The melon is structurally part of the nasal apparatus (the nose) and comprises most of the mass tissue between the blowhole and the tip of the snout. The function of the melon is not completely understood, but scientists believe it is a bioacoustic component, providing a means of focusing sounds used in echolocation and impedance matching. Impedance matching refers to the melon's function in creating a similarity between characteristics of its tissue and the surrounding water, so acoustic energy can flow out of the head and into the environment with the least loss of energy. Historically, some scientists believed that the melon had functions in deep diving and buoyancy, but these ideas have been discounted over the last 40 years, and are no longer considered valid by cetologists.
Melon size is unrelated to maximum dive depth in toothed whales. The particular characteristics of the melon probably have more to do with odontocete phylogeny, the taxonomic relationships over evolutionary time. In some species, melons are more specialized than others. The sperm whale has the largest nose of any animal in the world. The bulk of that nose is composed of two large, fatty structures, the spermaceti organ and the "junk". The junk is structurally the same as (homologous to) the melon. The melon is not homologous to the spermaceti organ.
The lipids (fats, oils, and waxes) within the foreheads of odontocetes are chemically distinct. They have the unusual characteristics that they transmit sounds at different speeds depending upon the chemical composition, and they are toxic to metabolism. It appears as if these specialized lipid structures evolved as components of the bioacoustical machine in toothed whales.
Even though the spermaceti organ of the sperm whale is not the same as the melon, it is noteworthy because the waxy substance within it was prized by the whalers of the 19th century, because it did not get rancid and burned brightly in their lamps and candles.
The melon of the beluga whale is also unique, in that it is capable of large deformations. These changes in shape probably have the effect of changing the size, shape, direction, and frequency composition of the echolocation beam.
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