Animal sacrifice dates back to ancient times. In the Bible, Adam's sons offer their flocks to God, whilst ancient Greeks and Romans sacrificed beasts to their own deities. To please these gods, it was believed that the animal offered should be the best of its kind; beasts were cleaned, robed in ceremonial garments, and the horns of an ox might be gilded before its death. Each god had his or her own requirements. The gods of the upper heavens required white, infertile animals of their own sex; for Jupiter a white ox, for Juno a heifer. Lower gods were offered fertile victims, whilst the gods of the underworld were given dark-coloured, sometimes pregnant animals, and those passing between the two worlds might receive a piebald or spotted sacrifice. It was believed that these offerings would reinforce the gods' divine power and incline them to use it on the giver's behalf.
In Christianity, the sacrifice of animals is thought to foreshadow the ultimate sacrifice of Christ -- 'the lamb of God'.
In traditional shamanism, animal sacrifice played a similar role to that in ancient Greece in appeasing the spirits and winning benefits to humans. Shamans played an important role in guiding the spirits of sacrificed animals through the upper world to the lord of the heavens. In Siberia, horses were often offered in sacrifice, their skins stretched out on poles to help guide them to the upper world. Sacrifice may have a very specific purpose - perhaps to quench the hunger of malicious spirits who bite human victims, causing illness - or be offered as a general invitation to the spirits to interact more closely with the shaman and his or her people. In modern shamanism, as in other religions, true sacrifice is discouraged although some contemporary teachers permit the letting of one's own blood.