One difference between monotremes and other types of mammals is

One difference between monotremes and other types of mammals is that monotremes secrete milk through pores on the surface of the mother's abdomen.

Explanation:

Monotremes are three different types of mammals. They are calsified by structural differences in their jaws, reproductive tract, brains, digestive tract, and some other body parts compared to the more common mammalian types. Monotremes secrete milk through pores on the surface of the mother's abdomen, they actually lactate from their mammary glands via openings in their skin.