Low‐frequency sounds and amphibious communication in Hippopotamus amphibious

Hippos make sounds in both air and underwater, and, with their heads in an amphibious position (eyes and nostrils above water but mouth and throat below), are able to transmit sounds to both media simultaneously. Hippos on the surface respond to the surface component by calling. Hippos underwater consistently surface and call in a chorus that can spread in air from one territory to the next for many kilometers. They produce several low‐frequency, high‐amplitude (100 dB re: 20 μPa) sounds. The grunt, their most common call, has a 30‐ to 60‐Hz fundamental, and the huff and some tonal sounds end with an abrupt drop in frequency to 20‐ to 30‐Hz. These sounds are usually given amphibiously, but the high‐pass filter characteristics of shallow water attenuates the low frequencies of the underwater component. Hippos also emit these and other sounds when they are completely submerged. These are inaudible in air, but they produce a fountain on the surface accompanied by a 10‐ to 20‐Hz sound. They also produce this effect with plosive blows underwater without other sounds. Similar ‘‘bubble blasts’’ have been reported in gray whales. The function of these sounds is not clear, but they may facilitate long‐distance ‘‘chain chorusing.’’