Tomboy mice have the most pups

NEVER write off a rank outsider. A female mammal with a “male” chromosome should struggle to reproduce, but for female African pygmy mice, a male chromosome spells more offspring. Mus minutoides has three sex chromosomes. On top of the usual X and Y seen in other mammals, there is a modified X called X*. In 2010, Frédéric Veyrunes of the Institute of Evolutionary Sciences at the University of Montpellier in France realised the X* is  “super-feminising”: it blocks the masculinising effect of the Y chromosome. So females can be XX, XX* or X*Y. Males are all XY. Now Veyrunes has found that X*Y females reproduce more than XX or XX*. They were more likely to have a litter during a six-month spell with a male, and had larger litters (Evolution, doi.org/rpr).