Why are horses and donkeys considered different species even tho they can produce offices?

Horses and donkeys are considered different species primarily due to their distinct genetic makeup; horses have 64 chromosomes, while donkeys have 62. When they interbreed, they can produce mules (a horse-donkey hybrid), which are typically sterile due to the mismatched chromosome numbers. This sterility prevents mules from reproducing and reinforces the classification of horses and donkeys as separate species. Additionally, they exhibit notable differences in behavior, morphology, and ecology.