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Melon – but not the kind you’re thinking of

Toothed whales (Odontocetes) are unique among mammals – they are able to echolocate underwater. Echolocation is the specialised acoustic adaptation by animals who use sound to forage for prey, navigate, and avoid predators by emitting sounds and listening to echoes as the sound waves reflect off different objects in the environment. This requires morphological and physiological adaptations. The group best known for echolocation is bats. Generally, bats make echolocation sounds by contracting their larynx (voice box), while others click their tongues. Bats ears are particularly sensitive to and adapted to receiving reflected sound waves and detecting frequency changes. While echolocation in bats was evidenced in 1938, dolphins were only conclusively found to use echolocation in 1960. Great steps have since been made in understanding echolocation in dolphins (as well as bats and other groups). The head of dolphins is a very complex structure, with unique air sacs, and specialised sound conducting fats. The exact location and mechanism of sound production for echolocation has been a perplexing issue in dolphins. Unlike bats, dolphins do not use their larynx for echolocation sounds, but use a structure in their nasal complex called the phonic lips. There are two sets of phonic lips associated with the two nares. Sounds produced by any source tends to radiate out spherically, and this is not useful for echolocation, where a more focussed beam is needed. The skull, various air sacs, and melon all play an important role in focussing echolocation signals into a beam. The melon is a globular fatty organ which gives shape to the domed forehead in odontocetes. It is less dense in the centre, and becomes denser in the outer layers – this non-homogenous density is important. Air is pushed through the phonic lips at pressure which produces the sound waves, which are reflected by airs sacs into the melon which refracts the waves narrowing and focussing the beam that then reaches the water. Interestingly, areas in the lower jaw of the dolphin are sensitive to the returning echolocation sound waves which are then transmitted to the ear.

 

Written by: Minke Witteveen

 

For further reading:

  • Parsons, E.C.M. 2013. An Introduction to Marine Mammal Biology and Conservation. Jones & Bartlett Learning, Burlington. Pp. 61-78.
  • Van Ryckegham, A. How do bats echolocate and how are they adapted to this activity? Accessed: 2017-02-13. URL: https://www.scientificamerican.com/article/how-do-bats-echolocate-an/
  • Whitlow, W.L. Echolocation. In: Perrin, W.F., Würsig, B. and Thewissen, J.G.M. (eds) Encyclopedia of Marine Mammals 2nd Academic Press, Amsterdam. Pp. 348.

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