Production and control of sound by the small sperm whales, Kogia breviceps and K. sima and their implications for other Cetacea

Abstract

The detailed anatomy of the noses of Kogia breviceps and K. sima (Kogiidae: Cetacea) is described in greater detail than previously. Probable functions of component parts of the nose are deduced from their geometry and physical properties of the tissues. It is concluded that control of the air making the sound is similar to that found in scottish bagpipes with a single reed, that sounds are produced by air being forced from the vocal valve and is resonated in an unusual chamber having cords and membranes, similar to human vocal cords. It is then reflected by air filled ‘mirrors’ and conducted and intensified through a megaphone-like ‘horn’ which is surrounded by air and contains tissues rich in spermaceti oil. Sound then passes through the melon where, by changes in temperature of the spermaceti oil, it is actively focused and possibly scanned. Thermal properties of Kogia spermaceti oil are described and related to previously made sound velocity and density measurements. The way sound production and reflection can continue while large variations of pressure are experienced during deep dives, is suggested. A structure for controlling the temperature of the melon is described. It is considered unlikely that the spermaceti oil is used for buoyancy control in Kogia. Some comparisons are drawn with Physeter catodon, Xiphius and Tursiops and some implications for research on other cetaceans are outlined.