They are the only mammals still in existence which lay eggs, rather than bearing live young The five extant monotreme species are the platypus and the four species of echidnas Monotremes are typified by structural differences in their brains, jaws, digestive tracts, reproductive tracts, and other body parts, compared to. After fertilization, eggs develop within the reproductive tract before being laid Monotreme eggs are small (about 1.5 cm in diameter), leathery, and contain yolk for nourishment We learn early on in school that what one of the distinguishing characteristics of mammals is that unlike birds, reptiles, and fish they bear live young
There are exceptions to every rule, some reptiles and fish bear live young, and some mammals lay eggs The only mammals that lay eggs are the platypus and the echidna, belonging to a group called monotremes The only mammals that lay eggs belong to the order monotremata, named for their single opening for waste and reproduction There are just five extant species, all endemic to australia and new guinea, divided into two distinct families The platypus and the echidnas. Out of over 6,000 mammalians, only five species actually lay eggs instead of giving birth to live young
