One may not breed a male of one species with a female of another, as the verse states: “You shall keep My statutes; you shall not breed your animal with a different species” (Vayikra 19:19). Although the verse says “your animal,” it refers to all living creatures, including birds and sea creatures. The Torah mentions animals by way of example because domesticated animals are commonly kept by people (Bava Kama 54b). By Torah law, one may not perform the mating directly, meaning actual insertion of the male’s organ into the female. The Sages further prohibited bringing about mating indirectly, such as by positioning the male atop the female in a way that leads them to mate. However, one may place different species together in the same enclosure. If they mate on their own, one need not separate them, since placing them in the same enclosure is not considered an act that caused the mating (SA YD 297:2–3).
One may not breed two species of domesticated or wild animals. If one transgressed and did so, however, one may derive benefit from the offspring. The best-known hybrid is the mule, born of a male donkey and a mare. Mules have long been prized for carrying loads and plowing. Since the prohibition is only to bring mules into being, one may purchase and use them.
Breeding two different species is prohibited, but breeding two breeds of the same species is permitted. Therefore, one may breed bulls and cows of different breeds with one another. Similarly, one may breed different breeds of sheep, cats, or dogs. However, one may not breed two different species, even if they are relatively similar to one another. The definition of a species is determined by the overall traits and form of each species, as explained below, section 4. As a general rule, the inability to produce fertile offspring is a sign that two animals are different species. Usually, breeding two different species produces no offspring at all. In the rare cases where offspring are born, as with a donkey and a mare, the offspring is sterile. If two types can produce fertile offspring, it appears that they are not two separate species, but two varieties within a single species. In that case, there is no prohibition against breeding them together.[2]
