Echidnas, also called spiny anteaters, are some of the weirdest mammals in the world. They're among the only mammals that don't give birth to live babies, and they also carry their young in a pouch.

Q. When we studied high school biology, our teacher said more than 99 percent of animals lay eggs and only a few give live birth. That percentage seems too high. Humans, dogs, cats and cows are ...