Vol.2, No.6, 646-653 (2010) Natural Science
http://dx.doi.org/10.4236/ns.2010.26081
Role of the mental foramens in dolphin hearing
Vyacheslav Ryabov
Karadag Natural Reserve NAS of Ukraine, Feodosia, Crimea, Ukraine;
Received 4 March 2010; revised 16 April 2010; accepted 13 May 2010.
ABSTRACT
The role of mental foramens in dolphin hearing
was studied in the present work. To this effect
the mental foramens’ morphology features wh-
ich are essential from acoustical viewpoint have
been studied. The patterns of relationship be-
tween the location of mental foramens and their
sizes are found. The affinity of the mental fora-
mens’ morphology and acoustics that the na-
ture had created testify acoustical expediency
of the mental foramens’ architecture. This nat-
ural inference in the main is confirmed in this
work by the experimental data. The mean values
of detection thresholds of short broadband sti-
muli with spectral maximum on frequencies 8,
16, 30 and 100 kHz at acoustical shielding the
mental foramens increased on 30, 34, 40 and 50 dB,
respectively. Results obtained testify that the
mental foramens are the unique sound-condu-
cting pathway into the fat body of the mandibu-
lar canal for the sounds of all frequencies used
in the experiment, approximately 6−150 kHz (in
view of stimuli broadbandness). The left and
right row of the mental foramens together with
respective mandibular canal plays the role of
pinna and external auditory meatus if to use
conventional terminology of a land mammal ear.
But it is already qualitatively the new external
ear implemented by the nature as the receive
array and acoustical horn. The new external ear
has apparently appeared in result of the dol-
phins’ ancestors’ adaptation to new environ-
ment conditions, as evolutionary adaptation of
the ear to the water and as functional adaptation
of the ear in order that to fulfill the new more
sophisticated functions in the structure of sonar.
The findings give good reason to suppose ex-
istence of the same external ear in Odontoceti.
Keywords: Dolphin; Hearing; Architecture of
Mental Foramens; Lower Jaw; Unique Pathway;
Sound Conduction; The Traveling Wave Antenna;
External Ear
1. INTRODUCTION
The mechanisms of Odontoceti hearing attract research-
ers for many years, therefore, a lot of works have been
performed in this study area and different hypotheses
about sound-conduction mechanisms to a cochlea were
suggested in the works. Some researchers assume that
the sound passes to the cochlea through the external aud-
itory meatus and the middle ear [1-3]. However, there is
an opinion that the auditory meatus cannot participate in
sound transmission to the middle ear in general [4-6], or
transmits sounds with frequencies under 30 kHz [7,8].
On the grounds of hypothesis related to the sound con-
duction through the external auditory meatus, authors
discuss the possibility of functionally specific subsys-
tems of passive (1-10 kHz) and active (about 100 kHz)
hearing [9].
Norris [10] supposed that the sound can be conducted
into the fat body of mandibular canal through the mental
foramens. Though, little later he suggests different hypo-
thesis [11] which is being generally accepted so far, to
the effect that the sound passes into the mandibular fat
body directly through the postero-lateral wall of mandi-
ble in the area which he called the “acoustic window”.
Two “acoustic windows” were determined in electro-
physiological experiments: one is for low frequencies
(16-22.5 kHz), which is located in the area of external
auditory meatus, and the other one - for high frequencies
(32-128 kHz) which is located at a distance of 9.3-13.1 cm
from melon tip (i.e. proximal part of a lower jaw) [12].
It has been also revealed that the fat body conducts the
sound to the lateral wall of the tympanic bone, where its
thickness is minimum, and the wall play a part of the
tympanic membrane, transmitting acoustical vibration to
the malleus of the middle ear [6,7,10,11,13-16]. Acoustic
stimulation of a lower jaw excites considerable evoked
potentials in the dolphin’s central auditory system [7].
However, the areas of maximum sensitivity of the man-
dible surface to sounds of the contact point emitter (size
smaller than wavelength) in each work are different, and
the test results do not explain the sound-conduction me-

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647
chanisms [6,7,17,18].
Acoustic shielding of the lower jaw significantly hin-
dered the dolphin’s ability to discriminate between tar-
gets in the echolocation task [19]. Results of these ex-
periments support hypothesis that the lower jaw has a
role in the reception of high frequency signals and their
transmission to the middle and inner ear. However, the
acoustical shield covers the lower jaw from the rostrum
tip up to the bases of pectoral fins, i.e. it covers both
mental foramens, and ”acoustic windows”, and ventral
area of the head, therefore the pathways and mechanisms
of sound-conduction so far remain unclear. Along with
this, the possibility of simultaneous participation of the
auditory meatus and “acoustic windows” for sound con-
duction to the cochlea at forming spatial auditory image
by a dolphin were also discussed [20].
Thus, results presented in the review are inconsistent
and the main questions about the pathways and mecha-
nisms of sound-conduction still remain without answers.
At the same time the results [21-23] give good reason to
consider the mental foramens as the unique sound-con-
duction pathway into the fat body of dolphin’s mandibu-
lar canal. The morphology analysis of the dolphin’s lo-
wer jaw and subsequent modeling confirm this assump-
tion and afford ground to assume that the left and right
row of mental foramens from acoustical viewpoint rep-
resents the traveling wave antenna which is located in
the throat of acoustical catenoidal horn (the left and right
mandibular canal plays a role of horn). The model with
two traveling wave antennas explains mechanisms of the
sound-conduction and directivity of the peripheral part
of dolphin’s auditory system [21-25]. In agreement with
this, anatomical structures of each half of the lower jaw
(mental foramens, mandibular canal and fat body) are
considered as the components of this part.
We study the role of mental foramens in the dolphin
hearing through experiments. The specific tasks of this
work are study of the morphology features of mental
foramens that are essential from acoustical viewpoint
and experimental measurement of the detection thresh-
olds dependence of wide-band acoustical impulses with
spectral maximum on different frequencies at mental
foramens’ acoustical shielding.
2. METHODS
2.1. Subject and Experimental Conditions
The mandible bones of adult bottlenose dolphin (Tur-
siops truncatus p.) were used for studying the morphol-
ogy of mental foramens. In order to perform the meas-
urements the mandible was sawn in the area of mental
foramens. Further in the text if it was necessary, mental
foramens was indicated with the number, as MFn, where
n—is the number of foramen, counting from the rostrum
tip; n = 1, 2, 3, 4. The cross section dimensions of men-
tal foramens were measured in the plane of maximum
width of each foramen, and then in the mutually perpen-
dicular plane.
The experimental studies were carried out with adult
Black Sea dolphin (Tursiops truncatus p.) which earlier
never participated in acoustic experiments, on the base
of Karadag Natural Reserve of NAS of Ukraine in the
indoor pool of 27.5 × 9.5 × 4.5 m.
2.2. Experimental Procedure
The experiments were carried out with using behavioral
response techniques (operant conditioning with food
reinforcement). A go/no-go response paradigm [26] was
used for reporting stimuli condition. The experimental
facilities were laid out as follows (Figure 1). The dol-
phin was trained to approach the testing platform (1)
upon a trainer’s signal, where the trainer was putting on
(or was not putting on) the acoustic shield (Figure 2) on
its rostrum (mental foramens’ area). After that the dol-
phin upon the trainer’s signal approached the start ball,
and stood touching it by the rostrum tip. Then the re-
searcher switched on the stimulus (or not switched on
the stimulus). The stimuli were presented over a period
of 4 seconds with repetition frequency 3/second. The
dolphin reported that detects the produced stimulus by
leaving the start position within 4 s of the trial beginning
and touching the signal ball with his rostrum, or if the
stimulus was not produced by remaining on the start
position within 4 s till a trainer’s signal.
Figure 1. The experiment configuration.
1-the testing platform, 2-the stimulus tra-
nsmitter, 3-the start ball, 4-the signal ball,
5-the dolphin in start position. The dol-
phin, start ball and stimulus transmitter
are located 1 m below the water surface.
Signal ball is located near the water surf-
ace. The distance between transmitter
and pool wall is 3 m, the distance betw-
een transmitter and start ball is 2 m.

V. Ryabov / Natural Science 2 (2010) 646-653
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In these cases the dolphin receives a fish reward for
the correct response. The acoustic shield was removed
by the trainer every time if it was needed for fish reward.
The errors (either dolphin was not approached to the sig-
nal ball when signal was produced, or was approached to
the signal ball when signal was not produced - false ala-
rm) were not rewarded.
The producing or not producing of stimuli in each trial
was determined in random order (but not more than
three similar stimuli in sequence). From trial to trial, the
stimuli level varied by a one-up-one-down staircase pro-
cedure. The session began from a stimuli level well
above the anticipated threshold the warming-up part of
the session. From trial to trial, the stimuli level varied
according to the animal’s response in the preceding trial.
If the animal detected the stimuli, the level in the next
trial decreased one step down. If the animal missed the
stimuli, the stimuli level in the next trial increased one
step up. The steps were 6 dB in the warming-up part of
the session, until the first miss. After that, the steps were
3 dB the measurement part of the session. Responses in
stimulus-absent trials did not influence the level in
stimulus-present trials. The last one or two trials were
again well above the threshold to make sure of ending
the session with rewards the cooling-down part of the
session. About twenty trials were performed at each step
of intensity variation. The threshold was calculated as
the mean of all reversal points’ maxima and minima of
the staircase succession.
2.3. Instrumentation
The short broad-band acoustical impulses were used to
decrease the effect on experiments results of the direct
and reflected signals interference in the pool. Spherical
acoustic transducers of 50 or 20 mm diameter were ex-
cited by rectangular electrical impulses with the duration
of 17 or 5 µs in order to obtain stimuli (the simulated
dolphin-like echo-location clicks) with the energy spec-
trum maximum on frequencies 30 or 100 kHz, respec-
tively. In order to obtain acoustical impulses with the
energy maximum on frequencies 8 or 16 kHz, the trans-
ducer of 50 mm diameter was excited by rectangular
electrical impulse through octave-band filter with center
frequency of 8 or 16 kHz, respectively. The duration of
each stimulus did not exceed 3 periods of the frequency
of its spectrum maximum. In this case the reflections
from pool walls and surface of water did not overlap the
stimulus, as they were coming with sufficient time delay
and were significantly weaker in comparison with the
direct signal. Thus, at the stimuli levels near the hearing
thresholds, the reflections levels were lower than the
detection thresholds of the stimuli. This allows perform-
ing the measurements without special sound absorbers.
The acoustically opaque shield of the mental foramens
(Figure 2) was made as per the shape of dolphin’s ros-
trum so that the shield was put on tightly on the rostrum;
the length of the shield was about 15 cm. The shield is
produced of the foamed (with closed-cell) neoprene sh-
eet of 5 mm thickness. This waterproof and oil-resistant
material has enough durability. Therefore, the efficiency
of sound-shielding that gas bubbles of the material pro-
vide remains the same for a long time. The acoustical
shielding efficiency of this material was tested before the
experimentation with using simulated dolphin-like echo-
location clicks with spectral maximum on frequencies
either 10 or 55 or 170 kHz. Shielding the hydrophone
with this material decreased the peak sound pressure
level of these dolphin-like clicks up to 28, 32 and 36 dB
respectively, for normal incidence of sound.
It is necessary to note, that in frequency range of
stimuli used in this experiment the sound wave-length
vary from 1.5 to 20 cm, i.e. sizes of the acoustic shield
on low frequencies are relatively small in comparison
with the wave-length. Considering it, the shield covers
the upper and lower jaw for providing the best sound
shadow in the region of mental foramens (Figures 2 and
3). If to make the shield form as external surface of the
mandible (i.e. shield only lower jaw), then necessary
shielding efficiency of the mental foramens, apparently,
cannot be received owing to sound diffraction even on
the frequency 100 kHz (the sound wave-length 1.5 cm),
because distance from edge of the shield up to the men-
tal foramens in this case will be comparable with the
wave length (Figure 3), let alone low frequencies.
Figure 2. The dolphin is near testing platform with the acous-
tically opaque shield on his rostrum.

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(a)
1cm
(b)
Figure 3. (a) The nasal part (symphysis) of
the lower jaw shown in ventral view. In the
cross section are presented the layout of MFs
of the left (L) and right (R) half of the lower
jaw and the region of lower jaw covered by
the acoustical shield. The arrows show the
projection of external orifices of MFs on the
adjacent halves of the lower jaw; (b) The
characteristic shape of the oblique endings of
mental foramens shown on the left lateral
side of the dolphin’s rostrum.
3. RESULTS
3.1. Acoustical Features of the Dolphin’s
Mental Foramens
The lower jaw’s morphology of Odontoceti is similar [11,
27]. The mandible is presented by two rectilinear hollow
bones, connected together (in shape of the V or Y letter)
in the nasal part along midline of the mandibular sym-
physis. The cavity of mandible is filled up with the fat
body and the corresponding neurovascular bundle. In the
area of symphysis the mandibular canal (MC) is pierced
by the mental foramens in the places where the branches
of mental nerve (n. mentalis) and blood-vessels go out of
the mandibular canal on an external surface of mandible.
The cross section size of the neurovascular bundle is
much smaller than the sounds wavelengths propagating
along the mental foramens and the mandibular canal.
The acoustic impedance of tissues filling lower jaw's
canals is close to the impedance of sea water [28]. Hence
these tissues do not introduce the acoustic heterogenei-
ties and they are transparent for sound. The walls of
mental foramens and mandibular canal are acoustically
elastic. Using these preconditions, the sound conduction
via mental foramens and mandibular canal has been
analyzed on the basis of lower jaw's canals geometry [21,
22,23,25].
Let’s consider in details the most essential features of
structure, shape and sizes of the mental foramens of the
dolphin (Tursiops truncatus p.) from acoustical view-
point. The most vivid feature is the amount of the mental
foramens. On the left half of the lower jaw there are
three of them (Figure 3); on the right half - four, what
are characteristically not only for the bottlenose dolphins,
but for the others species of Odontoceti [11,27].
The mental foramens have frontal direction and obli-
que ends. The oblique endings of first foramens are so-
mewhat shorter as compared to the others. The distances
between the mental foramens are decreasing with the
distance from rostrum tip (Table 1). It’s quite curious
Table 1. The mental foramens’ basic dimensions of Black Sea
dolphin (Tursiops truncatus p.). The first and second numbers
in column “MF cross section area” are the values from external
and internal end of the foramen, respectively. The lengths of
MFs are indicated without taking of the oblique ends into ac-
count.
MF
number
MF
length
(mm)
MF cross
section area
(mm2)
Distance to
the next
MF (mm)
Length of
oblique end
(mm)
1
left
right
10
11
8.04; 8.5
7.28; 8
50.2
36.1
7
8
2
left
right
27.5
35
5; 7.7
4.29; 6.04
31.2
31.5
11
13.5
3
left
right
12
20
5; 5.7
4.12; 4.94 19.15
11
11.5
4
right 10.5 1.53; 1.53 10

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650
that between the foramens’ location and their sizes there
are certain patterns of relationship. The mental foramens
of the right half of lower jaw is longer than the foramens
with the same number of the left half, i.e. if the length of
mental foramens is denoted as Lmn, where m is the left (ℓ)
or right (r) half of the lower jaw: m = ℓ or r; n is the
index of the mental foramens: n = 1, 2, 3 or 4; then,
obviously, Lr1 > Lℓ1, Lr2 > Lℓ2 and Lr3 > Lℓ3. Besides, if
the foramen is further from the rostrum tip, the length of
the foramen (except of MF1) is decreasing, i.e. Lr2 > Lℓ2
> Lr3 > Lℓ3 > Lr4. The length of MF1 in each half of the
mandible is essentially shorter than the distance between
external orifices of MF1 and MF2. At the same time the
length of MF2 and MF3 in the right half of the mandible
is few mm longer than the respective distances between
external orifices of MF2-MF3 and MF3-MF4 (Table 1).
Whereas, the length of MF2 in the left half is a few mm
shorter than the distance between the orifices of
MF2-MF3 (Figure 3). In the area of mental foramens
the cross-sectional dimensions of the mandibular canal
and its cross-sectional area gradually increases (caudal),
in respect to each mental foramen. Whereas the
cross-sectional dimensions of the mental foramens are
decreasing with the distance from rostrum tip, though
not as harmoniously as their lengths. Due to this the ratio
of cross-sections areas of MFn/МC in the area of mental
foramens is decreasing (caudal).
But the most obvious and interesting is that the mental
foramens of the left and right halves of the lower jaw
(except of MF1) are located asymmetrically relatively to
longitudinal axis of the animal, though the both halves of
the lower jaw are symmetric. Due to the non-equidistant
and asymmetrical locations of the mental foramens on
both halves of the mandible the projection of each men-
tal foramen (beginning from MF2) on the adjacent half
of the lower jaw is located between respective mental
foramens (arrows in the Figure 3), i.e. the location of
mental foramens and their lengths are mutually comple-
mentally. This vivid result of morphology study obvio-
usly shows to us the affinity of acoustics and morphol-
ogy of mental foramens that we see in their left-right
mutually complementary asymmetry.
3.2. Influence of Acoustic Shielding of
Mental Foramens on Dolphin Hearing
In the present study, we examined the effect of acousti-
cal shielding of the mental foramens on the detection
thresholds by the dolphin of the short broad-band sound
stimuli with different spectrum maximums of 8, 16, 30
and 100 kHz. The results of measurements are presented
in form showing up the relative impairment of the dol-
phin’s hearing at acoustical shielding of the mental fora-
mens in dependence on the frequency of the stimuli
spectrum maximum (Figure 4). In all tested frequency
range (6-150 kHz, taking into account the broadbandness
of stimuli), the shielding efficiency is high and increases
with the frequency from 30 to 50 dB.
The sound-wavelength in water in dependence on fre-
quency is presented on the same figure for best under-
standing of the results. The fact that the shielding effi-
ciency dependence on the stimuli frequency is a sort of
mirror reflection of the sound-wavelength dependence
on frequency (Figure 4) proves that the shielding effici-
ency is inversely proportional to the stimuli wavelength
and it means that the shielding efficiency is determined
by the stimuli wavelength.
The absolute values of the detection thresholds of
stimuli measured in this work agree with the bottlenose
dolphin audiogram [29], taking into account phenome-
non of the hearing energy summation [30,31]. Therefore,
the dolphin, which was used in our experiment, has got
normal hearing.
4. DISCUSSION
The studying of morphology features of the mental fo-
ramens was rather the successful but has created the new
question. With what purpose the nature has created such
complex harmony of mutually-complementary asym-
metric architecture of the mental foramens? For the an-
swer to this question it is necessary to take into account
the fact that the left and right row of the mental fora-
mens we considered like the traveling wave antenna and
the lower jaw as a peripheral part of a dolphin’s hearing
system. Therefore we consider the studied morphology
features of the mental foramens from viewpoint of the
linear arrays’ theory and physical acoustics. In this case
[21-25], the mental foramens being acoustically narrow
waveguides within the range of dolphin’s hearing fre-
quencies conduct the sound into the fat body of man-
dibular canal without distortion and define its intensity.
The length of mental foramens and their location defines
the delays of sound conduction in the traveling wave
antenna. In other words, the mental foramens set the
amplitude and phase distribution of particles velocities
of the left and right antenna arrays. The oblique ends of
the mental foramens, being the waveguides endings,
adjust the active acoustical lengths of the mental fora-
mens depending on frequency. The mental foramens are
functioning as elementary receivers of arrays and the
structure of their location defines the beam pattern of
each antenna array. Therefore, mutually-complementary
asymmetric architecture of the mental foramens is ap-
parently necessary in order that to form the features of
beam patterns of the left and right arrays. The prelimi-
nary calculations point out that beam patterns of the left

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δ, dB
λ
,cm
δ
λ
Figure 4. (δ) The ratios of mean values of the detection thr-
esholds of stimuli at shielding of the mental foramens to the
thresholds without shielding in dependence on the frequency of
the stimuli spectrum maximum (F). The sound wavelength in
water (λ) in dependence on frequency (F).
and right traveling wave antennas mutually intercross in
ventral direction [21]. Due to this we can assume that in
the medial plane of this area the beam patterns shape is
mutually-complementary too.
The morphology features of the mental foramens that
we considered in this study point out that their architec-
ture is subordinated to the acoustical expediency. This
natural inference in the main is confirmed by the ex-
perimental data reviewed below.
At shielding the mental foramens the mean values of
detection thresholds of stimuli with spectrum maximum
on frequencies 8, 16, 30 and 100 kHz increased on 30,
34, 40 and 50 dB, respectively (Figure 4). It means that
the shielding significantly impairs the dolphin’s hearing
both on the low frequencies and on the frequencies of
dolphin's whistling and in the echolocation frequencies
range. This new original results testify that the mental
foramens are the unique sound-conducting pathway into
the fat body of the mandibular canal in the frequency
range at least from 6 kHz and up to 150 kHz (in view of
stimuli broadbandness). This fact does not give any cha-
nces for the sound conduction along the other hypothe-
tical pathways [1-3,9,11,12,17,18,20].
At the same time the detection thresholds decreases by
20 dB with increase in the wavelength of stimuli. This
fact can be explained by increase in penetration of the
sound beyond the bounds of the shield as a result of dif-
fraction. Indeed, the sound wavelength significantly in-
creases when the frequency lowers (Figure 4), and if on
frequencies about 100 kHz the shield length 10 times
more of the wavelength, then for the frequencies about
16 kHz the linear dimensions of the shield is becoming
comparable to the wavelength and on frequencies of
about 8 kHz the dimensions of the shield are even
shorter than the wavelength. Therefore, as it follows
from the theory and the results (Figure 4, 100 kHz), in
this waves lengths range, the shielding efficiency is in-
creasing with increase of the ratio of the shield length to
the wavelength.
Thus, if to take into account the diffraction effect, so
becomes clear, that the shielding efficiency of the mental
foramens would be identical if the ratio of the shield
dimensions to the wavelength on the measured frequen-
cies was constantly. It is also confirmed by the fact that
the detection thresholds is vary in inverse proportion to
the wave length, (Figure 4). Unfortunately, on the low
frequencies it is difficult to perform the same ratio of the
shield dimensions to the wavelength, as for frequency of
100 kHz, because the shield dimensions must be too big
(for example, 1.875 m, for the frequency of 8 kHz).
Therefore, the efficiency of the used shield decreases
with lowering frequency.
In view of obtained results the assumption that the
morphological structures of lower jaw is the specialized
peripheral part of dolphin’s auditory system [21-25] that
was based on the morphology study and modeling has
now the additional experimental confirmation. Indeed,
the affinity of the architecture of mental foramens and
acoustics which the nature had created (Figure 3) ap-
parently points out for us that the left and right row of
mental foramens together with respective mandibular
canal plays a role of external ear, if to use conventional
terminology of a land mammal ear. But it already is
qualitatively the new external ear implemented by the
nature as the receive array and acoustical horn [21-25].
Moreover, each row of mental foramens being the re-
ceive array along with sound-conduction participates in
the hearing directivity creation similarly to a land mam-
mal pinna. Whereas, the mandibular canal being the
acoustical horn plays a role of both matching device and
part of the traveling wave antenna and like an external
auditory meatus transmits a sound through a fat body to
a tympanic bone's lateral wall [21-25] which plays a role
of tympanic membrane [6,7,10,11,13-16].
The new external ear has apparently appeared in result
of dolphins’ ancestors’ adaptation to new environment
conditions, as evolutionary adaptation of the ear to the
water and as functional adaptation of the ear in order that
to fulfill the new more sophisticated functions in the
structure of sonar.
Though the shielding efficiency of mental foramens
on frequencies lower than 8 kHz was not measured in
this work, we can assume that the mental foramens are
the unique way for the sound-conduction even on fre-
quencies lower than 8 kHz. The constant inclination
steepness of the low-frequency branch of a dolphin’s
audiogram (9-10 dB/octave) [29] from 50 Hz and up to

V. Ryabov / Natural Science 2 (2010) 646-653
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20-30 kHz is evidencing in favor of this.
5. CONCLUSIONS
It is known that the Odontoceti experienced a number of
functional and morphological modifications in the proc-
ess of secondary adaptation to the aquatic habitat. Ap-
parently, the new functions of mental foramens and
mandibular canal in quality of the external ear that were
discovered in this work belong to these modifications.
The findings of this work give good reason to suppose
the existence of the same external ear in Odontoceti.
6. ACKNOWLEDGEMENTS
Author would like to thank Sveta Yahno and Nadya Zhukova for the
dolphin training. The experimental part of study was supported by the
Ukrainian State fund of fundamental researches within the bounds of
the joint project “sffr - rfbr - 2009 Ф28.4/024” with Russian Founda-
tion for basic researches.
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