In dramatic contrast to most mammals which have a relatively simple two-chromosome sex determination system, the platypus has a ten chromosome sex determination system. Yet unlike most alternate sex determination systems known to biology, the platypus system uses the same X and Y chromosomes as the common mammalian XY system — platypi simply have five pairs of sex chromosomes instead of one pair.
This means that, in theory, platypi may have as many as twenty-five distinct genetic sexes with different numbers of Xs and Ys. The (relatively) straightforward cases are female with five XX pairs and male with five XY pairs; at this point, scientists don’t know how any other potential combinations work or how common they may be.
And then there’s the queer platypus who’s like, “F*** the pentavigesimal, I do what I want.”