Platypus

 Along stretches of its native rivers in eastern Australia, the female platypus digs a burrow near a stream and fills it with soft leaves as a place to lay eggs.
Jean-Phillipe Varin/Photo Researchers

The unique nature of the curiously constructed platypus starts even before birth and marches on from there. 

The platypus is a monotreme--a group where the females produce offspring by laying eggs. Giving birth this way is extremely unusual among living mammals--but normal for most other animals. Almost every other vertebrate, including most reptiles, amphibians, fish, and birds, reproduces by laying eggs.
 
Along stretches of its native rivers in eastern Australia, the female platypus digs a burrow near a stream and fills it with soft leaves as a place to lay eggs. Platypus babies cut their way out of the egg using a sharp "egg tooth" --a horny spike on the nose that is made of keratin, the same material as fingernails, that later falls off.

Fun Facts

• A female platypus usually lays only two eggs at a time and rarely leaves her stream-side den while nursing her young. When she does leave, she plugs the den opening with dirt.
• The platypus is one of just a handful of mammals that lay eggs. Another monotreme? Echidnas--commonly referred to as spiny anteaters.
• A platypus's bill can sense tiny electric currents produced by the bodies of small animals, helping it hunt in muddy water.

Like all mammals, monotreme mothers produce milk for their young. But unlike all other mammals, monotremes like the platypus have no nipples. Their milk oozes out of mammary gland ducts and collects in grooves on their skin--where the nursing babies lap it up or suck it from tufts of fur.