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I was taught in yeshiva that the only non-kosher animal which has split hooves but does not chew its cud is the pig. What about the hippopotamus which does not chew its cud but has split hooves? Is a hippopotamus somehow related to a pig?
Before answering this question we turned to Rabbi Dr. Ari Zivotofsky, professor at Bar Ilan University and an expert on the kashrus of animals and birds, who in turn consulted with his brother, Dr. Doni Zivotofsky, D.V.M., and we thank them for their help.
You mentioned that the Torah might consider hippopotami to be “related” to pigs. Some support this notion because scientists classify those two animals as being in the same “order”. However, this is not as significant as it seems because the order they share (Artiodactyla) refers to all mammals that have an even number of toes. Actually, they used to also share a suborder but scientists are now considering removing hippopotami from the pig suborder (Suina) and reclassifying them in a new suborder for hippos, dolphins and whales, which all seem to share certain DNA. These criteria may be significant to scientists but I think most non-scientists would agree that dolphins and hippos are not one “family” (even if they are in the same order and suborder), any more than pigs and giraffes are (even though they both are Artiodactyls)! Thus, whether pigs and hippopotami share an order and suborder does not seem to be a meaningful factor. From a lay perspective, there is some similarity between pigs and hippos, but it would seem that they are not similar enough for the Torah to consider a hippopotamus to merely be a water-based pig. [The Yiddish word for hippopotamus is vasser chazir, which means water-pig, but the English name means water-horse (in Latin, hippos means horse, and potamos means river).]
Rather, it seems that the answer to your question is that the hippopotamus does not have cloven hooves. A hippo has four toes which are covered and connected by thick skin which in turn produce a web-effect and aids the hippo in swimming. Thus, a hippopotamus is much like most non-kosher animals which do not chew their cud and do not have split hooves.
[A related side note from R’ Zivotofsky regarding the hippopotamus’ ruminant status: Without exception, every animal with a 4 chambered stomach is a ruminant. There are those who dispute this and assert that the hippo is an example of an animal that has 4 chambers and is not a ruminant., but this is erroneous. In fact, it has a three chambered stomach: parietal blind sac, the stomach (which can be considered simply connecting tissue) and the glandular stomach. For more on this see: E.T. Clemens and G.M.O. Maloiy, “The digestive physiology of three East African herbivores: the elephant, rhinoceros and hippopotamus”, Journal of Zoology, 1982, Vol. 198, pp 141-156.]