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.