Having a different number of chromosomes is a hurdle, but not an absolute barrier to offspring reproductive viability. It's not necessarily an issue for animal

Having a different number of chromosomes is a hurdle, but not an absolute barrier to offspring reproductive viability. It's not necessarily an issue for animal health. Horses & donkeys have a different number of chromosomes. They're also separated by an estimated 4m years of evolution, which is around the rule of thumb limit for large mammal hybridization so it makes sense that they're borderline hybridizable. They have shorter generations than us apes. Anyway, mules are healthy. No survival issues. They are mostly infertile, but not 100% of the time.