For mammals, gender is usually a simple affair. From mice to elephants, a single pair of chromosomes control sex—inheriting double Xs makes females, while inheriting a Y makes males. But for the platypus, the story is a bit more complicated.

Unable to locate a single Y chromosome in the platypus, researchers had long considered its gender-determining genes a mystery. By using fluorescent labeling to track platypus chromosomes during cell division, Frank Gruetzner, a molecular biologist at the Australian National University, solved the puzzle. Instead of a single pair of sex chromosomes, the platypus has five—a record for vertebrates. “It’s not as confusing as it might be,” Gruetzner says. “The sex chromosomes link up in a chain, so a male platypus is always XYXYXYXYXY.”

Even more intriguing, one of the platypus’s Y chromosomes shares genes with the ZZ/ZW sex chromosomes found in birds, which are thought to have evolved separately. “We’re not certain exactly how the two are related, because in birds the system is inverted,” says Gruetzner. “Males are the ones with two identical sex chromosomes.” The presence of both bird and mammal chromosomes in the platypus implies that sex determination evolved just once among a common ancestor of both groups, and then diverged into distinct systems.

The platypus has always looked a little ambiguous. Its signature duckbill and egg-laying are birdlike, but its fur and milk are mammalian. “Now,” says Gruetzner, “we know this ambiguity goes all the way down to its genome.”