In ancient Rome, when the impossible happened, people would react with the saying “When a mule foals.” It is common knowledge that a mule cannot produce offspring, but the reason why–that’s not so common. A horse and a donkey can birth a mule, but apparently two mules cannot have a baby mule. On the outside, this does not make any sense, but look closer–literally– and things will start to connect.
Ligers, Wholphins, and Pizzly Bears
These names may sound poorly made up, but they are, in fact, real animals. And what do they have in common with mules? They are all hybrid animals and cannot reproduce. A hybrid is the offspring of two different species mating. You can probably guess what the mix of these breeds are: Ligers are produced from a lion and tiger, Wholphins are the offsprings of whales and dolphins, and Pizzly Bears are the combination of their Polar and Grizzly bear parents.
Long Story Short
The short answer is that mules have chromosomes that do not match up, so they cannot make sperms or eggs. Thus, they are sterile and cannot produce offspring.
More specifically, a mule has 32 horse chromosomes and 31 donkey chromosomes. A total of 63 chromosomes. Normally, an organism would have two copies of one chromosome, one from the mother and the other from the father. However, for mules, they will get two different sets of chromosomes since the mother is a horse, and the father is a donkey. These genes will not exactly be matched sets, but they are still relatively similar (enough to produce the mule offspring in the first place). To reproduce, a process called meiosis will happen, in order to get one copy of each chromosome into the sperm or egg. During this process, the chromosomes will need to “match up” to another. Unfortunately, the donkey and horse chromosome are not similar enough to pair. Additionally, the mule will also have an extra unmatched horse chromosome, so sperm or eggs are unable to be made.
When a Mule Does Foal
While the Romans refer to mules foaling as impossible, sometimes the impossible is, well, possible. On a Grand Mesa Ranch in 1994, a female mule named Kate was able to give birth to a foal. Moreover, there are other recorded cases of mules giving birth.