In a darkened forest of central Colombia, a 2-foot-long mammal climbed a tree and paused to rest its “completely spiny” body.

Well, there's actually an important trade off for this nuisance. Only a handful of mammals can regrow teeth multiple times, compared to the 50,000 species of reptiles and fish. Take geckos ...

Echidnas are silent, hot water bottle sized mammals that don’t seem to have gotten the memo about doing all the things mammals are supposed to. They lay eggs like a reptile. They sense ...