Why can’t hybrid animals breed?

What are hybrid animals?

You may have heard of the tales of chimera, a fire-breathing lion with a tail ending with a snake’s head and the head of a goat emerging from its back. Even though this monstrous hybrid creature only exists in ancient Greek myths, hybrid animals do exist in our real world and some of them play an important role in our lives.

Chimera. Photo credit: Alexandra Korey via Flickr

Hybrid animals are the results of interspecies mating. To name a few, liger (the offspring of a male lion and a female tiger), beefalo (the offspring of a domestic cattle and an American bison) and mule (the offspring of a male donkey and a female horse). However, animals from different species rarely mate unless there is environmental stress or human interventions.

While many hybrid animals are kept in zoos, some of them especially mules have played an important role in mankind history. Historically, mules were widely used for transportation, agriculture and even fought alongside British soldiers during the Second Anglo-Afghan War. In modern days, although not as widely as in the past, they are still used in Central Asia and South America for agricultural purposes.

A mule transporting luggage. Photo credit: Jeroen Mirck via Flickr

All of those contributions of mules to mankind history are made possible due to their inherited patience, strength and long life-span from donkeys, and intelligence and speed from horses.

Although hybrid animals might look very cool and have many advantages over their parents, it is extremely rare for them to have babies.


Why can’t hybrid animals have babies?

To understand the infertility in hybrid animals, we must go deep and look into the world of chromosome. Chromosomes are thread-like structures consist of DNA and protein. They carry the genetic information that determines the body plan of animals. Chromosomes are arranged in pairs, one set from father, and one set from mother.

In order to have babies, animals need to produce sex cells. The production of sex cells requires paired chromosomes to exchange genetic information, so that the chromosomes from the father will carry some of the genetic information from the mother, and vice versa. For example, a blonde hair gene from the father exchanges for a black hair gene from the mother. This exchange process is called genetic recombination.

Genetic recombination. Image credit: yourgenome via Flickr

Genetic recombination is the process that goes wrong in hybrid animals and causes their infertility. In normal animals, because their father and mother are from the same species, the genetic information that are exchanged from their mother/father to their father/mother can still be processed. However, in hybrid animals, things could get really nasty. Because hybrid animals have parents from different species, the exchange of genetic information can cause many malfunctions in the chromosomes. This can result in the production of infertile sex cells and infertility.


There is an exception to every rule!

In ancient Rome, when “impossible” things happened, people often describe them with the saying “when a mule foals”. However, “impossible” things sometimes do happen.

For some unknown reasons, there are incidences  that some mules have skipped the process of genetic recombination and produced fertile sex cells. Although we cannot explain those incidences now, we can definitely say to the ancient Romans that sometimes mules do have babies.

One Response to “Why can’t hybrid animals breed?”

  1. Daniel Hutchinson says:

    Awesome post Xiaohan.
    Really awesome topic and I really like how you threaded in all the historical facts and trivia (I love biology and I love history so having both in one post was awesome).