The Torah sets out two criteria for determining whether a domesticated animal or wild animal is kosher, as it says: “These are the animals that you may eat from among all the beasts upon the earth: every animal that has a split hoof, completely divided into two hooves, and that chews the cud — that you may eat” (Vayikra 11:2–3).
The first criterion is a completely split hoof. A hoof is the hard covering, like a thick nail or soft bone, that grows over the bottom of the animal’s foot and allows it to walk on the ground without injury. A “completely split hoof” means that the foot and hoof are divided into two parts.
The second criterion is that the animal chews its cud. Since kosher species feed on grass, which is difficult and slow to digest, they have a special digestive system with four stomachs: 1) the rumen; 2) the reticulum; 3) the omasum; and 4) the abomasum. At first, these animals tear off grass quickly and swallow it after only minimal chewing. The food descends to the rumen, where digestion begins, and from there moves to the reticulum, where it forms into rounded pellets. It then returns to the mouth for extended chewing — this is what it means to chew the cud. Afterward, it descends to the omasum and finally to the abomasum.
Four species have only one of the two criteria and therefore resemble kosher species, but because they lack the other, they are non-kosher: the camel, the hyrax, the hare, and the pig. As the Torah states: “But these you shall not eat from among those that chew the cud or have split hooves” (Vayikra 11:4). The camel, hyrax, and hare are described as chewing their cud but not having split hooves: “The camel, though it chews its cud, does not have a split hoof; it is non-kosher for you. The hyrax, though it chews its cud, does not have a split hoof; it is non-kosher for you. The hare, though it chews its cud, does not have a split hoof; it is non-kosher for you” (ibid. 4–6). Biologically, however, the hare and the hyrax do not ruminate in the usual sense. Rather, their mouths appear to be constantly chewing, like those of ruminants, and the hare also eats its own droppings, which makes it appear to chew its cud. For this reason, the Torah states that they are non-kosher because they do not have split hooves (R. David Tzvi Hoffmann to Vayikra 11:4–6). The pig, by contrast, has split hooves but does not chew its cud, and is therefore non-kosher, as it says: “The pig, though it has a split hoof, completely divided, does not chew its cud; it is non-kosher for you” (ibid. 7).
The Sages gave additional identifying features of kosher species. Kosher species do not have front teeth in the upper jaw. Another feature appears in the flesh beneath the tailbone, which has a lengthwise-and-crosswise pattern, though this feature is also found in the arod, a non-kosher animal (Ḥullin 59a). The Sages also taught that the milk of kosher species can curdle into cheese, whereas the milk of non-kosher species does not curdle (Avoda Zara 35a). Horns, too, are mentioned as an indication that an animal is kosher, though not all agree that this is a definitive marker (Nidda 51b; Ḥullin 59a–b; Tosafot, Ḥullin 59a, s.v. elu).
In general, the Torah’s criteria for kosher species are also the traits of non-predatory herbivores. Grass is difficult to digest because it contains a great deal of cellulose, so God gave these animals a sophisticated digestive system and the ability to chew their cud, allowing them to continue breaking down their food after it has first been swallowed. Instead of claws for seizing prey, they have split hooves, suited to running through hills and climbing over rocks: they can find grass, eat quickly, and then escape predators to a safe place where they can chew at ease. The absence of front teeth in the upper jaw likewise reflects that they are not predators; their teeth are made for chewing grass. Their horns, too, are for defense, not attack. Even without the physical advantages of predators, kosher species thrive. This teaches that one who works hard and is content with what he has can find a good livelihood without preying on others.
