The Torah states: "in man and beast, he is Mine" (Exodus 13:2), declaring God's ownership of every first-born. The Mekhilta draws from this verse a principle of elegant symmetry: whatever rule applies to the man also applies to his beast. If a person is subject to the law of the first-born (bechor), then his animals are too. If the person is exempt, his animals share that exemption.
This principle leads directly to a surprising conclusion about the Levites. The tribe of Levi holds a unique status in Israel. Because the Levites were consecrated to God's service in place of all the first-born of Israel — a substitution that occurred after the sin of the Golden Calf — the Levites themselves are exempt from the first-born redemption laws. A first-born Levite does not need to be redeemed through the priests the way a first-born of any other tribe does.
The Mekhilta extends this exemption to the Levites' animals as well. Since the law of bechor does not apply to the Levite himself, it does not apply to his donkey, his cow, or his sheep either. The first-born of a Levite's flock carries no special sanctity requiring redemption.
The verse "in man and beast" thus functions as a linking mechanism. It binds the status of owner and property together, ensuring that the legal reality of one mirrors the legal reality of the other. The Levites' special standing before God radiates outward from their persons to encompass everything they own.