Researchers studying the Philippine tarsier, a tiny primate (4 or 5 inches high) with humongous orange-ish eyes, noticed the animal opening its mouth as if to make a call, but they could hear nothing. Were the animals yawning? Apparently not. Using ultrasound detectors, the scientists discovered that the tarsiers were busy making high-pitched calls. At about 70 kilohertz, the calls are well beyond the range of human hearing. Whales, dolphins, domestic cats, rats and mice, and some bats also communicate with very high frequency sounds.
In another bit of fascinating research, ecologists have made the first recording of what they believe are the sounds of deep-sea fish. Although scientists have suspected that deep-sea fish are capable of making sounds—they have the necessary anatomical structures—until now, no one has been able to "listen" to these creatures in their natural habitat. Rodney Rountree and colleagues, with the help of some deep sea fishermen, developed a special underwater microphone, with which they recorded 24 hours of deep-water sounds from the seafloor (about 2237 feet below the surface) of Welkers Canyon, off the New England coast. They were able to identify sounds of fin whales and other cetaceans, but there were also at least 12 other unique low-frequency sounds. They describe these are "drumming," "duck-like," and "unknown." You can listen to the sounds here (go to the bottom on the webpage). Cool stuff.