At just five inches long, tarsiers are among the world's smallest primates. And yet they communicate much like humans, using a wide range of calls to communicate. That's why it's so weird that some tarsiers apparently stay completely silent.
If you've ever seen a common domestic mouse, your first thought probably wasn't, "You know, I bet that guy has an amazing singing voice." But it turns out male house mice are rodent crooners, singing ultrasonic love songs to woo females.
China's concave-eared frogs are one of two amphibians that use ultrasonic frequencies to communicate. There's only one small problem: the males and the females have evolved along such vastly separate lines that females are completely deaf to the males' ultrasonic cries.