Melon (cetacean)

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The bisected head of a dolphin: The melon is just above the upper jaw. Dolphin head bisected.jpg
The bisected head of a dolphin: The melon is just above the upper jaw.
Three-dimensional models of various odontocete melons based on CT scans Cetacean melons.jpg
Three-dimensional models of various odontocete melons based on CT scans

The melon is a mass of adipose tissue found in the foreheads of all toothed whales. [1] [2] 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.

Contents

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. [3]

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. [4]

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. [1] [5] Research on the expression of genes such as that for the protein MYH16 show it originates from the masseter muscle. [6]

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. [1] 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. [7] 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). [8]

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. [9]

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. [10]

Kogia breviceps sagittal + coronal.svg

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.

Melon composition in K. breviceps [10]
Outer melonInner melonSpermaceti organ
Lipid content (weight)15–91%74–94%92–96%
Lipid
composition
Wax esters8–46%40–90%84–99%
Triglycerides54–92%10–69%1–16%
Average number
of carbons
Wax esters32–3529–3228–29
Triglycerides47–5141–4645

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 [11] , named because whalers dismissed it as worthless for extracting sperm oil. The junk originated from the odontocete melon. [12] It contains compartments of waxy oils separated by walls of connective tissue. [11] Together, the spermaceti organ and the junk add directionality and amplitude to biosonar clicks. [13] [14]

Beluga whale

The melon of the beluga whale is unique in that the whale can change the melon's shape at will. [15] These changes in shape probably change the size, shape, direction, and frequency composition of the echolocation beam.

References

  1. 1 2 3 Cranford, Ted W.; Amundin, Mats; Norris, Kenneth S. (June 1996). "Functional morphology and homology in the odontocete nasal complex: Implications for sound generation". Journal of Morphology. 228 (3): 223–285. doi:10.1002/(SICI)1097-4687(199606)228:3<223::AID-JMOR1>3.0.CO;2-3. PMID   8622183. S2CID   35653583.
  2. Harper, C.J.; McLellan, W.A.; Rommel, S.A.; Gay, D.M.; Dillaman, R.M.; Pabst, D.A. (July 2008). "Morphology of the melon and its tendinous connections to the facial muscles in bottlenose dolphins (Tursiops truncatus)" (PDF). Journal of Morphology. 269 (7): 820–839. Bibcode:2008JMorp.269..820H. doi:10.1002/jmor.10628. PMID   18473369. S2CID   206090478.
  3. Cranford, Ted W. (October 1999). "The sperm whale's nose: Sexual selection on a grand scale?". Marine Mammal Science. 15 (4): 1133–1157. doi:10.1111/j.1748-7692.1999.tb00882.x.
  4. Marine Mammal Biology: An Evolutionary Approach. pg 153
  5. Cranford, Ted W.; Mckenna, Megan F.; Soldevilla, Melissa S.; Wiggins, Sean M.; Goldbogen, Jeremy A.; Shadwick, Robert E.; Krysl, Petr; St. Leger, Judy A.; Hildebrand, John A. (April 2008). "Anatomic Geometry of Sound Transmission and Reception in Cuvier's Beaked Whale (Ziphius cavirostris)". The Anatomical Record: Advances in Integrative Anatomy and Evolutionary Biology. 291 (4): 353–378. doi: 10.1002/ar.20652 . PMID   18228579.
  6. Takeuchi, Hayate; Matsuishi, Takashi Fritz; Hayakawa, Takashi (2024). "A tradeoff evolution between acoustic fat bodies and skull muscles in toothed whales". Gene. 901 148167. doi: 10.1016/j.gene.2024.148167 . PMID   38224921.
  7. Litchfield, Carter; Ackman, R. G.; Sipos, J. C.; Eaton, C. A. (September 1971). "Isovaleroyl triglycerides from the blubber and melon oils of the beluga whale (Delphinapterus leucas)". Lipids. 6 (9): 674–681. doi:10.1007/BF02531529. PMID   5141491. S2CID   4023319.
  8. Litchfield, Carter; Greenberg, Anne J.; Caldwell, David K.; Caldwell, Melba C.; Sipos, J. C.; Ackman, R. G. (April 1975). "Comparative lipid patterns in acoustical and non-acoustical fatty tissues of dolphins, porpoises and toothed whales". Comparative Biochemistry and Physiology Part B: Comparative Biochemistry. 50 (4): 591–597. doi:10.1016/0305-0491(75)90095-4. PMID   1122741.
  9. Wedmid, Yuri; Litchfield, Carter; Ackman, R. G.; Sipos, J. C.; Eaton, C. A.; Mitchell, E. D. (December 1973). "Heterogeneity of lipid composition within the cephalic melon tissue of the pilot whale (Globicephala melaena)". Biochimica et Biophysica Acta (BBA) - Lipids and Lipid Metabolism. 326 (3): 439–447. doi:10.1016/0005-2760(73)90144-6. PMID   4776442.
  10. 1 2 Karol, R.; Litchfield, C.; Caldwell, D. K.; Caldwell, M. C. (1978). "Compositional topography of melon and spermaceti organ lipids in the pygmy sperm whale Kogia breviceps: Implications for echolocation". Marine Biology. 47 (2): 115–123. Bibcode:1978MarBi..47..115K. doi:10.1007/BF00395632. S2CID   84443478.
  11. 1 2 Clarke, Malcolm R. (February 1978). "Structure and Proportions of the Spermaceti Organ in the Sperm Whale". Journal of the Marine Biological Association of the United Kingdom. 58 (1): 1–17. Bibcode:1978JMBUK..58....1C. doi:10.1017/S0025315400024371. ISSN   1469-7769.
  12. Heyning, John E.; Mead, James G. (1990), "Evolution of the Nasal Anatomy of Cetaceans", Sensory Abilities of Cetaceans, Boston, MA: Springer US, pp. 67–79, doi:10.1007/978-1-4899-0858-2_5, ISBN   978-1-4899-0860-5 , retrieved 21 August 2025
  13. Møhl, B.; Wahlberg, M.; Madsen, P. T.; Miller, L. A.; Surlykke, A. (1 January 2000). "Sperm whale clicks: Directionality and source level revisited". The Journal of the Acoustical Society of America. 107 (1): 638–648. Bibcode:2000ASAJ..107..638M. doi:10.1121/1.428329. ISSN   0001-4966. PMID   10641672.
  14. Møhl, Bertel (1 June 2001). "Sound transmission in the nose of the sperm whale Physeter catodon. A post mortem study". Journal of Comparative Physiology A. 187 (5): 335–340. doi:10.1007/s003590100205. ISSN   1432-1351. PMID   11529477.
  15. "Beluga Whale wiggling its Melon at Vancouver Aquarium". YouTube. 31 May 2010. Archived from the original on 21 December 2021.