The Torah forbids many types of mixtures, among them is the prohibition against mating different species of animals together (see Vayikra 19:19).
With this in mind, we find on today’s daf that Rabbi Yehuda teaches: if a mule was in heat it may not be mated with a horse or a donkey, but only with one of its own kind. The Gemara explains that Rabbi Yehuda forbids mating it with any kind of horse or any kind of donkey, since we do not know the mule’s true species.
The Gemara then suggests that we should determine what “its kind” must be by examining it by means of specific signs taught by Abaye. These include: If its voice is deep, it is the offspring of a female donkey; if its voice is shrill, it is the offspring of a mare. In addition we have the tradition of Rav Pappa that if its ears are large and its tail small, it is the offspring of a female donkey; if its ears are small and its tail large, it is the offspring of a mare!
Since Rabbi Yehuda does not consider the possibility of checking these physical traits, the Gemara concludes that this must be dealing with a case where the animal was mute and its ears and tail were lopped off, and therefore could not be examined.
The offspring of a horse and a donkey – equus mulus – will either be a mule (if its mother was a horse) or a hinny (if its mother was a donkey). A mule can be either male or female, but even though externally it appears to have fully formed sexual organs it is infertile because it has an odd number of chromosomes. Mules are known as strong, hard-working animals, and they have served as work and pack animals for thousands of years. Hinnies are generally smaller than mules and are relatively uncommon.
In contrast with the descriptions in the Gemara – perhaps contemporary mules are from a different stock of animals –common mules today have long ears similar to those of donkeys and their tails are bald at the top like donkeys; their voice vacillates between the braying of a donkey and the neighing of a horse. In contrast, a hinny appears similar to a horse in the structure of its head and tail and its ears are shorter than those of a donkey.