Echolocation by marine mammals
Principle
Echolocation, also called biosonar, is the biological sonar used by mammals such as dolphins, most whales,
shrews and some bats. Technical sonar (Sound
Navigation And Ranging) is a technique that uses sound (ranging from infra- to
ultrasound) propagation (usually underwater) to navigate, communicate or to
detect underwater objects. Two bird groups also
employ biosonar for navigating through caves.
The physical principle is the same as applied in
medical echography: sender (small-angle sound beam), reflecting object,
receiver and analysis. The latter is basically the measurement of the time
delay between sending and receiving to calculate the object distance and other
characteristics. To obtain a well defined time delay echolocating animals emit
very short calls (a few ms) out to the environment. The calls can be constant
frequency (single or composed) or for instance frequency modulated (FM). They
use these echoes to locate, range, and identify the objects. Echolocation is
used for navigation and for foraging (or hunting) in various environments.
Animal echolocation relies on both ears which are
stimulated by the echoes at different times and at different loudness levels,
depending on the source position. These differences are used to perceive
direction and distance, object size and other features (e.g. kind of animal).
Toothed whales (suborder Odontoceti),
including dolphins, porpoises, river dolphins, orcas and sperm whales, use
biosonar that is very helpful when visibility is limited, e.g. in estuaries and
rivers due to light absorption or turbidity.
Echoes
are received using the lower jaw and especially its filling of
waxy tissue as the primary reception path, from where
they are transmitted to the inner ear (Fig. 1).
Lateral sound may be received though fatty lobes surrounding the ears with a
similar acoustic density to bone. Some researchers
assume that when they approach the object of interest, they protect themselves
against the louder echo by quieting the emitted sound. In bats this is known to
happen, but here the hearing sensitivity is also reduced close to a target.
Fig. 1
Diagram illustrating sound generation, propagation and reception in a
toothed whale. Outgoing sounds are drak gray (red) and incoming ones are light
grey (green).
Toothed whales emit a focused beam of
high-frequency clicks in the direction that their head is pointing. Sounds are
generated by passing air from the bony nares through the phonic lips. These
sounds are reflected by the dense concave bone of the cranium and an air sac at
its base. The focused beam is modulated by a large fatty organ, the 'melon'.
This acts like an acoustic lens because it is composed of lipids of differing
densities. Most toothed whales use a click train for echolocation, while the
sperm whale may produce clicks individually. Toothed whale whistles do not
appear to be used in echolocation. Different rates of click production in a
click train give rise to the familiar barks, squeals and growls of the
bottlenose dolphin. In bottlenose dolphins, the auditory EEG response resolves
individual clicks up to 600 Hz trains, but yields a graded response for higher
repetition rates.
Some smaller toothed whales may have their
tooth arrangement suited to aid in echolocation. The placement of teeth in the
jaw of a bottlenose dolphin, as an example, are not symmetrical when seen from
a vertical plane, and this asymmetry could possibly be an aid in the dolphin
sensing if echoes from its biosonar are coming from above or below. (A similar
asymmetry for elevation localization has been found in the barn owl.)
Fig. 2 Parasagittal slice in a plane lying slightly
to the right of the midline of the common dolphin forehead. This diagram includes
the skull and jaw bone (gray hatched), the nasal air sacs (black or in colour
red), the right MLDB (monkey lip/dorsal bursae) complex, the
nasal plug (np), the epiglottic spout (es) of the larynx, and the melon tissue. Other tissues, including the
muscle and connective tissue surrounding many of the labeled forehead tissues,
are not shown.
Table 1 summarizes the most
important differences between cetaceans and human.
|
|
Cetaceans (Fully Aquatic Ears) |
Human (Aerial Ears) |
|
Outer ear |
Temporal bone is
not part of skull |
Temporal bone is
part of the skull |
|
No pinnae |
Pinnae |
|
|
Auditory meatus is
plugged |
Air-filled auditory
meatus |
|
|
Middle Ear |
Middle ear is
filled with air and vascular tissue |
Middle ear
completely air-filled |
|
Inner Ear |
Basilar membrane
thin and broad at apex of odontocetes |
Basilar membrane
thick and narrow at the basal end |
|
Mysticetes basilar
membrane thinner and wider than odontocetes and humans |
|
|
|
Strong support of
basilar membrane for odontocetes, less for mysticetes |
Little support of
basilar membrane |
|
|
Long basilar
membrane length |
Short basilar
membrane length |
|
|
Semi-circular
canals are small |
Semi-circular
canals are average to large |
|
|
Large number of
auditory nerve fibers |
Average number of
auditory nerve fibers |
From http://www.dosits.org/animals/produce/ceta.htm.
Dolphins use a narrowly focused
sound beams which mainly emanates from the forehead and rostrum during
echolocation as illustrated in the simulation of Fig. 3. The beam formation
results primarily from reflection off the skull and the skull-supported air sac
surfaces. For the frequencies tested, beam angles best approximate those
measured by experimental methods for a source located in a region of the MLDB
complex. The results suggest that: 1) the skull and air sacs play the central
role in beam formation; 2) the geometry of reflective tissue is more important
than the exact acoustical properties assigned; 3) a melon velocity profile of
the magnitude tested is capable of mild focusing effects; and 4) experimentally
observed beam patterns are best approximated at all frequencies simulated when
the sound source is placed in the vicinity of the MLDB complex.
Examples of
vocalizations can be found at http://neptune.atlantis-intl.com/dolphins/sounds.html.
Application
Dolphins
are used as reconnoiterers in navies of various counties. Also their echolocation system was of
interest for technical military applications.
More Info
In echolocating mammals, the
cerebral analysis of the echo’s may be complicated since the animal has to
correct for the speed of self-motion and that of the (living) object. Suppose
that a marine animal is at a distance of
Fig. 3 A 2D computer simulation of sound production
in the common dolphin. The dolphin's head (based at parasagittal CTs)
is indicated by a dotted outline. Sonar pulses from a spot (just beneath the
uppermost air sac) below the dolphin's blowhole reflect and refract through
these structures. The lines around the dolphin's head represent the direction
and intensity of sound waves emitted from the model. Most of the acoustic
energy is emitted in a forward and slightly upward-directed beam in this 100
kHz simulation.
References
Aroyan JL, McDonald MA, Webb SC, Hildebrand JA,
Clark D, Laitman JT, Reidenberg JS (2000) Acoustic Models of Sound Production
and Propagation. In: Au WWL, Popper AN, Fay RR (eds), Hearing by Whales and Dolphins.