The spermaceti organ is an organ that commonly appears in the heads of toothed whales of the family Physeteroidea, in particular the sperm whale. This organ contains a waxy liquid called spermaceti and is involved in the generation of sound. In the modern sperm whale, the spermaceti organ is far larger in proportion to the animal’s body than what would be explained by simple allometry, and the evolution of the organ has caused changes in basal skull morphology, which may implicate tradeoffs were made that compromised the functionality of other nearby features. The high investment in this organ suggests that the organ must have adaptively advantageous functionality. However, the nature of its function hasn’t been clearly understood historically. The two prevailing hypotheses proposed regarding the spermaceti organ’s function are that 1) it assists in controlling buoyancy via manipulation of the contained spermaceti oil's temperature and, in consequence, its density, facilitating deep diving by cooling and surfacing by warming, as well as allowing the animal to remain motionless at great depth and 2) it essentially functions as sonar, aiding the whale in echolocation. The hypotheses that the spermaceti organ is involved in buoyancy manipulation has been challenged by many authors, with the following points raised as problematic: the change in density that could be achieved by manipulation of spermaceti oil temperature would likely have a negligible impact on the animal’s overall buoyancy; the anatomical features that would be needed for heat exchange with the spermaceti organ don’t appear to be present; a mechanism of temperature regulation would necessitate high physical exertion while at great depth, which deep-diving animals tend to avoid; sperm whales appear to be highly active during dives, countering the suggestion that buoyancy manipulation would be advantageous because of its benefit in remaining motionless while diving; and the evolution of the spermaceti organ with buoyancy as a selective pressure would be very difficult and is unlikely due to the fact that the organ wouldn’t have any impact on buoyancy until it became extremely large in proportion to the body. Thus, the hypothesis that the spermaceti organ is an adaptation aiding in echolocation is generally the best accepted one. Under this hypothesis, the spermaceti organ assists in echolocation for the purpose of foraging activity during deep dives, allowing the whale to manipulate the directionality and power of output sound waves for improved sensory perception of its surroundings and increased ability to actively capture prey. While nasal complex morphology is believed to be homologous in all of the echolocating Odontoceti, the hypertrophied quality of the sperm whale's nose, which contains the greatly enlarged spermaceti organ can be interpreted as an adaptation for deep diving activity uniquely evolved in the Physeteroidea. The spermaceti organ in sperm whales is shaped like an elongated barrel and sits on top of the whale's melon. Historically, the spermaceti oil found within it was used in a variety of products – including lamp oils, candles, and lubricants – providing the economic basis for the sperm whaling industry. A sperm whale may contain as much as 1,900 litres of this oil.
