Unlike most mammals, which rely on the familiar X and Y chromosome combo (with the Y-linked SRY gene triggering male development), monotremes—aka platypuses and echidnas—don’t follow the rules. They have ten sex chromosomes, not two. And until recently, no one could find the gene responsible for steering embryos toward male or female development.

That changed with a study just published in Genome Biology, where researchers finally identified the long-missing puzzle piece: a gene called AMHY, a modified version of the anti-Müllerian hormone (AMH). This gene, located on one of the Y chromosomes, appears to direct male development by flipping on during the critical window when testes start to form.

Scientists Just Solved a 100-Million-Year-Old Mystery About Platypus Sex

“We showed that AMHY is turned on in the right tissue and at the right time to direct development of the testes,” researchers wrote. And while AMH plays roles in many species, the platypus version is special. AMHY functions as a hormone rather than a traditional gene regulator, marking the first known instance of a hormone playing the lead role in determining sex in mammals.

Even weirder, AMHY doesn’t act directly on DNA. Instead, it works on the surface of cells, triggering pathways that influence what genes get expressed. That puts monotremes in strange company—closer to certain fish and amphibians than any other mammal walking the planet today.

The idea that platypus sex determination might have diverged from other mammals more than 100 million years ago is more than just a fun evolutionary fact. It explains why previous attempts to find an SRY-like gene in these creatures kept failing. They don’t have one. Instead, they’ve evolved their own system, where a modified hormone now runs the show.

This breakthrough solves a long-standing mystery and raises a new set of questions. How exactly did AMHY evolve? Why did monotremes stick with this unusual system while other mammals adopted SRY? And what could this reveal about how flexible sex determination really is across species?

One thing’s for sure: the platypus continues to be nature’s favorite outlier. And now, we finally understand how it decides who gets testes.