One of the world's smallest primates, the Philippine tarsier, can communicate in ultrasound, a range of high-pitched noise inaudible to humans, according a new study published in the British Royal Society journal Biology Letters. The study, Primate communication in the pure ultrasound, notes that the tarsier is the only primate known to communicate in this way.

Any sound above 20 kilohertz (20,000 hertz) is considered ultrasound, and is above the range of human hearing, due to limitations of the middle ear, which acts as a low-pass filter, cutting off sound above that range. Many animals have the ability to not only hear ultrasound, but produce it as well. Click through the slideshow to see what some of those animals are.

Smallest Primate
The Philippine tarsier is one of the world's smallest primates and mammals, growing only up to about 4.5 inches, no bigger than a man's hand. The tiny, furry creature with wide, saucer eyes is related to monkeys and lemurs, and the is a nocturnal animal that mostly feeds on insects. It is also one of the handful of mammals that can send and receive vocal signals in the ultrasound range. Wikipedia
Bats
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Animal rights activists from Peta back proposal for animal rights to be recognised
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Tiger Moth
The tiger moth produces ultrasound not to communicate, but as a defense mechanism. By producing ultrasound of its own, it is able to ward off bats by acting as a radar jammer, confusing the bats so that the moths cannot be located. By doing this, they add another weapon to their survival repertoire that already includes poison. Wikipedia