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Anatomy
Reptiles have an inner and a middle ear.
Crocodiles are the only ones with rudiments
protect the tympanum during underwater
stays. Snakes, on the other hand, have very
simple ears that leave them practically deaf
to all sounds but those at very low frequen-
cies. They perceive mainly vibrations in the
ground by means of the skull bones and,
to a lesser extent, the rest of the skeleton
( 1992).
Chelonians lack an outer ear, and their
hearing apparatus shows on the outside only
as an enlarged, round membrane. In Testudo
hermanni, this tympanum is approximately
0.25 mm in thickness. Behind it lies the
middle ear that leads over to the inner ear
and contains various structures that serve the
perception of vibrations.
The middle ear consists for the largest part
of the tympanic cavity that is connected to
the oral cavity via the Eustachian tube (see
Figs. 1 & 2). It is closed off to the outside
by the membranous tympanum. Vibrations of
this membrane are transferred to the inner ear
via a bony rod, called the columella.
A close look at the inner ear reveals that the
skinny and bony labyrinth is connected to the
middle ear on the one, and to the skull cavity
on the other hand. The inner ear is separated
from the middle ear by a large, pericapsular
fairly large in some species and then will
establish contact with the endolymphatic
cavity ( 1996).
Stéphane Gagno
The Auditory Sense in Tortoises, using Hermann’s Tor-
toise, Testudo hermanni (Gmelin, 1789), as an Example
Fig. 1.
Anatomy of a turtle ear
(Emys orbicularis).
A: endolymphatic cavity,
B: utriculus, C: sacculus,
D: tympanic cavity,
E: columella, F: Eusta-
chian tube, G: pericap-
sular sinus, H: cisterna
perilymphatica, I: ductus
perilymphaticus, J: saccus
perilymphaticus, K: brain,
IX and X: cerebral nerves
(after 1970);
terminology partly after
(1983).
4 RADIATA 22 (2), 2013
The Auditory Sense in Tortoises
Attributes of sound waves
If sound is to be produced, there needs
to be a vibrating body whose vibrations can
bring the adjacent air molecules out of their
balanced state. This will create an alternating
series of zones of increased pressure in which
the air molecules are closer together, and
zones with reduced pressure in which the air
molecules are farther apart. This succession
facilitates the spatial spread of sound. It also
implies that sound cannot be generated or
dispersed in a vacuum. Sound is illustrated
as a sinus-shaped curve.
Sound waves have two major attributes:
- Amplitude, which is the volume, quanti-
- Frequency (f), which is the number of
vibrations of air molecules per second, quan-
sometimes substituted with the number of peri-
ods per second (p.p.s.), with a period (T) being
the duration of one complete vibration.
f = 1/T
The higher the frequency, the higher
a sound is perceived, and the lower the
frequency, the lower a sound is perceived.
Sound travels at a speed of 340 metres per
second in air.
Fig. 2. Dissection specimen of the upper head region of a tortoise, from below on the left, with
the openings of the Eustachian tube pointed out, and on the right, with a catheter channelled
Fig. 4. Visualisation of a low and a high
sound.
Fig. 3. Attributes of sound waves.
RADIATA 22 (2), 2013 5
low sound high sound
Eustachian tube
period
amplitude/
sound intensity
Stéphane GaGno
Previous studies in chelonians
(1978, 1980, 1983, 1986)
described the responsiveness to vibrational
cues that impacted directly on the cara-
pace. These studies were conducted using
specimens of Testudo graeca and Testudo
hermanni from which the brain had been
removed. Nervous impulses caused by sound
from a loudspeaker that was in direct contact
with the carapace were then measured with
an electrode implanted in the spinal marrow.
Various frequencies were tested, and it sho-
wed that the range of perception contained
gaps. On the bottom line, the experiments
demonstrated that the carapace of a tortoise
had to be equipped with sensory receptors
that were able to perceive vibrations and that
these sensory receptors had to be situated
close under the surface. Differences were
found in the intensity of responsiveness,
depending on where exactly the cue was
applied. Frequencies of 150 Hz or more did
not trigger nervous impulses in the spinal
marrow at all; the strongest responses were
noted in the range between 80 and 100 Hz.
For this was sufficient evidence
that a tortoise that was fully retracted into
its shell would mainly perceive vibrations in
the ground and much less airborne sound.
(1981, 1982, 1983) conducted
labyrinth experiments with live specimens of
Trachemys scripta and Terrapene carolina,
in which the exit of a maze was marked
with a source of sound waves. His results
suggest that turtles would complement their
acoustic perception with cues perceived
and transferred via the bones of their shell
and were thus able to both perceive sound
waves from their immediate environment
and locate their point of origin. This type
of sensing is of major advantage for the
turtles when they have to retract the head
with its actual hearing organs within the
shell to protect it against attacks from
predators. To support this conclusion, -
, like in his experiments,
applied vibrations right onto the shell and
measured the nervous impulses on the brain
stem. He was convinced that the transfer
of sound waves via the bones created the
actual perception of sound.
In 1983, a team headed by
conducted similar experiments with marine
turtles (Caretta caretta and Lepidochelys
kempii) and found that the surface of skull
and shell had to contain the sensory re-
ceptors required to perceive sound waves.
These turtles are thus able to perceive the
low frequencies of the beach on which they
hatch, memorise their pattern, and find their
way back to exactly this beach for nesting
in later years. Anthropogenic activities on
and in the sea produce a lot of noise and
investigations revealed that this underwater
noise falls for the largest part within a range
of 80 to 110 dB. The top values of these
disturbances thus meet with the maximum
sound sensitivity of marine turtles. It is
therefore incontestable that a noisy environ-
ment cannot but affect their behaviour and
represents a particular type of environmental
pollution ( et al. 2005).
There are a great number of studies on the
auditory nerves of chelonians (
1978, 1980,
1981a, 1981b, et al. 1984,
1986,
1988, 2003,
et al. 2003, et
al. 2012). (1980)
researched the electrical impulses in the
nerves and the sensory cells of the cochlea
in Trachemys scripta elegans. Cues with a
frequency range of 70 to 670 Hz produced
nervous impulses, and the ear of these turtles
was most sensitive to sound wave pressures
between about 30 and 40 dB. Within this
frequency range, with top values between
400 and 500 Hz,
et al. (2012) also measured the optimal
vibration of the tympanum of Trachemys
scripta elegans. As a consequence, the latter
6 RADIATA 22 (2), 2013
The Auditory Sense in Tortoises
team of researchers hypothesised that the
resonance of the air-filled middle ear could
also play a role in underwater hearing.
Using a similar methodology, et
al. (1969) managed to specify the maximum
sensitivity of Chelonia mydas as 300 to 400
Hz. While the upper limit of hearing was
2,000 Hz, frequencies up to about 1,000 Hz
could be perceived reliably. Outfitted with
ciliaries, the auditory sensory cells play
an important role in the amplification of a
signal and the differentiation of perceivable
frequencies. Response times are less than
one millisecond ( et al. 2001).
et al. (2004) described in detail the
molecular processes within the auditory
sensory cells. The mechano-electrical si-
gnal converters have pores through which
certain molecules transport information.
These porous canals have a diameter of a
mere 12.5 Å (1 Ångström = 10-10 metres)
over a length of 31 Å.
et al. (2007) described the embryo-
nic ontogenesis of the relevant areas of the
hearing organs in Pelodiscus sinensis.
1981) elaborated on
electro-physiological measurements taken
in Chrysemys picta and Trachemys scripta,
which revealed distinct impulses in the vesti-
bular window, semicircular duct and canal,
and the outer wall of the tympanic cavity
at soft volume to a maximum frequency of
500 Hz. Above this limit, impulses could
be recorded to about 3,000 Hz, but this was
a result of increased sound wave pressure,
or in other words, “over-stimulation”, with
a risk of damage to the hearing system.
The two researchers tapped the electrical
impulses directly in the hearing centre of
the brain in four species of chelonians.
It demonstrated that Testudo graeca was
the least sensitive of the species studied.
The scientists measured responses in this
species from 40 dB in a frequency range
from about 100 to 200 Hz. For Mauremys
caspica, a perceivable frequency range
from 300 to 600 Hz was established, with
these pond turtles being most sensitive to
300 Hz. In contrast, Emys orbicularis and
Testudo horsfieldii were significantly more
sensitive to sound, showing responses from
a volume of only 30 dB and within a wider
frequency range, so that electrical impulses
were still recorded at as little as 1.5 KHz
and as much as 3.5 KHz.
The literature available on the subject of
hearing capabilities in chelonians shows that
these animals almost exclusively perceive
deep sounds. The studies mentioned were
for the largest part dedicated to researching
electro-physiological signal transfer from
the sensory organs and via the nerves. What
remains to be investigated is how these
signals are fed into, and processed by, the
central nervous system and which responses
they trigger. In other words, it needs to be
studied whether and how chelonians exhibit
recognisable responses to sounds.
As early as in 1982, had expo-
sed eight species of chelonians representing
three families (Emydidae, Testudinidae
and Chelydridae) to sounds up to 100 dB
in amplitude. If the sounds were emitted
via a loudspeaker and airborne, there was
no behavioural response by the turtles. It
was only when the turtles were impacted
directly by sounds – on the dorsal shell
– that they would respond with movements
of the head.
The present study investigates the ef-
fects exclusively airborne sound waves
may have on Testudo hermanni hermanni.
The results were evaluated on the basis of
directly observed responses in the studied
specimens.
Material and methods
Tortoise each was placed in a soundproof
environment, i.e., a completely closed
container that was thickly insulated with
soundproofing material all around. The walls
RADIATA 22 (2), 2013 7
Stéphane GaGno
coated with a layer of cork. A small window
with one-way glass facilitated observing
the tortoise without its being able to see
the observer. The container was placed on
rubber feet so that the tortoise was fully
shielded from any exterior vibrations. A 40
W loudspeaker was mounted in the upper
portion of the tank in a manner so that it
could not transfer bodily vibrations to the
container. It was connected to a frequency
generator for sinus waves from 0 to 100
kHz. Amplitudes (volume) within the tank
were measured with a sonometer relative to
the frequency applied (Fig. 5). Temperatures
within the experimental setup ranged from
25 to 30°C. All experiments were conducted
at the facilities of the Turtle Village (SO-
The frequency generator was activated
only after the tortoise had calmed down and
assumed a relaxed posture by resting the
head on the ground. Starting from a certain
frequency, the frequency of the sound was
gradually changed. It was either raised to
a maximum of 100 kHz or lowered to a
minimum of 0 Hz (see Fig. 6). Whether
a certain frequency was perceived by the
tortoise was evaluated by its behaviour
and assumed to be affirmative if it lifted
the head, for example. This frequency was
then recorded. The experiment was succes-
sively duplicated with ten adult tortoises,
with each being exposed to a different
starting frequency. The recorded results
were summarily plotted in a graphic and
represent the perceived frequency range of
the tested tortoises.
Healthy adult tortoises were randomly
taken from the pens of the Turtle Village SO-
-
tric data are summarised in Table 1.
Fig. 6:
Experimental
setup.
Fig. 5.
Sound pressure
level of the ex-
perimental setup
as a function of
the generated
frequency (in
decibels).
8 RADIATA 22 (2), 2013
89142938 109 700720 1026
81738540 9950 11000
1890019411 2173222050 23210
64 687280
70
8286 908088
6080
901008010390
80
72
hertz (Hz)
decibels (Db)
The Auditory Sense in Tortoises
Results and Discussion
After having adjusted to the environment of
the experimental setup, the tortoises responded
with different behavioural expressions to the
sounds produced:
- Startled waking up, followed either by
retracting the extremities or extending the
neck. This was the most commonly observed
response.
- Distinct pumping motions with the front
legs in order to increase breathing activity.
- Yawning, which almost made an impres-
sion of embarrassment over the disturbance.
The frequencies at which every one of the
responses were plotted along a graphic axis.
Figure 7 illustrates this using tortoise No. 1 as
an example. Every red dot marks a frequency
at which this specimen displayed a response.
The directions of the arrows indicate whether
this response was triggered while the frequen-
cy was rising or falling.
Now the responses of all tortoises were
pooled in a common graphic (Fig. 8). Each
response is here marked with a blue rectan-
gle. The frequency range perceivable to the
tortoises lies between 10 and 182 Hz. It is
thus much narrower than that of humans,
which ranges from about 20 to 20,000 Hz.
A part of the frequency range perceived
by these tortoises lies within the range of
infrasound (between 10 and 20 Hz), which
cannot be heard directly by the human ear.
It is evident that the tortoises can only hear
deep sounds.
The tortoises in our study responded with
lifting their heads to frequencies that were
different from those in the study by
Tortoise No. Sex Length (mm) Width (mm) Height (mm) Weight (g)
1110 85 59 236
2141 110 74 550
3 F 171 131 85 990
4 F 200 148 95 1,523
5 F 185 134 90 1,200
6 F 158 127 88 726
7 F 170 125 89 1,014
8 F 155 112 88 790
9 F 151 114 74 660
10 F160 117 82 786
Table 1: Data on the tortoises used in the experiment
Fig. 7. Responses of tortoise No. 1 to frequencies.
Fig. 8. Summary of the experimental frequencies that triggered a response in the tested tortoises.
RADIATA 22 (2), 2013 9
frequency (Hz)
Stéphane GaGno
(1982). He had conducted his experiments
with frequencies above 250 Hz, which were
clearly above the maximum perception ca-
results demonstrate that these tortoises can
hear sounds only within a fairly limited range.
They are adapted to hearing deep sounds, even
within the infrasound range that the human
ear cannot perceive any more. Sounds as deep
as these are perceived via an adapted bone
structure that contains special sensors for the
capable of hearing high and even very high
frequencies: Dogs to about 35 KHz, cats to
about 25 KHz, and bats even to 80 KHz. This is
in contrast to other, mainly very large animals
such as the elephant, which are capable of
exploiting even infrasound for communicating
Fig. 9.
Frequency
spectra of various
musical instru-
ments.
10 RADIATA 22 (2), 2013
The Auditory Sense in Tortoises
over distances of several kilometres. It may
be presumed that by perceiving low-frequency
ground vibrations, tortoises are able to detect
the approach of a potential predator already
from a great distance.
Tortoise keepers have again and again been
reporting that their tortoises came running
from their outdoor enclosures when they
played the piano. Studying the overview of
frequencies in Fig. 9 shows that the piano is
one of the musical instruments that can also
produce very deep sounds. The above results
also explain why tortoises will respond more
readily to the deeper male voice than to the
higher voice of a woman.
Studies on the hearing capabilities of tor-
toises have demonstrated that they can mainly
perceive deep sounds. Their range of hearing
is a direct consequence of the presence of
apparatus. For example, ciliary-bearing cells
are responsible for distinguishing different
sounds according to their sensitivity. The
structure of the visible tympanum likewise
limits the transfer of sound waves. Viewed
from a purely mechanical perspective, it is
a specialised, round membrane that is sur-
rounded by inelastic bones. The vibrational
amplitude of membranes with the same surface
area is smallest in membranes with a round
shape. This vibrational amplitude is calculated
after the following formula:
For a round membrane with a solid margin
depend on a variety of factors, i.e. density
of elasticity after (E), and the strain
-
rameters are about equal for all chelonian spe-
cies. There are, however, two more variables
that also play a role here, i.e., the thickness
of the tympanum (h) and its diameter (a),
which are different in the various species
of chelonians. These two parameters could
explain the deviant measuring results in the
study by (1981), because
the tympani of Testudo horseldii and Emys
orbicularis are about four times as thick as that
of Testudo graeca. This parameter functions
as a numerator in the formula given above so
that it is entirely plausible that these species
frequencies and are more sensitive to sound
than the Spur-thighed Tortoise.
It should be clear by now that chelonians
have by all means a well-developed sense
of hearing that is complemented by sensory
receptors in their shell that are sensitised to a
wide range of the frequencies a tortoise would
respond to. While this probably enables a tor-
toise to detect an approaching predator early,
the results of other studies render it likely that
the auditory capabilities also plays a role in
reproduction. The publication by et
al. (2005a) suggests that the sounds emitted
by male Testudo hermanni during copula-
tion can be evaluated by females and may
These sounds may even serve the purpose
of inter-male communication (to keep rivals
and intruders away). With these sounds be-
male (1995, quoted in et
al. 2005a). The courtship and mating gestures,
but also the mating sounds of male Testudo
hermanni, Testudo marginata and Testudo
graeca may be indicative of the physiological
et al. 2003,
et al. 2005b, et al. 2011).
The mechanism of the production of mating
sounds of Testudo spp. were described by
et al. (2004), and in some tortoise
species, it was even demonstrated that they
truly communicate via sounds ( 2007).
In the case of Gopherus polyphemus, “con-
RADIATA 22 (2), 2013 11
Stéphane GaGno
versations” of up to ten minutes in duration
were documented between residential caves
that were in some instances several kilome-
tres apart, with the produced and perceived
frequency lying between 3 and 40 Hz in this
species ( 2008).
et al. (2013) recorded the sounds emitted by
adult and newly hatched Podocnemis expansa
during various situations in and outside of the
water. This team of researchers even stated
that there was an acoustic communication
system in place between adult and hatchling
turtles that enabled the females to “call” the
juveniles together prior to starting off on one
of the long migrations typical of this spe-
cies. Only once assembled would adults and
juveniles set out on their journey together.
In Chelodina oblonga, evidence for acoustic
underwater communication was found (
2005, et al. 2009). Their repertoire
of sounds proved to be very complex, with
most sounds having frequencies below 1 KHz
while ranging from 100 Hz to 3.5 KHz. When
reviewing the literature it is evident that the
auditory senses of chelonians vary consider-
ably, according to species. We must be aware
that the ability of the different chelonians to
hear and to emit sounds is not limited to the
auditory sense enables the animal to perceive
signals from their environment and react
accordingly, for example to the presence of
predators. Even though chelonians have no
externally visible ears, their sense of hearing
plays a major role in their day-to-day life, is
crucial for their survival, and acts as a trigger
for some of their responses.
Acknowledgements
I am particularly grateful to Dr. -
for the critical review of my manuscript,
and appreciation of this work. I furthermore
thank and
for their assistance with taking measurements
in the course of their extracurricular internship.
I am indebted to for her translating
the original French manuscript into German and
the extraordinary courteousness she afforded
go to for his translation into
English and to for accepting
my manuscript for the Radiata.
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Author
Stéphane Gagno
Village des Tortues (SOPTOM)
83590 Gonfaron
France
Homepage: www.villagetortues.com
journal of the German Society for Herpetology
and Herpetoculture. It publishes the results of
including systematics, faunistics, ethology,
ecology, physiology, conservation biology and
captive breeding.
BioSciences
Information Service (BIOSIS) of Biological
Abstracts and Zoological Record as well as
Herpetological Contents and other review
organs.
www.salamandra-journal.de
RADIATA 22 (2), 2013 15