Most mammals have routine distributions of sex chromosomes but not the creeping vole:
For example, while most placental and marsupial mammals have two X chromosomes in most of their cells, female creeping voles have just the one. Confusingly, where our sex cells halve their chromosome numbers, inside the tissues that produce ova in creeping voles you’ll find a double-X arrangement.
The males, at least according to Ohno, are more like typical mammals with an X and Y in each of the body’s non-sex cells and a single chromosome in the cell lines that give rise to sperm. Only for some reason it’s always the same ‘Y’ chromosome.Mike McRae, “These Little Creatures Have The ‘Weirdest Sex Chromosome System Known to Science’” at ScienceAlert (May 21, 2021)
We are told this “weirdest sex chromosome system known to science” happened in the past couple of million years but it is not clear why.
The paper is closed access.