In several decades of zero-g research, they didn't even manage to put a few mice up to see whether they could successfully reproduce without gravity. Are they afraid of the results?
It would be embarrassing if the mouse babies come out crippled, even if it doesn't immediately generalize to humans and the low-g environment of Mars.
They've successfully hatched small fish on ISS. For animals, IIRC embryos need gravity to determine heads and tails or something. We do already know. The answer is that we Earthian lives generally face great difficulty reproducing in zero-g.
I meant to say larger animals - it's been tried. Small fish and frogs were fine, but chicken eggs showed issues developing past some early stages under microgravity[1]. Therefore we know that animals significantly more complex than amphibians can not reproduce normally under zero-g.
> Twenty out of 30 eggs (nine out of ten 10-day-old eggs; 10 out of ten 7-day-old eggs; 1 out of ten 0-day-old eggs) were recovered alive after landing. The only living embryo of the 0-day-old egg died 24 days after launch, and was comparable to a 16-day-old embryo when it died. The high mortality of the 0-day-old eggs appeared to be related to the specific inner structure of the egg. The yolk (specific gravity, 1.029) would not have separated from the albumen (1.040) during space flight.
Still, trying it with mice (or other mammals) would have been worth it, because we can't rule out mammals being more robust during embryogenesis than birds. The ISS actually has still a few years left, so there would be some time left for such an experiment. Though I doubt it will happen.
It would be embarrassing if the mouse babies come out crippled, even if it doesn't immediately generalize to humans and the low-g environment of Mars.