In outlining the rules for animal sacrifice, the Torah presents a regulation about male animals with damaged or disfigured sexual organs.
"You shall not offer these to God, and in your land you shall not do so" (Leviticus 24:22). Careful readers cannot help noticing an ambiguity in the words "You shall not do so." At first glance "not doing so" means not making a sacrifice of this sort of animal, in which case the phrase adds very little to the sentence.
The ancient rabbis rather understood the phrase to prohibit castrating cattle, or any other sort of animal or even a man, for that matter (Sifra Emor 121).
This specific prohibition must add something to the general prohibition on causing pain to animals (Talmud Shabbat 128b), and indeed it does. For the rabbis permit causing pain to animals when doing so meets a real human need (comments of Rabbi Moshe Isserles to Shulchan Aruch Even HaEzer 5:14) — for drug research, for example.
They note that the Torah even permits us to kill animals, in a relatively gentle way, for food (Deut. 12:20) and, as this verse itself shows, for specific religious ceremonies. But the rabbis do not find grounds for leniency in our verse about castration.
In the course of history, men have been castrated to produce guards free from the usual disqualifications for protecting a harem, or to produce government officials with no children to advance dishonestly, or to produce adult singers with powerful, high boy-soprano voices. The Torah, according to traditional interpretation, forbids Jews from doing so.
People have castrated animals to produce docile beasts of burden, or especially tame pets or mild-tasting meat. The Torah, according to traditional interpretation, forbids Jews from doing so. And it would seem, in the absence of other indications, that the Torah prohibits us from castrating our pets to avoid contributing to the population of unwanted dogs and cats.
The Talmud records that when the students asked Ben Zoma, "What about castrating a dog?" He replied in the negative: "Anything 'in your land you shall not do'" (Talmud Hagigah 14b). Thus the Torah comes into direct conflict with a contemporary civic virtue. Experts advise us to neuter our cats and dogs. We should do so, they say, as responsible pet owners.
Jewish thinkers have speculated about the goals of this commandment against castration.
The anonymous author of the Book of Education in 13th-century Spain, offers this analysis: The will of the Creator "blessed all living beings, to be fruitful and multiply, and even commanded the male of the species of the human on this, that it might endure.
"For otherwise, a species would die out after death ended all of its kind. Therefore, if someone incapacitates the organs of generation, he shows himself to be as one who cannot tolerate the work of the Creator and desires the destruction of his good world" (Sefer HaHinnukh, No. 291).
The Spanish rabbi shows himself a sort of early environmentalist. Bringing about extinctions impoverishes our rich, diverse, various, complex world, a world with many species, which the Creator called "good" and "very good" (Genesis 1:25, 31). Neutering one dog does not make dogs extinct, but it does frustrate that dog's ability to procreate — the Darwinian imperative that the Torah calls divine blessing and that protects species against extinction.
Certainly no one wishes to see packs of starving feral dogs roaming our cities, or desperate bony cats massacring our remaining song birds.
Perhaps, indeed, we should see it as an act of kindness to choose a specific dog or cat and give it a comfortable life, with plenty of nutritious food, with expensive medical care, with heated shelter, with our love, but without the opportunity to father offspring. Perhaps, indeed, if the dog or cat had a choice in this matter, it would choose a neutered life of luxury.