What is the melon for in odontocetes?

The bulging forehead so characteristic of dolphins, their pleasingly rounded profile, the expression often described as “smiling”, all of it comes down to a single strange organ with no equivalent in the terrestrial animal kingdom: the melon. The word, of old standing, comes from the appearance this structure takes on when examined in anatomical section: a rounded, yellowish, dense mass filling the whole space between the nasal passages and the skin of the forehead. Its function long remained mysterious. It was taken in turn for a shock-absorbing cushion, a fat reserve, a hydrodynamic guide. The reality, worked out from the 1960s onwards, is subtler and finer: it is a living acoustic lens, with no equivalent.

The geometry of a lens made of fats

To understand what makes the melon a lens, it is worth taking the time to examine its chemical composition. Unlike ordinary subcutaneous fat (blubber), which is rich in conventional triglycerides, the melon contains a majority of waxes: esters of fatty acids and fatty alcohols, and in particular isovalerates, compounds that are rare elsewhere in the animal kingdom. These specific lipids have one peculiarity: their acoustic velocity: the speed at which sound travels through them, varies with the exact composition.

The melon, however, is not chemically homogeneous. The researchers Kenneth Norris and George Harvey, in a pioneering study of 1974, showed that the concentration of waxes varies gradually between the core and the periphery. At the core of the melon, the concentration of lipids with a low acoustic velocity is at its highest: sound travels slowly there. At the periphery, close to the bones, the composition is nearer to that of an ordinary fat, with a faster acoustic velocity. Sound passing through this structure is therefore deflected, in the same way that light is deflected by a glass lens whose refractive index varies from the centre towards the edges.

The net effect is focusing: the acoustic waves emitted in a relatively omnidirectional way by the phonic lips are gathered into a directional beam, projected forwards. Without this lens, echolocation would be unusable: there would be no way of telling where the echo came from.

Where is the sound produced?

One received idea is persistent: that the echolocation sound is produced in the melon itself. This is false. The melon is a transmitter, not an emitter. The sound originates in the phonic lips, muscular and fatty structures located just beneath the blowhole, in the nasal passages. When the animal drives a stream of pressurised air through these lips, they snap rapidly, producing a click of about fifty microseconds. This primary click is then injected into the melon through its posterior base, travels through the fatty mass, and emerges focused at the front. See our detailed explanation of echolocation.

The extreme case of the sperm whale

In the sperm whale, evolution has taken this system to an almost caricatural extreme. The “melon” has there been multiplied into two massive structures: the spermaceti organ in the upper part and the junk in the lower part of the snout. Together, these two organs occupy a third of the animal’s body length. The spermaceti organ holds up to one thousand nine hundred litres of clear oil, the famous spermaceti that made the fortune of the nineteenth-century whaling industry.

The arrangement is not only a matter of size. The sound produced by the phonic lips of the sperm whale, located right at the front of the nose, is reflected backwards by a cranial air sac, travels once more through the spermaceti and the junk, then emerges projected forwards. This zigzag path amplifies the signal considerably. It is what allows the clicks of the sperm whale to reach two hundred and thirty decibels, the most powerful biological sound ever measured. At that power, the ocean can be probed over several kilometres and a squid detected at five hundred metres.

A mobile melon

A more recent discovery has made the picture more complex still. The melon is not a passive structure. Several anatomical studies (Harper et al., 2008) have identified peripheral muscles able to alter its shape. In the common bottlenose dolphin, these muscles allow a subtle adjustment of the focusing. In the beluga (Delphinapterus leucas), the effect is particularly visible: its melon is so mobile that it can literally be seen changing shape through the skin while the animal echolocates. The beluga can “point” its acoustic beam at a precise target, as a hunter would aim a torch.

An old hypothesis about buoyancy

An old hypothesis, put forward by the biologist Malcolm Clarke in the 1970s, proposed that the spermaceti organ of the sperm whale might serve as an organ of variable buoyancy. The idea: the spermaceti wax would melt at body temperature and solidify on cooling (in this case, when the animal dives into colder deep water), thereby altering the density of the head and assisting the descent. It was an appealing hypothesis, and it long featured in textbooks.

Today, this hypothesis is largely discounted. Several studies (notably Madsen et al., 2002) have shown that the temperature of the spermaceti remains stable during dives, and that the energy savings associated with the proposed mechanism would in any case be only marginal. The function of the melon and of the spermaceti appears to be essentially acoustic. A secondary function of mechanical protection (the melon acting as a cushion during fights between males) remains debated but is a minority view.

A taxonomic signature

The shape of the melon is one of the criteria for rapid identification between odontocete families. The common bottlenose dolphin has a rounded melon and a well-marked rostrum. The pilot whale has a globular melon that overhangs the rostrum. Risso’s dolphin has a massive melon with no rostrum (hence its characteristic “blunt-faced” silhouette). Cuvier’s beaked whale has a discreet, cone-shaped melon. The sperm whale has an entire head that is nothing but a hypertrophied melon. This morphological diversity reflects different acoustic strategies, hunting at short or long range, in shallow or abyssal water, for stationary or mobile prey.

Sources

• Norris, K.S. & Harvey, G.W. (1974). “Sound transmission in the porpoise head”. Journal of the Acoustical Society of America, 56(2), 659-664.
• Cranford, T.W., Amundin, M. & Norris, K.S. (1996). “Functional morphology and homology in the odontocete nasal complex”. Journal of Morphology, 228(3), 223-285.
• Madsen, P.T., Payne, R., Kristiansen, N.U., Wahlberg, M., Kerr, I. & Møhl, B. (2002). “Sperm whale sound production studied with ultrasound time/depth-recording tags”. Journal of Experimental Biology, 205, 1899-1906.
• Harper, C.J. et al. (2008). “Morphology of the melon and its tendinous connections to the facial muscles in bottlenose dolphins”. Journal of Morphology, 269.
• Aroyan, J.L. (2001). “Three-dimensional modeling of hearing in Delphinus delphis“. JASA, 110.

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Further reading