Underwater, whales produce loud noises: known as songs. Scientists use whale songs to study the movements for migrations, of groups of whales. Recently, scientists discovered something unusual: a single, solitary whale whose song is unlike that of all other known whales. The most notable difference between this unusual whale's song and those of other whales is its high pitch or frequency This unique whale is called the 52-hertz whale because it sings at the unusual frequency of 52 hertz, a much higher pitch than normal. When the 52-hertz whale was first detected, the cause of its uniquely high-pitched song was unknown; however, scientists now have several theories to explain it. One theory holds that the 52-hertz whale may be a hybrid: the offspring of two different whale species Whales of different species are known to interbreed and produce hybrid offspring that combine characteristics from each of their parents' species. As a hybrid, the whale may have a unique song, different from that of either of its parents because it resulted from a combination of the two. A second theory is that the 52-hertz whale may have a damaged sense of hearing Just as people learn to speak by copying the sounds they hear, whales may learn to sing by listening to the sounds of other whales* songs. When people are born deaf, their speech may sound different from that of people born hearing. Similarly, the 52-hertz whale’s songs may sound different simply because it cannot hear the songs of other whales. A third theory holds that the 52-hertz whale may be the only known member of a rare species Perhaps there were once many more whales of this species, but most are now gone. It seems to be entirely unique only because most of its species has died out.