How Dolphins Produce Sounds
Dolphins produce sounds differently in air and underwater.
Wild dolphins mainly make sounds underwater, although
they have been known to emit high-pitched whistles both
under the water and above the surface when in distress.
The dolphins at
Dolphin Research Center, on the other hand, have developed
quite a repertoire of sounds produced in the air. These
sounds, such as "giggle" (Click here to
hear a dolphin “giggle”.) and "raspberry," (Click here to
hear a “razz” sound.) were invented by the
dolphins and encouraged by the staff. The dolphins often
make up new sounds that are mimicked by their pool mates
and passed around the facility.
Dolphins make sounds in air
by releasing air through their blowholes. The dolphins
have great muscle control over their blowholes and
can alter the sounds by changing the size and shape
of the blowhole opening. At DRC, the dolphins apparently
vocalize to get people’s attention
as they walk by the lagoons. It works very well; there
is no way you can walk past a "screaming" dolphin
and not reply in some fashion!
The sounds that a dolphin makes underwater serve to
help them navigate, locate food, glean information about
the environment, and to communicate
with other dolphins. These sounds are generated
inside the dolphin’s head, under the blowhole,
and, generally, without air escaping from the dolphin's
blowhole. There are two hypotheses about how dolphins
produce sound underwater.
The most widely accepted
hypothesis is the nasal sac theory. Sounds are produced in three pairs of air
sacs located underneath the blowhole. After the
dolphin takes a breath, it closes its blowhole, and air
returns from the lungs into the channel leading to the
blowhole, and, into one or more of the air sacs. The
air inflates the sacs. At the opening of each air sac
is a nasal plug. Air is forced out of the air sac
and over the nasal plug, producing the various sounds. The
action of these air sacs is similar to filling up a balloon
and then squeezing the end to let out the air.
The second hypothesis for
sound production is that the air sacs act as an acoustical
mirror, focusing sound produced by small knobs of fatty
tissue just beneath the blowhole. This tissue may act
like lips that flap together when air is pushed past
them, creating a pulse that is transmitted out into
the water (Au, Popper, & Fay,
2000).
The reason that scientists
are not sure which hypothesis is correct is that no
one has ever been able to see inside a dolphin’s head while it is making sounds. The
medical equipment available today that could allow researchers
to see what is happening is too difficult to use with
dolphins, and in some cases may present some danger to
the dolphin’s well-being. As medical technology
progresses, we might one day be able to directly observe
the dolphin’s sound producing mechanism.
All of the measurements done
with live vocalizing dolphins seems to indicate the
nasal sac area of the dolphin’s
head. The site of sound production appears to be
the museau de singe or “monkey lips” which
resemble the lips of monkeys. Tests using an endoscopy
with a high speed video camera observed the synchronous
movements of the monkey lips and the production of pulses
(Cranford et al., 1997). These sounds produced
by the monkey lips will travel in the forward direction
through the melon. The fatty tissue melon located
in the forehead area of the dolphin acts like an acoustic
lens directing the sound out (Au et al., 2000).
SOUND RECEPTION
The current research seems
to indicate that specialized fatty tissues in the jaw
region serve as the primary route for picking up sound
to the middle and inner ears. This
is the pan bone theory first proposed by Norris (1968).
TYPES OF SOUNDS
Dolphins make many types of underwater sounds. The
three that are most well-known are their whistles, clicks,
and burst pulses.
Whistles
Dolphins produce many different types of whistles. One whistle which dolphins
use most frequently, called a signature whistle, appears to serve as identification
of individual dolphins since each dolphin develops a signature whistle that
is uniquely its own. Dolphins do not appear to be born with a signature whistle.
Calves develop their signature whistles over a four to six month period after
birth. Each dolphin’s signature whistle pattern is distinctive, yet certain
parts of the whistle stay constant while other parts of the whistle alter with
changing circumstances. This suggests that whistles may communicate other information
or serve other purposes. Dolphins have an exceptional ability to mimic sounds
and have been recorded making the signature whistles of other dolphins (Caldwell,
Caldwell, & Tyack, 1990; Tyack, 2000). The approximate frequency range
of bottlenose dolphin whistles is .2 to 24 kHz (Reynolds & Rommel, 1999).
(Click here to hear
a dolphin whistle.)
Clicks/Echolocation
Clicks emitted by dolphins are thought to be exclusively used for echolocation,
the dolphin's amazing ability to gather information about its world through
sound. (Click here to
hear dolphin clicks.) Clicks are produced in rapid sequence, called "click
trains," that sound to us like a creaking door or loud buzz. The
clicks are produced so rapidly, you have to have special equipment to hear
just one of them. The frequency range for echolocation clicks is 0.2
to 150 kHz (Reynolds & Rommel, 1999). Because lower frequency sounds
travel further, dolphins tend to use lower frequencies when echolocating on
objects that are at a distance. Lower frequency clicks, however, do not deliver
as much detailed information about an object as higher frequency clicks. Thus,
as the dolphin moves closer to an object, it can increase the frequency of
its echolocation to learn more about the object.
Dolphins have a waxy, lens-shaped structure in their
forehead called the melon that focuses the clicks into
a tight beam forward. When dolphins are examining an
object or scanning their environment, their heads move
rapidly from side to side as they direct the echolocation
beam back and forth across the object or through the
environment. Dolphins may be able to accurately echolocate
on objects as far away as 100 yards. However, dolphin
echolocation is useless in air.
The clicks emitted by a dolphin
strike objects in its underwater world and bounce back
as echoes to be picked up through the dolphin's lower
jaw From the returning
echoes, a dolphin can tell the size, shape, distance
from, speed, direction of travel, and density of the
object. Thus, dolphins can tell the difference between
materials of different densities, even if they look the
same. Dolphins are particularly good at detecting air
spaces within objects. Since most fish have a swim bladder
filled with air to maintain the fish's equilibrium, dolphins
can easily detect fish with their echolocation. Dolphins
have an exquisite anti-jamming ability associated with
their echolocation. Even in a large group of dolphins
all echolocating at once, each dolphin seems to be able
to pick out its own echolocation echoes and not collide
with another (Au et al., 2000).
Interestingly enough, scientists
now speculate that a dolphin’s teeth may play a part in receiving
incoming echolocation pulses. The teeth are perfectly
spaced one tooth space apart from each other, and the
teeth on one side of the jaw are aligned one half of
a tooth space forward than the other side of the jaw.
It is believed that this arrangement of the teeth acts
as an array or antenna focusing the incoming sound and
helping the dolphin pinpoint the exact location of an
object (Goodson & Klinowska, 1990).
Dolphins do not echolocate
constantly, especially if they are in a familiar area
or if the visibility is quite good. When not echolocating,
dolphins rely on their extremely sensitive hearing
for information about their environment, including
sounds made by other dolphins. Often
dolphins catch prey by listening for it rather than with
echolocation. The sound emitted by a dolphin when
echolocating may give away their presence. Dolphins
can control the loudness of their echolocation clicks,
adjusting this based on their surroundings and how far
they want the clicks to travel. The concern that
dolphins living in aquariums with concrete walls may
be harmed by the sounds of their echolocation bouncing
off the walls is completely unfounded (Au
et al., 2000).
Burst Pulse Sounds
Burst pulse sounds are a general classification given to such sounds as barks,
mews, chirps, and pops. Dolphins apparently make these sounds only under emotional
duress, when they are angry, frightened, upset, or frustrated. These sounds
can be directed towards humans, other dolphins, and inanimate objects.
Researchers have thought for a number of years that
dolphins could stun or kill fish and squid with sounds
they produce, having observed fish in dolphin
pools swimming one minute and lying on the
bottom the next. At first, researchers thought that dolphins
used echolocation as the stunning sound, since dolphins
are capable of echolocating very loudly (see below).
Testing revealed that dolphins trained to echolocate
powerfully at fish and squid did not stun them, so researchers
began looking elsewhere for the stunning sound. Recordings
of bottlenose dolphins and orcas in the wild have led
researchers to now look at loud, low frequency sounds,
or even burst pulse sounds, as the source of the
dolphin's ability to stun prey (Reynolds & Rommel,
1999).
Noise Pollution
In recent years, noise pollution has become a serious
issue affecting marine mammals. Anthropogenic sounds (man-made noise) can inhibit a cetacean’s
ability to navigate, find food, rest, locate another for protection or mating,
among a number of other essential activities. Man-made noise arises from
a variety of sources such as oil drilling, sonar testing, explosives, and seismic
surveys. However the majority of man-made sound in the ocean comes from
ships.
According to research conducted
by scientists at Scripps Institution of Oceanography,
noise levels in 2003-2004 were 10 to 12 decibels higher
than in 1964-1968. The
main cause of this is increased global shipping trade. According
to John Hildebrand of Scripps, the noise is more powerful
by a factor of 10. This noise increase is due to
more and noisier ships. Recently there has been
much discussion and concern over the Navy’s testing
of sonar. There is mounting evidence to suggest
that sonar has indeed caused adverse affects on marine
mammals. Sound deployed with such intensity may
even be severe enough to cause stranding and death. The
Navy itself has voiced concern about the affects of sonar
and has begun funding research to study the affects of
man-made noise on marine mammals.
The diagram below illustrates
the frequency range of some man-made sounds to the
frequency range of sounds made by different marine
mammals. From this you
can see where they overlap and interfere.
Chart courtesy of Dr. Brandon Southall, NOAA
Currently there have not
been enough scientific studies conducted to understand
the impact of any type of man-made sound on marine
mammals to determine what is detrimental to their survival. Continued scientific study is
desperately needed to investigate this issue in order
to maintain the safety and protection of marine mammals
and their environment. Until we have a better understanding
of the affects of man-made sound on the marine environment
and its inhabitants, noise pollution, regardless of the
source should be limited and avoided in areas that marine
mammals frequent.
References
Au, W. 1993. The Sonar of Dolphins.
New York: Springer-Verlag.
Au, W., A.N. Popper, and R. F. Fay,
ed. 2000. Hearing by Whales and Dolphins. New York:
Springer-Verlag.
Cranford TW. Van Bonn WG, Chaplin MS, Carr JA, Kamolnick TA, Carder DA, Ridgway
SH (1997). Visualizing dolphin sonar signal generation using high-speed
video endoscopy. Journal Accoustics Society of America 102:3123
Caldwell, M. C., D. K. Caldwell,
and P. L. Tyack. 1990. A review of the signature whistle
hypothesis for the Atlantic bottlenose dolphin, Tursiops truncatus.
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