Are horses and donkeys reproductively isolated from one another?

Horses and donkeys are not completely reproductively isolated, as they can interbreed to produce mules (from a horse mare and a donkey stallion) and hinnies (from a donkey mare and a horse stallion). However, these hybrids are typically sterile due to the differing number of chromosomes in horses (64) and donkeys (62), resulting in mules and hinnies having 63 chromosomes. This sterility ensures that while they can produce offspring, those offspring cannot reproduce, maintaining a degree of reproductive isolation between the two species.