What characteristic of monotremes sets them apart from other mammals?

Monotremes

Monotremes are one of the three forms of mammals. All mammals suckle their young, have three middle-ear bones, and have hair.

Monotremes are the only mammals that lay eggs. They also have extra bones in the shoulder girdle. Their legs have spurs at the sides. These have no function excepting for in male platypuses; in male platypuses they contain venom. Monotremes have no nipples and no corpus callosum. Their body temperature is only about 90 degrees Fahrenheit (as opposed to 95 in marsupials and 99 in placental mammals). They are toothless as adults. They are the only mammals that can electrolocate and the only mammals with full cloacas.

An example of a monotreme is the duck-billed platypus.

Written by a placental mammal...