The Sperm Whales Sonar Head
Among the whales or Caetaeans there are two prominent families: the balean whales (Mysticeti) and toothed wales (Odontoceti). Dolphins, killer whales and sperm whales are all toothed whales. The sperm whale (Physeter macrocephalus) thanks its name to its enormous head, almost one third of the size of its body (15-18 meters). Its weights is around 40 tons. The sperm whales reputation comes from the writer Herman Melville and his famous novel describing Moby Dick, a great albino sperm whale and its deadly struggle with the obsessed captain Ahab. Whale hunters of former centuries must have feared the confrontation with the large headed whale while chasing it in the choppy seas in their small boats. Sperm whales are now less mystical than in the days of Melville and can even be visited by snorkelers floating at the surface with their cameras. Favourite sites are Dominica, Norway, the Azores and Portugal. Sperm whales can dive to depths of more than 100 meters to find their prey Which means that they can hold their breath for long periods of time, often more than one hour.
The orca seem to be the only enemy of the sperm whale. In a NOAA study a group of Orcas was filmed while attacking a herd of sperm whales, in a “wound and withdraw” strategy. The attacks were violent and lasted for hours. The sperm whales appeared largely helpless: their main defensive behavior was the formation of a rosette (“marguerite”-heads together, tails out). ** Anothe marine study verified that during sleep the whales lie motioness in a vertical position: head or tail up. Their sleeps seem to last (on the average) only 15 minutes.**
The large head of the sperm whale contains two enormous flexible cavities filled with fluid. On top is the spermaceti organ filled with milkish sperm-like substance. It may contain 1500 liter of fluid wax that was used for candles, lamp oil and ointments. The cavity beneath it, the ‘junk’ or melon, is filled with orange colored fatty substance (see picture). It is assumed that these spaces may serve to control the buoyancy of the whale. For instance by letting cold water enter the air tubes that run through the head before a dive, the fluid in the spermaceti organ solidifies and reduces in volume. But later it became clear that they were also important for sound transmission. Through the spermaceti organ run two air canals, one to the blowhole and another to the phonic lips in front of the head that connect with the lungs. The wax like fuid of the spermaceti organ probably adds internal echo of the clicks emitted by the respiratory organs, while the melon functions like a transmission and direction station of reflected sounds (see further).
The sperm whales brain (red spot at the left in figure above) has the largest weight of all mammals. It weights around 8 kg which is more than the elephant brain (4.7kg) and much more than the human brain (1.35 kg). But sperm whales have a lower encephalization quotient (EQ) which is an index estimating the part of the brain that may be used for intelligent behavior, when body mass is taken into account. Here the sperm whale ‘scores’ much lower than its smaller toothed whale nephew the dolphin. Little is known about the specific functions of the sperm whales brain. Its main 'intelligent' function as described in current literature is to serve and steer its refined echolocation system located in that enormous forehead. Used either to locate its prey, or to communicate with members of its pod or clan. Most conspicuous in the sperm whale brain are the thick cranial nerves that innervate muscles in its huge forehead and the facial nerve that controls the muscles of the blow hole and the large structure in the lower forehead the melon.
In fact, the large head of the sperm whale functions as one big sound producing or ‘sonar’ system. The sperm whale produces ‘clicks’ (sound burst of short duration) with a pair of phonic lips (also known as "monkey lips" or "museau de singe") at the front end of the nose, just below the blowhole. The sound then travels backwards along the length of the nose through the spermaceti organ. Most of the sound energy is then reflected by the back wall of the frontal sac at the cranium (a sort of sound mirror) and projected back into the melon, whose lens-like structure further directs and amplifies it (see picture). So the temporal pattern of multiple clicks produced by the sperm whale results from reverberations within the nasal complex of the whales. High frequency clicks are used for location and homing in on the prey at larger distances, and low frequency clicks for social communication at closer distances. The echoes are received in fatty structures around the lower jaw, from where they are transmitted to the middle ear via a continuous fat body to the auditory cortex in the brain.
Sperm whales swim in small social units or pods and several pods may form larger groups or clans distributed over a much larger area. Recent studies have shown that families and clans have their own 'dialect': typical signatures of sound bursts called codas. For example, a ‘5R’ coda is one in which five clicks are regularly spaced, while a ‘1+1+3’ coda sounds like ‘click-[PAUSE]-click-[PAUSE]-click-click-click’ with longer gaps between the first two clicks followed by three clicks in rapid succession. http://rsos.royalsocietypublishing.org/content/3/1/150372
Using these codas sperm whales recognize vocalizing individuals of other social units that share a similar dialect. Next to clan codas sperm whales also produce and recognize individual codas to identify individuals in their own pod and smaller families within a clan. Calves learn to produce and recognize codas in their infancy, similar to human babies learning to babble. This learning may take years to perfection.
*Marine Mammal Sciene, 2011 (NOAA)
**Current Biology, 2008.
Rendell L., & Whitehead H. Vocal clans in sperm whales (Physeter macrocephalus). Proc Biol Sci. 2003 Feb 7, 225–231.
Oelschläger HH1, Kemp B. Ontogenesis of the sperm whale brain. J Comp Neurol. 1998 Sep 21;399 (2):210-28.