Only two kinds of egg-laying mammals are left on the planet today—the duck-billed platypus and the echidna, or spiny anteater. These odd “monotremes” once dominated Australia, until their pouch-bearing cousins, the marsupials, invaded the land down under 71 million to 54 million years ago and swept them away.
Turtles don’t have vocal cords and their ears are internal, so scientists believed that turtles were deaf and didn’t communicate through sounds. However, research has found that turtles actually communicate at an extremely low frequency that sounds like “clicks, clucks, and hoots” that can only be heard through a hydrophone (a microphone used underwater). These sounds even come from the egg before the turtle hatches. Researchers hypothesize that this helps all the turtle siblings hatch at once.