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Melon (cetacean)
The melon is a mass of adipose tissue found in the forehead of all toothed whales. It focuses and modulates the animal's vocalizations and acts as a sound lens. It is thus a key organ involved in communication and echolocation.
==Description==
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 that acoustic energy can flow out of the head and into the environment with the least loss of energy. In the past, 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.
The varying composition of the melon creates a sound velocity gradient that refracts sound directionally. Sounds also bounce off the skull and air sacs that surround the melon.〔(''Marine Mammal Biology: An Evolutionary Approach'' ). pg 153〕
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 in 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 the melon (homologous to it). The melon is ''not'' homologous to the spermaceti organ.〔〔Cranford, T. W., et al. (2008). "Anatomic Geometry of Sound Transmission and Reception in Cuvier's Beaked Whale (Ziphius cavirostris)." Anat Rec 291(4): 353-378.〕
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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