This may be a stupid question, but why are they only storing semen? Shouldn't they be storing eggs as well? Unless they are projecting that females of the breed will survive but not male, won't they need both in case they decide they need to reintroduce the species?
If they had both, then they could do in vitro fertilization and then implant the embryo into a horse of another breed. However, if they only have the sperm, but there are no females of the breed left, or if the females that are left are unable to breed for some reason, then all they will be able to produce is a hybrid.
Just wondered.
