Smith's Bible Dictionary (1863)
Under the law, in memory of the exodus (when the first-born of the Egyptians were slain), the eldest son was regarded as devoted to God, and was in very case to be redeemed by an offering not exceeding five shekels, within one month from birth. If he died before the expiration of thirty days, the Jewish doctors held the father excused, but liable to the payment if he outlived that time. (Exodus 13:12-15,16; Leviticus 27:6) The eldest son received a double portion of the father’s inheritance, (21:17) but not of the mother’s.
Under the monarchy the eldest son usually, but no always, as appears in the case of Solomon, succeeded his father in the kingdom. (1 Kings 1:30; 2:22) The male first-born of animals was also devoted to God. (Exodus 13:2,12,13; 22:29; 34:19,20) Unclean animals were to be redeemed with the addition of one-fifth of the value, or else put to death; or, if not redeemed, to be sold, and the price given to the priests.