14. Mar, 2016

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 onbly 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.

Sources:

*Marine Mammal Sciene, 2011 (NOAA)

**Current Biology, 2008.

https://en.wikipedia.org/wiki/Sperm_whale

https://en.wikipedia.org/wiki/Animal_echolocation

Rendell L.,  & Whitehead H.   Vocal clans in sperm whales (Physeter macrocephalus). Proc Biol Sci. 2003 Feb 7, 225–231.

Whitehead, H. (2003). Sperm Whales: Social Evolution in the Ocean. Chicago: University of Chicago Press. p. 4. 

Oelschläger HH1, Kemp B. Ontogenesis of the sperm whale brain. J Comp Neurol. 1998 Sep 21;399 (2):210-28.