Jan 31, 2013

Mammals that lay eggs and suckle their young

The duck-billed platypus (Ornithorhynchus anatinus) and the echidna or spiny anteater (family Tachyglossidae), indigenous to Australia, Tasmania, and New Guinea, are the only two species of mammals that lay eggs (a non-mammalian feature) but suckle their young (a mammalian feature). These mammals (order Monotremata) resemble reptiles in that they lay rubbery shell-covered eggs that are incubated and hatched outside the mother's body. In addition they resemble reptiles in their digestive, reproductive, and excretory systems, and in a number of anatomical details (eye structure, presence of certain skull bones, pectoral [shoulder] girdle, and rib and vertebral structures). However they are classed as mammals because they have fur and a four-chambered heart, nurse their young from gland milk, are warm-blooded, and have some mammalian skeletal features. (The Handy Science Answer Book, compiled by the Science and Technology department of the Carnegie Library of Pittsburgh)