The Mekhilta addresses a practical problem. First-born animals that are consecrated cannot be nursed by their consecrated mothers, because the mother's milk has sacred status. But newborn animals need to nurse. How is this resolved?
The solution is funded by Temple money. Funds are taken from the Temple treasury, and a non-consecrated animal is purchased — an ordinary, unconsecrated mother who can nurse the first-born lambs. This surrogate mother "takes pity on" the young animals and nurses them, even though they are not her biological offspring.
The Mekhilta notes that some donors contribute animals with exactly this purpose in mind — providing wet-nurses for consecrated first-borns. The system thus combines institutional funding (Temple treasury) with private generosity (individual donors) to solve a problem that the sacred status of the animals creates.
This passage reveals the remarkable practicality of the Temple economy. Sacred law created a situation where newborn animals could not access their own mothers' milk. Rather than allowing the animals to starve or creating an exception to the rules of consecration, the rabbis devised a commercial solution. They bought a non-sacred animal to do the nursing. The holiness of the law was preserved, the welfare of the animals was maintained, and the cost was borne by the institution that the law served. Sanctity and practicality worked together, not against each other.