New Defender's Study Bible Notes
13:13 firstling of an ass. As an unclean animal, the ass was not suitable as a sacrifice, but was a valuable animal for service. Since every firstborn animal was henceforth to be sacrificed, the firstborn of an ass must either be slain and discarded, or else redeemed by a sacrificial lamb. The firstborn son of Israelite parents also had to be redeemed by a lamb.