Well, I started to type that I think that when hybrids are sterile due to mismatched chromosome counts, it's usually both sexes. However, good ol' Wikipedia refutes me by saying that, in fact, of the scattered accounts of mules producing young, all have been female. So, maybe ...

However, if this were more of a "mystery morph," I'd be a bit more inclined towards that hypothesis -- but don't we have pretty good "origin stories" on two separate lines of Desert that are both female-sterile?