Egg-laying mammals are known as monotremes, and they include the platypus and the echidna. Their characteristics are:
- unlike placental mammals and marsupials, they do not give live birth, but lay eggs in order to reproduce
- monotremes nurture their young with mothers' milk as all mammals do
- they have four limbs
- they are warm-blooded vertebrates
- they have a four-chambered heart
