Gender Identity for Mice

“African pygmy mouse (Mus minutoides)” by Ian White.


A female African pygmy mouse is not necessarily a female African pygmy mouse. There are noticeable differences in behavior between the females, and in this case, you can’t just blame it on personality or individual differences.

What does this mean? When you look at the DNA of the mice, things become a little clearer. The meaning here is that there are three possible genetic varieties to a female individual of this species: XX, XX* or X*Y. The asterics symbolize an unidentified variation of X, that makes the individual, despite of having a “Y” chromosome, have female sex organs.

This is a fascinating phenominon, considering that in other species, mutant individuals with the chromosomes XXY are determined as male. The female pygmy mouse with the X*Y, however, has fully functioning female sexual organs and are, most of all, naturally occuring. What is more surprising is that they even have better reproductive success than the other, more “regular” mice.

So what exactly makes them so successful? According to studies, it’s not that they’re more attractive to males. It appears to be much more about this: the females with the X*Y chromosomes did have some male-prominent behavior such as aggressiveness and anxiety, and more curiosity towards new situations. Scientists hypothesize that the territorial and aggressive behaviors of X*Y females provide better chance of survival for the litter, as well as the lesser level of anxiety on creating a bond between the parents.

It’s difficult for now to tell how exactly these unusual females act and live differently from the others, but it will certainly be very interesting to keep an eye on further research–how do these natural mutants fare and function in their natural environment, and most of all, how did they come to be?


Saunders, P. A. et al. Masculinised Behaviour of XY Females in a Mammal with Naturally Occuring Sex Reversal. Scientific Reports. 6, 22881; doi: 10.1038/srep22881 (2016).

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