Over a twenty year period, entire songs are slowly transformed. In contrast, signature whistles of dolphins are stable over periods of many years. Tyack compares this change to the sexual appeal of a bird with a large repertoire of songs. Sound travels through water a lot differently than it does through air. Because water is relatively more dense, sound travels through it very easily.
Velocity also increases with increasing salinity and temperature.
Because of this, an area of low-velocity sound transmission exists at the base of the thermocline. Marine mammals may use this phenomena to a much greater extent than we presently realize as a means of long-distance communication. Other animals in the ocean also take advantage of this phenomena to transmit sound. The way that we may identify the direction of sound in air is completely lost when we enter the ocean. Instead, we hear sounds within our own heads.