Cuvier's beaked whale (Ziphius cavirostris) head tissues: physical properties and CT imaging
1 Scripps Institution of Oceanography, University of California, San Diego,
9500 Gilman Drive, La Jolla, CA 92093-0205, USA
2 San Diego State University, 5500 Campanile Drive, San Diego, CA 92182,
USA
* Author for correspondence (e-mail: mhock{at}ucsd.edu)
Accepted 29 March 2005
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Summary |
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Key words: Cuvier's beaked whale, Ziphius cavirostris, physical property, sound speed, density, Hounsfield unit, elastic modulus
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Introduction |
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Sound production systems in beaked whales may be similar to those found in
other odontocetes. Comparative anatomic studies highlight homologous
structures responsible for sound generation associated with echolocation,
which are unique among mammals (Au,
1993; Cranford et al.,
1996
; Heyning,
1989a
; Heyning and Mead,
1990
; Lawrence and Schevill,
1956
; Mead, 1975
).
Convincing evidence suggests that echolocation clicks are produced at the
phonic lips [a structural complex previously known as the monkey lips/dorsal
bursae (MLDB) region; Cranford et al.,
1996
; Aroyan et al.,
1992
] and then progressively focused by the skull, air spaces,
connective tissue sheaths and gradients within the melon, before propagating
into the external environment. The melon, a complex connective tissue and
lipid structure located in the forehead region of all odontocetes
(Cranford et al., 1996
), is
likely to be important in the sound production system of Z.
cavirostris (Fig. 1).
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Morphological studies of homology across odontocetes suggest that the sound
reception system in beaked whales is similar to the system found in
better-studied odontocete species (Au,
1993). The odontocete sound reception pathway involves
transmission through bone and lipids of the lower jaw, rather than through the
tympanic membrane, as is typical in terrestrial mammals. Sound waves are
directed towards the inner ear complex through a system that includes fat
bodies lining the interior and exterior mandibles, thin translucent regions of
bones in the posterior portion of the lower jaws, and air sinuses
(Aroyan, 2001
;
Brill et al., 1988
; Figs
1,
2). Norris (1968) coined the
term `acoustic fats' to describe these mandibular fat bodies, as well as the
fats in the melon, due to their proposed role in sound propagation. The air
spaces are excellent acoustic reflectors that may function to shield the ear
complex from internally produced sounds and isolate them acoustically to
facilitate directional hearing (Norris,
1964
).
|
Odontocete melon and mandibular fat bodies exhibit structural and chemical
complexity that could have important implications for sound emission and
reception. Structurally, the melon is comprised of gradations of tissues, with
a central core of acoustic lipids grading into complex musculature, dense
connective tissue (theca) and blubber, with notable topographical asymmetry
(Heyning, 1989a;
Cranford et al., 1996
;
Fig. 1). Studies of the
chemical composition of melon acoustic lipids and mandibular fat bodies
suggest complex chemical topography associated with acoustic functionality
(Blomberg and Lindholm, 1976
;
Koopman et al., 2003
;
Litchfield et al., 1973
;
Varanasi et al., 1982
). Sound
speed in melon lipids has been shown to vary with chemical composition and
topography in the melon (Blomberg and
Jensen, 1976
; Blomberg and
Lindhom, 1976
; Flewellen and
Morris, 1978
; Goold et al.,
1996
; Goold and Clarke,
2000
; Litchfield et al.,
1973
,
1979
;
Norris and Harvey, 1974
;
Varanasi et al., 1982
). Sound
speed in the tissues surrounding the melon has not been studied. Understanding
structural and compositional heterogeneity, and the resulting tissue physical
properties, is invaluable for developing a good representative model of the
acoustic pathways in the head of an odontocete whale.
To build representative acoustic-structural models of a Cuvier's beaked whale, high-resolution sound speed, density and elasticity measurements are needed; however, no data exist in the literature for this species. Fortunately, in 2002, we obtained a freshly stranded neonate Cuvier's beaked whale specimen for study. From the head of this specimen, we present approximately 200 measurements for tissue sound speed, density, CT scan Hounsfield unit (HU) values and elastic modulus that provide a basis for which to build an acoustic-structural model. We show that our methods provide comparable results to those from aquatic and terrestrial mammals. We show that sound speed values and topographical variations in a neonate Cuvier's beaked whale melon are similar to those in more-studied odontocete species. Finally, we provide a predictable relationship between the Hounsfield units from CT scans and measured tissue sound speed and density values.
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Materials and methods |
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We analyzed the CT scan data after the dissection to determine HU values
for the volumes representing the tissue samples used for physical property
measurements. We used E-film, a specialized software package (Merge-efilm,
Milwaukee, WI, USA), to convert CT units of electron density into calibrated
HUs. HUs are scaled from 1000 to >1000, where air is at 1000,
room temperature water is at 0, and hard bone can be found at >1000
(Robb, 1998). Mammalian soft
tissues generally range between 100 and 100, with fatty tissues at the
low end and denser connective tissues at the high end
(Duck, 1990
). HUs are
standardized by defining water as zero and are calibrated such that one unit
is equal to the change in the density of 1 cm3 of water raised
1°C at standard temperature and pressure. We referred to photos taken
during the dissection to locate the position of dissected tissue samples in
the CT scan images. We chose the CT image that corresponded to the center of
each dissection slice by referring to landmarks such as the skull, eyes and
ears. Samples were picked by visually correlating the CT image with the photo
of each slice. HU values within a 50 mm3 region were used to
calculate a mean HU and standard deviation at the approximate center of each
sample.
Dissection
A dissection was performed after the CT scan, with extra attention and
finer scale sampling in the forehead and mandibular regions (see
Table 1 for sample
descriptions). The forehead was cut transversely into 2 cm-thick slices for a
total of 15 slices (Fig. 3A,B).
Each slice was further cut in a grid-like fashion into 2 cm cubes
(Fig. 3C) to provide
high-resolution topographical data. The left mandibular region was also cut
transversely into 2 cm-thick slices for a total of 12 slices. For each slice,
one sample was taken from the blubber and the exterior mandibular fat, and
15 samples were taken from the interior mandibular fat. Each cubic
sample was numbered, notched on the anterior right dorsal corner, and stained
on the anterior side with Basic Fuchsin
(Lillie, 1977
) to provide
sample location and orientation reference during post-dissection evaluation.
The dissection was recorded photographically with a 5 megapixel digital color
camera (Sony Cyber-shot DSC-707).
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Tissue samples were visually categorized by color, texture and location within the head into the following broad categories: blubber, acoustic fats (including melon and mandibular fat), muscle, and connective tissue. All samples containing a mixture of these categories were discarded from further analysis to provide homogeneous tissue samples for physical property measurements. Note that the tissue samples were categorized subjectively and, while more accurate descriptions of tissue could be determined by histological analysis, such an investigation was not within the scope of our analysis.
Sound velocity
Sound velocity was measured in each tissue sample using a Krautkramer
Branson USD10 Ultrasonic Digital Flaw Detector and two Krautkramer Branson
longitudinal acoustic transducers (Alpha series, 10 MHz, 0.25 mm;Krautkramer,
General Electric Inspection Technologies, Hverth, Germany) attached to digital
calipers (model no. CD-8''CS, Mitutoyo America Corp., Aurora, IL, USA).
The Krautkramer velocimeter system measured the transmission time of 10 MHz
broadband pulses through the various tissue samples. The calipers were used to
measure the sample thickness to the nearest millimeter, and velocity was
calculated by dividing the thickness by the travel time. Prior to measuring
the samples, the velocimeter was calibrated using distilled water at room
temperature (22.5°C), assuming a sound speed of 1490 m
s1 (Chen and Millero,
1977). Sound velocity was measured along three directions of each
sample cube (anteriorposterior, dorsalventral, lateral) to test
for anisotropy. Temperatures of the samples were recorded for later
temperaturesound velocity normalization. Ultrasonic attenuation
measurements were also made. Results of sound velocity anisotropy and
ultrasonic attenuation are reported by Soldevilla et al.
(2004
). Sound velocity
measurements were made at ultrasonic (10 MHz) frequencies, because short
wavelengths are necessary to measure small tissue samples, and dispersion
effects are expected to be small (see below).
Temperature and sound velocity
Temperature affects sound velocity in tissues. We were interested in
estimating in vivo (37°C) sound velocity rather than room
temperature (21°C) sound velocity so that future models can be
representative of a living animal. A sample of each type of tissue was placed
in consecutive water baths at temperatures ranging from 10 to 37°C until
the sample reached equilibrium, and its sound velocity was measured for each
temperature.
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Density
Density was calculated using Archimedes' principle. We measured volume by
immersing each sample in distilled water and weighing the displaced water.
Water temperature was monitored to allow accurate calculations of density from
water volume measurements. Wet mass was determined by direct measurement with
a Mettler PM 460 Delta Range mass balance (Mettler-Toledo Inc., Columbus, OH,
USA). Density was calculated by dividing mass by volume.
Statistical analysis
Sound velocity measurements were averaged across orientations, resulting in
non-directional sound speed. We performed one-factor analyses of variance
(ANOVAs) to compare values among tissue types for sound speed, sound
attenuation, density, HU and hysteresis, followed by two-factor ANOVAs to
compare values between locations (forehead or mandible) for blubber and
acoustic fats (Zar, 1999).
ANOVAs were followed up with a Scheffe post-hoc analysis
(Zar, 1999
). All ANOVAs were
performed using SYSTAT (Systat Software Inc., Point Richmond, CA, USA).
Student's t-tests were run to compare tissue type values with those
of seawater at 15°C, 1 atm. (=101 kPa) and 35
salinity. Linear
regressions and a principal components analysis (PCA) were run to investigate
the relationships between the various measures. First, we averaged the values
from the three orientations for sound velocity and elasticity. This enabled us
to compare these measures to the density and HU measures that only had one
value per sample. We performed linear regressions to determine the
relationship between HUs and each of the other measures. We used PCA to
determine the independence of the four physical property variables and to
isolate the most important modes of variability in the data. PCA seeks linear
combinations of variables that maximally explain the variance in the data
(Jackson, 1991
). The variance
of a large data set is concentrated into a small number of physically
interpretable patterns of variability the principal components.
Patterns among the physical property variables are illustrated by loadings on
the principal components (Jackson,
1991
).
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Results |
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![]() | (1) |
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The organizational structure of the forehead tissues has a strong effect on sound speed. Across a lateral plane, sound speeds are lowest in the center of the forehead. However, in the posterior region, the low sound speed center shifts toward the right (Fig. 6). This change in sound speed is evident within the melon tissue as well as between tissue types. From anterior to posterior, the sound speeds are higher toward the rostrum where the melon fat grades into blubber. This increase is also evident toward the front of the forehead in the dorsalventral direction. In general, a low sound speed core is present that converges posteriorly, exhibiting spatial asymmetry and strong sound speed gradients. Anteriorly, the core is broader in size/shape, spatially symmetric and exhibits a low gradient toward the dorsal rostral side. Laterally, the gradient is sharp throughout the melon.
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Mass density
Cuvier's beaked whale tissue densities ranged from 854 to 1175 kg
m3. All tissues exhibited significantly different density
values from one another except blubber and acoustic fats. Connective tissues
were densest, followed by muscle and then the fatty tissues
(Table 3;
Fig. 5). Acoustic fats in the
mandibular region were significantly lower than forehead acoustic fats and all
blubber tissues (P<<0.001). Seawater with a salinity of 35,
at 15°C and 101 kPa has a mean density of 1026 kg m3
(Pilson, 1998
). Fatty tissues
(P<0.05) were significantly less dense than seawater, while muscle
was not significantly different, and connective tissue was significantly
denser than seawater (P<0.05).
Hounsfield units
Hounsfield units in Z. cavirostris tissues ranged from 106
to 108 HU, with sampling standard deviations ranging from 1 to 30 HU. All
tissues exhibited significantly different HU values from each other
(P<<0.001), except for acoustic fats and blubber. Again, connective
tissues exhibited the highest values, followed by muscle and then fatty
tissues (Table 3;
Fig. 5). In the mandible,
blubber HU values were significantly lower than those of acoustic fats and
forehead blubber (P<<0.001). Blubber, acoustic fats and muscle
values were significantly lower than those for water, while connective tissue
exhibited HU values significantly higher than those of water
(P<0.05).
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Percent hysteresis values in Z. cavirostris tissues ranged from 0.6 to 88.8%. Connective tissues exhibited significantly higher percent hysteresis than lipids (P<<0.001), while acoustic fats exhibited significantly higher percent hysteresis than blubber (P=0.01; Table 3). Lipids in the mandibular region had lower percent hysteresis than lipids in the forehead (P=0.03).
Correlations
Principal components analysis was run to search for combinations of tissue
properties that vary together systematically. The results show that the first
principal component explained 72% of the variance, while the second component
explained 23%. Sound speed, density and HU were explained by the first
principal component and are therefore dependant measures
(Fig. 9). Elasticity was an
independent measure and is explained by the second component. The samples are
distributed in a nonrandom pattern, with similar tissues clumping together as
shown in Fig. 10. The plot
shows that the connective tissues, muscle and lipids separate out along the
first component. On the other hand, melon and blubber can be distinguished by
the second component.
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Discussion |
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Previous studies of sound speed in the odontocete forehead have reported a
low sound speed inner core within the melon, consistent with the results of
our study (Flewellen and Morris, 1987;
Litchfield et al., 1979;
Norris and Harvey, 1974
). The
presence of a low sound speed core supports the hypothesis that the beaked
whale melon helps to produce a directional sound beam. The structure of the
forehead, involving different tissues components, may give rise to stages of
directional sound beam formation. For example, air spaces, dense connective
tissue layers and bony regions can act as acoustically reflective surfaces
that function to direct sound anteriorly. Another stage of beam formation
results from the chemical topography in the melon, which tends to focus the
beam along the low sound speed core.
The increase in sound speed between melon and blubber may be important in
impedance matching of the sound waves to the water to maximize the acoustic
energy transfer out of the forehead. Impedance matching occurs when the
acoustic impedances (the product of density and sound speed) of two adjacent
materials are similar, resulting in greater transmission of acoustic energy at
the boundary between the two materials. Another factor that influences sound
speed, and thereby impedance matching, is the change in temperature from the
interior to exterior boundaries of the blubber. At the interior, sound speed
averages 1401 m s1 for 37°C blubber, which increases to
1492 m s1 at the exterior blubber boundary in 15°C water
(see Table 2), which has a
sound speed of 1507 m s1. These results support acoustic
impedance matching between the melon, blubber and seawater as a proposed
function for the Cuvier's beaked whale melon, as proposed for the bottlenose
dolphin melon (Norris and Harvey,
1974). Sound speed in the fats in the mandibular channel also
exhibit low sound speeds compared with surrounding tissues. Again, the change
in sound speed from seawater to blubber to acoustic fat is probably important
in impedance matching for sound reception and the channeling of sound from
seawater to the ear complexes.
Sound speed values for cetacean blubber, muscle and connective tissue are
not present in the literature; however, values for terrestrial mammal fats,
muscle and collagen fibers (a main component of connective tissue) are
available. Our Z. cavirostris sound speed values for blubber at
21°C (ranging from 1453 to 1489 m s1) are consistent
with those reported for terrestrial mammals (14121489 m
s1 for human, cow, dog and pig fat; reviewed by
Duck, 1990). The sound speed
values for most Z. cavirostris muscle samples at 21°C (ranging
from 1461 to 1594 m s1) are also similar to those reported
for terrestrial mammals (15421631 m s1 for human,
cow, dog, pig and rabbit muscle; reviewed by
Duck, 1990
;
O'Brien, 1977
), but values for
muscles found in the forehead region extend lower. Forehead region muscles
appear to have increased lipid content compared with muscle tissues found
throughout the body (Soldevilla et al,
2004
), which leads to a decrease in sound speed
(Duck, 1990
). Very little has
been published on connective tissue sound speed; however, one study reports
collagen sound speed at 1570 m s1 (reviewed in
Duck, 1990
;
Lees and Rollins, 1972
).
Ziphius cavirostris connective tissue values exhibited a large range
(15381708 m s1 at 21°C), which is probably due to
the high variability in connective tissues that exhibit complex collagen
structures. Collagen content in tissues affects sound speed significantly
(O'Brien, 1977
) such that the
range of sound speed values may be associated with relative proportions of
water and collagen in the samples.
The results of our temperaturesound speed experiments are similar to
those reported by others. Our study of fats shows a linear decrease in sound
speed with increasing temperature of 4 m s1
deg.1, while muscle and connective tissue had a nearly
constant slope. Duck (1990
)
reports that sound speed in fats decreases linearly with increasing
temperature (at
10 m s1 deg.1) until
35°C, where it exhibits nonlinear behavior and decreases at a lower
rate. Studies have investigated sound speed changes with temperature in
forehead acoustic fats in marine mammals. Lipids extracted from melon tissues
decrease with increasing temperature, in the order of 113 m
s1 deg.1
(Blomberg and Jensen, 1976
;
Goold et al., 1996
;
Goold and Clarke, 2000
;
Litchfield et al., 1979
).
These studies (except Litchfield et al.,
1979
) have shown a nonlinear effect due to the phase change of the
oils near body temperature. This is an important finding since beaked whale
melon/acoustic fats have a different lipid composition, particularly in
neonates, yet the lipids exhibit the same pattern of properties. We were not
able to investigate nonlinear changes around body temperature since the
tissues in our samples exhibited a phase change. It would be important to
study this effect in beaked whale tissues in the future to better understand
how the melon functions for sound conduction.
Mass density
Mass densities of the Z. cavirostris tissue types are similar to
those found in other mammals. Fatty tissues in this study ranged from 854 to
987 kg m3, averaging 922 kg m3. Duck
(1990) reports mean human fat
densities ranging from 917 to 939 kg m3 (human breast fat)
and 990 to 1060 kg m3 (human breast including all tissues).
The lower values that we found therefore correspond to pure fatty tissues,
while the higher values include more structural tissues such as collagen in
blubber. Our Z. cavirostris muscle values (ranging from 909 to 1066
kg m3) are similar to those reported for terrestrial mammals
(ranging from 1038 to 1056 kg m3;
Duck, 1990
) but extend lower,
suggesting the presence of lipids in forehead muscle tissues.
Hounsfield unit
Ziphius cavirostris fats, including acoustic fats and blubber, had
CT scan HU values ranging from 106 to 46. Robb
(1998) found that mammalian
fat values typically lie between 100 and 70. Most of our samples
lie within this range; however, some blubber and melon fat samples from the
forehead exhibited higher HU values than Robb found. This may be due to human
error in matching the locations of the samples in the CT scan to their
geometric space from dissection. It may also indicate that changes are due to
the unique lipid chemistry of odontocete blubber and acoustic fats compared
with common mammal fats or the presence of collagen in the blubber. This is a
topic that merits further investigation.
Muscle HU values typically fall between 25 and 60 HU
(Robb, 1998). Our values
ranged from 61 to 2. These are lower than those reported by Robb, which
may be caused by lipid content in the forehead muscle, which would decrease
the HU value. Goodpaster et al.
(2000
) found that human
skeletal muscle containing high lipid concentrations exhibited lower HU values
than muscle that contained low lipid concentrations. The differences in our
values might also be due to the amount of postmortem time and
consequent decomposition in muscle tissues, perhaps the most susceptible to
these changes.
Elasticity
Elastic modulus values for Z. cavirostris tissues range between
0.1 and 1.5 MPa at stresses from 2.5 to 50 kPa. Connective tissue and blubber
exhibit the full range of values above, with elastic modulus increasing with
increasing stress. Melon, on the other hand, exhibits lower values, with a
high of 0.93 MPa at 50 kPa. The higher values for blubber and connective
tissue suggest that these tissues have greater structure than melon, which is
reasonable since connective tissue and blubber are held together by collagen
fibers. O'Brien (1977) notes
that the elastic properties of soft tissue are determined primarily by the
content of collagen and other structural proteins. Elastic moduli for tissues
vary nonlinearly with applied stress, making comparisons with previously
published values difficult as these measurements may have been carried out on
different portions of the stressstrain curve. Percent hysteresis values
for Z. cavirostris fall in the typical range for mammalian tissues.
Collagen exhibits low values (7%), while materials like silk exhibit high
values (65%), with the majority of tissues falling around 20% (Vogel,
1988).
Correlations
The results of the PCA show that connective tissue, muscle and lipids
separate distinctly along the first principal component
(Fig. 10), which represents
sound speed, density and HU (Fig.
9). This separation shows that sound speed, density or HU can each
be used to determine the type of tissue in a Cuvier's beaked whale, since each
tissue type's physical properties are distinct. The high explanatory power of
the first principal component illustrates that sound speed, density and HU are
measuring similar characteristics. This supports the notion that HUs can be
used to model density and sound speed parameters needed for acoustic modeling,
which is beneficial to modeling efforts, since HUs can be collected throughout
a whole specimen non-destructively. Hounsfield units cannot be used to predict
elastic modulus values. Elasticity, primarily represented by the second
principal component, is the only measure that distinguishes between acoustic
fats and blubber (Fig. 10).
Elasticity is important for understanding the behavior of these tissues under
high stress conditions.
The linear correlations between HUs and sound speed and density were high (0.85 and 0.76, respectively). The linear relationships we found for these measures provide a model to determine the sound speed and density when only the HUs from CT scanning are available (Fig. 11). Some of the noise in the data may be due to human error in correlating the samples to their location in the CT scans. Future studies should work to improve this method.
Limitations
Our analysis of the physical properties of Cuvier's beaked whale tissues
provides improvements on past research and insight into sound generation,
reception and acoustic trauma modeling. However, the results do have some
limitations. (1) Sound speed measurements were made at 10 MHz, while the
frequencies of interest range from 1 to 200 kHz. Sound speed dispersion for
biological soft tissues and marine mammal oils ranges from 1 to 10 m
s1 between 1 and 10 MHz (Cartensen and Schwan, 1959;
Kremkau et al., 1981;
Kuo and Weng, 1975
;
O'Donnell et al., 1981
). While
our values may be a few m s1 too high, this is within the
error of our measurements. (2) The specimen we studied was a neonate. H. N.
Koopman, S. M. Budge, D. R. Ketten and S. J. Iverson (personal communcation)
have described ontogenetic changes in the chemical makeup of acoustic fats in
beaked whales that may affect physical properties of tissues. This is an
important distinction for sound generation and reception modeling. However,
Ziphius cavirostris specimens are not readily available. The present
study is the first to present any physical property data from a Cuvier's
beaked whale. (3) Temperaturesound speed measurements were not carried
out at 37°C, our estimated in vivo temperature. Sound speed in
lipids exhibits nonlinear changes around body temperature, suggesting that our
calculated sound speeds may be a few m s1 too low. It would
be important to study this effect in beaked whale tissues in the future to
better understand how the melon functions for sound conduction. (4)
Histological analysis of tissue categories was beyond the scope of our study.
Variations in tissue content of each sample could have led to the variation we
see in our results. (5) Measurements were made on thawed, postmortem
tissues. Changes in tissue physical properties may arise after death, and with
freezing. No significant changes were found for melon fat, blubber and
connective tissue CT scan HU values in live, recently deceased and frozen
bottlenose dolphin specimens (M. F. McKenna, personal communication). No
significant differences have been found for sound speeds in bottlenose dolphin
tissues as a function of time after death and freezing (M. F. McKenna,
personal communication). The same results were found for sound speeds in fresh
and frozen mammalian liver (Van der Steen
et al, 1991
), myocardial tissues
(Dent et al., 2000
) and human
breast tissue (Foster et al.,
1984
). Elastic properties of thawed non-contractile tissues,
including human intervertebral discs and rabbit ligaments, are not
significantly different from fresh samples
(Smeathers and Joanes, 1988
;
Woo et al., 1986
). Fitzgerald
(1975
) and Fitzgerald and
Fitzgerald (1995
) suggest
there is a life-to-death transition in visco-elastic compliance for beef fat,
porcine intervertebral disc and whale blubber around 511 h
postmortem. Significant changes in the elastic properties of
contractile muscle tissues result from the postmortem and freezing
process (Leitschuh et al.,
1996
; Gottsauner-Wolf et al.,
1995
; Van Ee et al.,
2000
). In summary, sound speed and CT scan HU values are not
significantly affected by postmortem and freezing processes. Elastic
properties of non-contractile tissues may exhibit postmortem
differences but are not affected by freezing, and elastic properties of
contractile tissues are significantly affected by postmortem and
freezing effects. For this reason, we have discarded muscle from our analysis
of elastic properties.
Conclusion
This study provides high topographical resolution data on sound speed, mass
density, CT scan Hounsfield units and elasticity for Z. cavirostris
tissues. This has been the first attempt at gathering this breadth of physical
property data from beaked whale tissues. We have examined the similarities and
differences that exist between these properties from Cuvier's beaked whale
tissues, which have unique structural and chemical characteristics, and those
found in other marine and terrestrial mammals. Evidence for a relationship
between CT scan HUs and sound speed and mass density is presented. This may
allow future studies to relate CT scan values from Z. cavirostris to
these physical properties without extracting tissue samples. While there are
limitations to our methods and results, the data we present show promise for
use in acoustic-structural models of a beaked whale that may provide insight
into sound emission and reception paths and into possible mechanisms for
Z. cavirostris' sensitivity to sound.
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Acknowledgments |
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