Herod also got together a great quantity of wild beasts, and of lions in very great abundance, and of such other beasts as were either of uncommon strength or of such a sort as were rarely seen. These were trained either to fight one with another, or men who were condemned to death were to fight with them. And truly foreigners were greatly surprised and delighted at the vast expenses of the shows, and at the great danger of the spectacles, but to the Jews it was a palpable breaking up of those customs for which they had so great a veneration. -Josephus, Antiquities of the Jews.
A righteous man knows the soul of his animal - Proverbs 12:10
Judaism places great stress on proper treatment of animals. Unnecessary cruelty to animals is strictly forbidden, and in many cases, animals are accorded the same sensitivity as human beings. This concern for the welfare of animals is unusual in Western civilization. Most civilized nations did not accept this principle until quite recently; cruelty to animals was not outlawed until the 1800s, and even now it is not taken very seriously.
The primary principle behind the treatment of animals in Jewish law is preventing tza'ar ba'alei chayim, the suffering of living creatures. Judaism expresses no definitive opinion as to whether animals actually experience physical or psychological pain in the same way that humans do; however, Judaism has always recognized the link between the way a person treats animals and the way a person treats human beings. A person who is cruel to a defenseless animal will undoubtedly be cruel to defenseless people. Modern psychology confirms this understanding, with many studies finding a relationship between childhood animal cruelty and adult criminal violence. Sadly, the converse is not always true, and those who love animals do not always value human life: Hitler loved animals; the animal rights group PETA wrote a letter to Arafat telling him, when he blows up a bus full of Israelis, could he please not hurt any donkeys. (the letter is no longer on their website, but remains in the Internet Archive)
In the Bible, those who care for animals are heroes, while those who hunt animals are villains. Jacob, Moses and King David were all shepherds, people who cared for animals (Gen. 30, Ex. 31, I Sam. 17). A traditional story tells that Moses was chosen for his mission because of his skill in caring for animals. "The Holy One, Blessed Be He, said 'Since you are merciful to the flock of a human being, you shall be the shepherd of My flock, Israel.'" Likewise Rebecca was chosen as a wife for Isaac because of her kindness to animals. When Abraham's servant asked for water for himself, she volunteered to water his camels as well, and thereby proved herself a worthy wife (Gen. 24).
On the other hand, the two hunters in the Bible, Nimrod and Esau, are both depicted as villains. The Talmud tells the story of a great rabbi, Judah Ha-Nasi, who was punished with years of kidney stones and other painful ailments because he was insensitive to the fear of a calf being led to slaughter; he was relieved years later when he showed kindness to animals. (Talmud Baba Metzia 85a)
In the Torah, humanity is given dominion over animals (Gen. 1:26), which gives us the right to use animals for legitimate needs. Animal flesh can be consumed for food; animal skins can be used for clothing. The Torah itself must be written on parchment (animal hides), as must the scrolls for mezuzot and tefillin, and tefillin must be made out of leather.
However, dominion does not give us the right to cause indiscriminate pain and destruction. We are permitted to use animals in this way only when there is a genuine, legitimate need, and we must do so in the manner that causes the animal the least suffering. Kosher slaughtering is designed to be as fast and painless as possible, and if anything occurs that might cause pain (such as a nick in the slaughtering knife or a delay in the cutting), the flesh may not be consumed. Hunting for sport is strictly prohibited, and hunting and trapping for legitimate needs is permissible only when it is done in the least painful way possible.
Under Jewish law, animals have some of the same rights as humans do. Animals rest on Shabbat, as humans do (Ex. 20:10). We are forbidden to muzzle an ox to prevent it from eating while it is working in the field (Deut. 25:4), just as we must allow human workers to eat from the produce they are harvesting (Deut. 23:25-26). Animals can partake of the produce from fields lying fallow during the sabbatical year (Ex. 23:11).
Several commandments demonstrate concern for the physical or psychological suffering of animals. We may not plow a field using animals of different species (Deut. 22:10), because this would be a hardship to the animals. We are required to relieve an animal of its burden, even if we do not like its owner, do not know its owner, or even if it is ownerless (Ex. 23:5; Deut. 22:4). We are not permitted to kill an animal in the same day as its young (Lev. 22:28), and are specifically commanded to send away a mother bird when taking the eggs (Deut 22:6-7), because of the psychological distress this would cause the animal. In fact, the Torah specifically says that a person who sends away the mother bird will be rewarded with long life, precisely the same reward that is given for honoring mother and father (Ex. 20:12; Deut. 5:16), and indeed for observing the whole Torah (Deut. 4:40). This should give some indication of the importance of this law.
We are permitted to violate Shabbat to a limited extent to rescue an animal in pain or at risk of death. For example, we can move them if they are in pain, move objects that we would not otherwise be permitted to touch to relieve their pain, we may give them medicine, and we may ask non-Jews to do things that would violate Shabbat to help a suffering animal.
In the Talmud, the rabbis further dictated that a person may not purchase an animal unless he has made provisions to feed it, and a person must feed his animals before he feeds himself (interpreting Deut. 11:15).
Jewish law does not prohibit keeping pets, and indeed many observant Jews have dogs, cats or other household pets, though Jewish law does raise some complications for pet owners.
As with all animals, we are required to feed our pets before ourselves, and make arrangements for feeding our pets before we obtain them. Also, like all animals, household pets are entitled to Sabbath rest, thus you cannot have your dog retrieve the paper for you on Shabbat, etc.
Some sources consider pets are considered to be muktzeh, within the category of objects that cannot be handled on Shabbat. I haven't been able to get a clear idea of what exactly is and is not permitted with an animal on Shabbat. I have seen several sources say that walking a dog is permitted, but if an animal runs away on Shabbat, it is not permitted to trap the animal.
It is permissible to feed non-kosher food to pets, as long as you do not consume it yourself. This falls under the general rule that it is permissible to use products of non-kosher animals as long as you don't eat them; for example, it is permissible to use a toothpaste that contains non-kosher ingredients as long as the toothpaste is not fit for human consumption, and it is permissible to wear gloves made from pig suede. However, it is not permissible to derive any benefit from a mixture of meat and dairy; therefore, any food you feed your pet cannot contain both meat and dairy.
Similarly, during Pesach, there are rules for pets, but they are not as strict as for people. It is impermissible to have any chametz (leavened grain products) in your home, or to derive any benefit from chametz, thus you cannot use chametz to feed your pets. However, you can feed your pets food that contains kitniyot. Star-K's website usually has a good list of KFP pet foods when the time comes. You can also feed your pets Passover table scraps, and you can feed matzah meal or farfel to fish or rodents. I used to have a hamster who loved Passover: carrot and potato peelings, celery tips, and all the matzah farfel he could eat! If you cannot find suitable food, you must temporarily sell the pets to a non-Jew, as you temporarily sell your other chametz to a non-Jew during the holiday.
It is a violation of Jewish law to neuter a pet. The Torah prohibits castrating males of any species (Lev. 22:24). Although this law does not apply to neutering female pets, neutering of females is prohibited by general laws against tza'ar ba'alei chayim (causing suffering to animals). Please note that, while the law prohibits you from neutering your pet, it does not prohibit you from owning a pet that is already neutered. If you want a neutered pet, I strongly encourage you to adopt from one of the many reputable shelters, such as Spay and Save (where I adopted a cat), Kitty Cottage (where I adopted two others) or the Delaware Humane Association. I also heard an amusing story about an Orthodox Jewish woman who gave her unaltered female cat birth control pills, but I don't know how much truth there is to that story. It certainly would not be a violation of Jewish law to do so.
It is a violation of the general prohibition against tza'ar ba'alei chayim to have your pet physically altered in any way without a genuine, legitimate need. For example, declawing cats and docking the ears or tails of dogs are forbidden. Again, there is no law against owning an animal in this condition, so you should look into adopting from a shelter if you want such an animal. The cat that I adopted from Spay and Save was neutered and declawed by her previous owner.
The vegetarian diet was considered the ideal for humanity. Note that in Genesis 1:29, G-d gives humanity all fruits and vegetables for food, but no meat. Meat is not permitted until after the Flood (Gen. 9:3). Even offerings before that time did not involve the death of animals: a traditional midrash indicates that Abel and Cain's offerings (Gen. 4) were wool and flax.
Certainly, a vegetarian diet simplifies the process of keeping kosher, as it eliminates the need to separate meat and dairy! (see Kashrut) In fact, I once heard a joke about a vegan who decided to keep kosher, but he thought it was too easy for him, so he designated certain vegetables "meat" and certain vegetables "dairy," and certain vegetables "pareve," and he wouldn't eat "meat" vegetables with "dairy" vegetables!
Most vegetarian foods are kosher; in fact, many vegetarians who do not keep kosher rely on kosher certifications to make sure that the foods they buy are vegetarian! Beans, grains, fruits, vegetables, and dairy products can all be eaten and in any combination, and do not require kosher certification if they are not processed. If they are processed, you should seek reliable kosher certification to make sure that they weren't prepared on the same equipment as non-kosher food and that they don't have any hidden non-kosher ingredients.
It is traditional to eat meat to celebrate on Shabbat and festivals, but it is not a requirement. There is no holiday or observance for which it is a mitzvah (commandment) to eat meat, and most symbolic foods eaten for holidays are not meat. In fact, on Shavu'ot, it is traditional to eat dairy meals (which could not be eaten with meat). Meat is a traditional part of Shabbat and festival meals to make them more festive, but as long as you eat something special, something out of the ordinary, that should be sufficient to create the necessary festive atmosphere.
The one area that may cause concern for vegetarians is the use of animal parts for ritual purposes. The Torah is written on parchment (animal skins), as are the scrolls in a mezuzah and the tefillin. The tefillin are made of leather. The shofar blown at Rosh Hashanah is a ram's horn. Jewish law requires all of this. What is a religiously observant vegetarian to do? The Jewish vegetarian website JewishVeg points out that only a very small number of animals are used for this purpose, and also notes that Jewish law allows us to make these ritual items using animals that died of natural causes (though no one is currently advertising that they are doing so).