Do pregnant mules die? Why can't mules give birth? The culprit is...

Expert of this article: Zhao Xumao, Young Researcher of Lanzhou University

I believe everyone knows the horse, which is one of the twelve zodiac signs. The idioms related to it can be recited by everyone: immediate success, vigorous spirit of dragon and horse, etc.

Everyone has heard of the donkey, and the term "stubborn donkey" is often used to describe a person with a very stubborn character.

However, some people may not know much about mules. This animal looks a bit like a horse and a bit like a donkey. Where did it come from?

There is also a saying among the people: "A pregnant mule will die." Is there any scientific basis for this? With the curiosity of many people, let's embark on the road of exploring mules.

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Can't tell the difference between a horse, a donkey and a mule?

From a biological point of view, wild horses and wild donkeys both belong to the genus Equus of the family Equid. Their common ancestor is Eohippus, which lived in the Eocene about 50 million years ago.

The biggest difference between Eohippus and modern horses lies in their toes. They had four toes on their forelimbs and three toes on their hind feet, while modern horses have only one toe.

Wild horses and wild donkeys began to differentiate about 4 million years ago and have now become two reproductively isolated species.

Humans domesticated horses and donkeys about 6,000 years ago. The difference is that the domestication of donkeys began in Africa, while the domestication of horses began in Asia. After wild horses and donkeys were domesticated, wild horses gradually disappeared, but there are still a certain number of wild donkeys in nature.

According to research by archaeologists and paleontologists, several species of wild horses have lived in my country since the Pleistocene, but have become extinct one after another. The existing Przewalski's horse also became extinct at the beginning of this century. The wild horses that are now galloping in the Junggar Basin in my country are actually the descendants of Przewalski's horses that were plundered from China by Europe and then released into the wild.

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After domesticating horses and donkeys, humans made a bold attempt, which was also a breakthrough in ancient breeding technology: crossbreeding horses and donkeys. Mules are the offspring of the crossbreeding of horses and donkeys.

How did the mule come about?

In my country, the history of hybridization between horses and donkeys can be traced back to the Spring and Autumn Period, but the technology was not perfect at that time, and mules as hybrid individuals were only occasional behaviors and could not be bred on a large scale. It was not until the Ming Dynasty, with the improvement of hybrid breeding technology, that mules began to be "produced" on a large scale.

If a mare and a male donkey are crossed, the resulting mule is called a "horse mule". It looks like a horse, sounds like a donkey, has more endurance than a horse, and is stronger than a donkey. In reality, a mare and a male donkey are more likely to produce offspring.

If a male horse and a female donkey are crossed, the resulting mule is called a "donkey mule". It looks like a donkey and brays like a horse. Although it is not as strong as a mule, it is good at running.

The fact that mules take their mother's surname is in line with the natural habits, because in nature, horses and donkeys live in groups in a matriarchal society.

Will a pregnant mule die? Will it be infertile?

Generally, when cross-species hybridization occurs, it is difficult for offspring to inherit the advantages of their parents, but mules are different. Whether it is a horse mule or a donkey mule, they have inherited the advantages of their parents, such as horse mules are strong, donkey mules are good at running, and in some periods they are also responsible for carrying tasks.

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Although mules have many advantages, they also have an obvious flaw - they cannot reproduce.

Why can't mules reproduce? This has to do with reproductive isolation.

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The so-called reproductive isolation refers to the inability of different populations to mate under natural conditions, and even if they mate, they cannot produce fertile offspring. Generally speaking, the same organism is divided into different populations due to geographical barriers, which prevents gene exchange between populations and forms geographical isolation. Long-term geographical isolation will cause genetic variation between different populations, and over time, reproductive isolation will form.

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In nature, horses and donkeys are independent species and have long been reproductively isolated.

A horse has 64 chromosomes, a donkey has 62 chromosomes, a horse's egg has 32 chromosomes, a donkey's sperm has 31 chromosomes, and the new individual developed through the fusion of sperm and egg - a mule - has 63 chromosomes.

If mules can produce offspring, the number of chromosomes in their sperm or eggs will become 31.5, which is obviously impossible because the sperm and eggs of horses and donkeys are haploid, and their formation is the so-called cell division, which is reduced from 2n to n, and it is impossible to form homologous chromosomes, let alone produce gametes.

Therefore, mules do not have the conditions to become pregnant, and the saying that "a mule will die if it becomes pregnant" has no scientific basis.

So, since mules cannot reproduce, why haven’t they become extinct? The answer is simple: as long as horses and donkeys are not extinct, mules will not become extinct.

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