DUCK-BILLED PLATYPUS (MONOTREME)
Among all the monotremes, or egg-laying mammals, the duck-billed platypus (subclass Holotheria, genus Ornithorhynchus) is perhaps the most familiar species. Like all mammals, the duck-billed platypus is endothermic, meaning it generates its own internal body heat. However, its metabolic rate and body temperature are lower than those of most other mammals.

The duck-billed platypus is also one of the rare venomous mammals. A spike on its ankle contains a poison that is injected when the meat-eating platypus attacks a small animal.

Functioning similar to birds, monotremes incubate their one to three eggs outside the body of the mother. Another similarity to birds is that the platypus does not possess teeth.

Incubation lasts about 12 days, after which the monotreme young use a "milk tooth" to carve themselves free from within an egg.

Milk produced by a mother's mammary gland is secreted onto the skin within the pouch and sucked or lapped up by the babies. Weaning of the young happens at 16 to 20 weeks of age.