"You shall not plow with an ox and the donkey together." (Deut. 22:10)
The classical commentator, Rashi, writes that the ox and donkey mentioned in the above verse are to serve as examples, for "the same is true for any two species in the world." This mitzvah therefore prohibits us from forcing two different species to work together in any way.
The Sefer Ha-Chinuch is a classical work on the Torah's 613 mitzvos. The author of this work suggests that one of the reasons for this particular mitzvah is to avoid the suffering which is caused to animals when they are forced to work together with members of another species. He writes:
"It is known that the various species of animals and fowl have great anxiety in dwelling with others not of their kind, and all the more certainly to do work with them…Every bird will dwell with its own kind, and so all animals and other species will always cling to their own kind as well." (Mitzvah 550).
The author of the Sefer Ha-Chinuch adds that each wise-hearted person can learn from this mitzvah the following insight: One should not appoint two human beings to work together if they would experience a clash, due to their being radically different in nature and/or differing in their conduct. If we are to have understanding and respect for the different nature of animals, we should surely have understanding and respect for the differing nature of human beings!
Yosef Ben Shlomo Hakohen (See below)
1. Rabbi Samson Raphael Hirsch mentions the following related laws in Horeb (chapter 57): "You must not allow one task to be done together by animals of two species. You may not allow them to carry the smallest thing together, even if it be only the seed. Therefore you may not even use the voice in order to drive forward animals of differing species that are yoked together. You may not sit in a wagon which is drawn by animals of differing species. (Yorah Deah 297b)
2. The Sefer Ha-Chinuch explains that the ban against forcing animals of different species to work together is an example of the Torah's general prohibition against causing suffering to animals. With the help of Hashem, we shall discuss this general prohibition at a later stage of this series, and we shall cite some specific examples which are mentioned in the Torah. We shall also discuss the exceptions where the Torah does permit us to cause some suffering to animals; however, even in these special cases, we are obligated to ensure that the suffering is kept to the minimum that is needed. For example, we are allowed to plow with an ox, but we are obligated to treat it in a humane way. And we are forbidden to cause the ox extra discomfort by forcing it to plough together with a donkey or a member of any other species.
3. The Sefer Ha-Chinuch also cites the opinion of Maimonides (the Rambam). According to Maimonides, the ban against forcing animals of different species to work together is related to the ban against mating animals of different species, for it is the way of farmers to bring the working pair into one stall, and the farmers may therefore decide to mate them. (Guide to the Perplexed 3:49)