Melon (cetacean)
The melon is a mass of adipose tissue found in the foreheads of all toothed whales.<ref name="cranford1996">Template:Cite journal</ref><ref name="harper2008">Template:Cite journal</ref> 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 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 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. It was once hypothesized that the melon had functions in deep diving and buoyancy, but these ideas are no longer considered plausible by cetologists.<ref name="cranford1999">Template:Cite journal</ref>
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.<ref>Marine Mammal Biology: An Evolutionary Approach. pg 153</ref>
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 Template:Em homologous to the spermaceti organ.<ref name="cranford1996" /><ref name="cranford2008">Template:Cite journal</ref> Research on the expression of genes such as that for the protein MYH16 show it originates from the masseter muscle.<ref name="Takeuchi Matsuishi Hayakawa 2024 p. 148167">Template:Cite journal</ref>
Composition
The melon is a mixture of triglycerides and wax esters. The exact composition varies throughout the melon. Typically, the inner core of the melon has a higher wax content than the outer parts and conducts sound more slowly. This gradient refracts sound and focuses it like a lens.
The lipids in the melon cannot be digested by the animal, as they are metabolically toxic. A starving dolphin has a robust melon even if the rest of its body is emaciated.<ref name="cranford1996" /> The lipids in the melon tend to be of lower molecular weight and more saturated than the blubber.
The melons of the Delphinidae (dolphins) and Physeteroidea (sperm whales) have a significant amount of wax ester, whereas those of the Phocoenidae (porpoises) and Monodontidae (narwhals and beluga whales) contain little or no wax.<ref name=Litchfieldetal1971>Template:Cite journal</ref> The speed of sound in the melon is lowest in the Delphinidae, Phocoenidae, and Monodontidae, intermediate in the Ziphiidae (beaked whales), and highest in the Physeteridae and Platanistidae (South Asian river dolphins).<ref name=Litchfieldetal1975>Template:Cite journal</ref>
Pilot whales
The melon of pilot whales (Globicephala) is a mixture of wax esters and triglycerides. The inner core of the melon is about 33% wax esters, while the outer layer is about 5% wax esters. Most of the fats are saturated.<ref name=Wedmidetal1973>Template:Cite journal</ref>
Pygmy sperm whale
In the pygmy sperm whale (Kogia breviceps), the melon consists of an outer layer and an inner core, which has a generally larger proportion of wax esters than the outer layer.<ref name=Karoletal978/>
Behind the melon is a cornucopia-shaped organ that many scientists refer to as the "spermaceti organ". This organ is different in form and composition from the spermaceti organ of the sperm whale.
| Outer melon | Inner melon | Spermaceti organ | ||
|---|---|---|---|---|
| Lipid content (weight) | 15–91% | 74–94% | 92–96% | |
| Lipid composition |
Wax esters | 8–46% | 40–90% | 84–99% |
| Triglycerides | 54–92% | 10–69% | 1–16% | |
| Average number of carbons |
Wax esters | 32–35 | 29–32 | 28–29 |
| Triglycerides | 47–51 | 41–46 | 45 | |
Sperm whale
Sperm whales have two large oil-filled sacs, stacked one on top of the other: the dorsal spermaceti organ or spermaceti case and the junk,<ref name=":0">Template:Cite journal</ref> named because whalers dismissed it as worthless for extracting sperm oil. The junk originated from the odontocete melon.<ref>Template:Citation</ref> It contains compartments of waxy oils separated by walls of connective tissue.<ref name=":0" /> Together, the spermaceti organ and the junk add directionality and amplitude to biosonar clicks.<ref>Template:Cite journal</ref><ref>Template:Cite journal</ref>
Beluga whale
The melon of the beluga whale is unique in that the whale can change the melon's shape at will.<ref>Template:Cite webTemplate:Cbignore</ref> These changes in shape probably change the size, shape, direction, and frequency composition of the echolocation beam.