fatty acid composition of polar bears-diet, licencjat, potrzebne publikacje
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Vol. 265: 275–282, 2003
MARINE ECOLOGY PROGRESS SERIES
Mar Ecol Prog Ser
Published December 31
Fatty acid composition of the adipose tissue of
polar bears and of their prey: ringed seals,
bearded seals and harp seals
Otto Grahl-Nielsen
1,
*
, Magnus Andersen
2
, Andrew E. Derocher
2,4
,
Christian Lydersen
2
, Øystein Wiig
3
, Kit M. Kovacs
2
1
Department of Chemistry, University of Bergen, 5007 Bergen, Norway
2
Norwegian Polar Institute, 9296, Tromsø, Norway
3
Zoological Museum, University of Oslo, PO Box 1172 Blindern, 0318 Oslo, Norway
4
Present address:
Department of Biological Sciences, University of Alberta, Edmonton T6G 2E9, Canada
ABSTRACT: Polar bears
Ursus maritimus
are predators of phocid seals, but they also forage oppor-
tunistically on a variety of other species. In the Barents Sea — Svalbard area, their diet is thought to
consist almost exclusively of ringed seals
Phoca hispida
, bearded seals
Erignathus barbatus
and harp
seals
P. groenlandica
. When a seal is killed, polar bears preferentially consume the blubber. The fatty
acid (FA) compositions of the outer, middle and inner adipose layer of 18 polar bears were compared
to each other and also with the FA composition of the blubber of their prey, represented by 10 ringed
seals, 10 harp seals and 9 bearded seals. The composition of the FAs in the inner layer of the bear adi-
pose tissue differed from the composition of the outer layer, and was also distinctly different from the
composition of the blubber from the prey. Fifteen of the 28 FAs analysed were found in lower relative
amounts in the polar bears than in any of the 3 seal species. Eight of the FAs were found in higher rel-
ative amounts in polar bears when compared to the 3 prey species. Only 5 of the FAs in polar bears
were within the range of relative values found in the prey. This strongly suggests that polar bear adi-
pose tissue has a unique FA composition that is not a straightforward mixture of what they consume,
but rather is the result of selective processes prior to and during deposition of lipids in the tissue.
KEY WORDS: Polar bear · Adipose tissue · Fatty acid composition · Prey · Seal · Blubber
Resale or republication not permitted without written consent of the publisher
INTRODUCTION
Phoca vitulina
(Iverson et al. 1997), gray seals
Hali-
choerus grypus
(Walton et al. 2000) and beluga whales
Delphinapterus leucas
(Dahl et al. 2000). However,
some studies suggest that FA profiles in the blubber or
adipose tissues of a species cannot be used reliably as
a means to determine its diet (Grahl-Nielsen et al.
2000).
Polar bears are at the top of the marine food chain in
the arctic, and feed largely on ringed seals
Phoca hisp-
ida
, bearded seals
Erignathus barbatus
, and harp seals
P. groenlandica
(Lønø 1970, Stirling & Archibald 1977,
Derocher et al. 2002) in the North Atlantic region.
Polar bears feed little from late summer through win-
The use of fatty acid (FA) signature analyses has
been proposed as a method for determining the diet of
marine mammals (e.g. Iverson 1993, Iverson et al.
1997). The underlying principle of this method is the
assumption that FAs in prey species are incorporated,
more or less unmodified, into the adipose tissue of the
predator. To assess the diet of a predator, the FA sig-
nature from its adipose tissue can be compared to the
FA signatures from potential prey species. This
method has recently been used to study the diet of a
variety of marine mammals, including harbour seals
*Email: otto.grahl-nielsen@kj.uib.no
© Inter-Research 2003 · www.int-res.com
276
Mar Ecol Prog Ser 265: 275–282, 2003
ter, with the peak feeding period occuring in spring
and early summer when substantial adipose deposits
are formed (Watts & Hansen 1987, Ramsay & Stirling
1988).
No detailed studies on polar bear diets are available.
Most of our knowledge regarding the polar bear diet is
derived from field observations of animals killed by
bears (Stirling & McEwan 1975, Hammill & Smith
1991, Derocher et al. 2002) or from stomach contents of
polar bears (Lønø 1970).
Polar bears preferentially consume the blubber of
seals (Stirling & McEwan 1975). Sampling of the bears
adipose tissue and of blubber from various seal species
could provide an opportunity to apply the FA method
to identify the bears’ diet. Iverson et al. (1999) reported
preliminary results on a study of geographic variation
in the diet of polar bears by the use of fatty acids.
FA composition of polar bear adipose tissue has not
been well described, but some investigations on wild
and captive bears have been conducted (Pond et al.
1992, Colby et al. 1993). The proportions of FA in polar
bear adipose tissue were found to be significantly dif-
ferent from those reported in the blubber of ringed
seals, considered to be their principal prey (Pond et al.
1992). The biochemical properties of the adipose tissue
in polar bears was found not to be adapted to thermal
insulation. But there was some evidence that the FA
composition was adapted to low skin temperatures
(Pond et al. 1992).
The aims of the present investigation were: (1) to
describe the FA composition of adipose tissue of polar
bears in greater detail than has been previously
reported; (2) to determine whether the FA composition
varies with depth within the adipose layer of polar
bears, as is the case for the blubber of seals and
whales; and (3) to compare the FA profiles of polar
bears with the FA profiles of their 3 main prey species
of seals, in order to explore the possibility of using FAs
to determine the diet of polar bears.
sterile biopsy punch (6 mm diameter) from the rump fat
depots to the side of the tail. Efforts were made to
obtain as deep a fat core as possible from each bear,
but the tissue samples were often loosely connected at
depth, and in a few instances the cores broke. This
increased the possibility of cross-contamination of lipid
material along the core, and made it difficult to keep
track of the various core layers. The samples were
wrapped in tin foil, and placed in a sterile glass
container and stored frozen until analysis.
Harp seals (n = 10), ringed seals (n = 10) and bearded
seals (n = 9) were collected from seals that were shot
along the ice edge in the Greenland Sea (west of Sval-
bard), Barents Sea (east of Svalbard) and the Arctic
Ocean (north of Svalbard) during 1999 and 2000. Blub-
ber samples were taken from 5 to 10 cm
2
sections col-
lected through the whole layer of the blubber, from skin
surface down to the muscle, mid-dorsally at approxi-
mately 40% of the distance between snout and tail. The
samples were wrapped carefully in aluminium foil,
then plastic, and stored frozen until analysis.
All animal handling methods were approved by the
National Animal Research Authority of Norway
(NARA; Norwegian Animal Health Authority).
Laboratory methods.
Subsamples of adipose tissue
from the bears and blubber from the seals were taken
for chemical analysis while the tissues were frozen.
From the bears, 2 parallel sets of subsamples, each
individual sample weighing between 10 and 20 mg,
were taken next to the skin, next to the muscle and
midway between. From the seal blubber, a minimum
of 8 subsamples, 4 from the inner and 4 from the outer
blubber layer, to cover the total blubber depth, were
taken from each individual. All subsamples were sub-
jected to methanolysis in thick-walled glass tubes with
Teflon-lined screw caps, using 0.5 ml dry methanol
containing 2 M HCl, for 2 h at 90°C. After methanoly-
sis, approximately half the methanolic HCl solution
was evaporated off using nitrogen gas. Distilled water
(0.5 ml) was added to the sample and the FA methyl
esters were extracted twice with 1 ml hexane.
Of the mixed hexane extracts, 1 µl were subjected to
chromatography on a 25 m × 0.25 mm fused silica col-
umn with polyethyleneglycol as the stationary phase,
with a thickness of 0.2 mm (CP-WAX 52CB Chrom-
pack), and helium at 20 psi as the mobile phase. The
column was mounted in a Hewlett-Packard 5890A gas
chromatograph, equipped with a Hewlett-Packard
7673A autosampler and a flame-ionisation detector.
The injector temperature was 260°C. The temperature
of the column was kept at 90°C for 4 min after splitless
injection, and thereafter increased to 165°C at a rate of
30°C min
–1
, followed by an increase of 3°C min
–1
to
225°C. This temperature was maintained for 10.5 min.
The flame ionisation detector was set at 330°C. Samples
MATERIALS AND METHODS
Field methods.
Bears were captured in the Svalbard
area (76–80° N, 22–29° E) in mid-April 1999 by remote
injection of a drug-filled dart (Palmer Cap-Chur
Equipment) fired from a helicopter. The drug Zoletil
vet
®
(Virbac International) was administered in a solu-
tion of 200 mg ml
–1
at a dosage of 5 to 10 mg kg
–1
of
body mass (Stirling et al. 1989). The sex and reproduc-
tive status were documented for each bear. A rudimen-
tary premolar tooth was extracted from all bears for
age determination (Calvert and Ramsay 1998). The 18
bears in this investigation were between 2 and 21 yr of
age. Adipose tissue samples were collected using a
Grahl-Nielsen et al.: Fatty acid composition of polar bear adipose tissue
277
were analyzed in random order, with a standard solu-
tion, GLC-68D (Nu-Chek-Prep), that contained 20 FA
methyl esters between every 8 h sample. Altogether, 28
FAs were identified in the samples using the standard
mixture and mass spectrometry. The peaks were inte-
grated by Atlas 2000 software (Thermo Labsystems),
and the peak areas were corrected according to re-
sponse factors determined from the 20 FA methyl esters
present in the standard mixture, and by estimation via
use of chain length and number of double bonds for the
8 FA methyl esters not present in the standard mixture.
The amount of each FA in a sample was expressed as a
percent of the sum of all FAs in the sample.
Statistical methods.
To obtain the combined infor-
mation from all 28 FAs simultaneously, the data were
subjected to multivariate principal component analysis
(PCA). The relative values of the FAs were logarithmi-
cally transformed, thereby leveling out the quantita-
tive differences among fatty acids. With each sample
positioned in the multi-dimensional space described
by the log-transformed variables (fatty acids), the 2
axes (principal components, PC) that described the
largest (PC1) and second largest (PC2) sources of vari-
ance among the samples were computed using SIRIUS
software (Kvalheim & Karstang 1987).
To quantify the observed difference between the
various prey samples and the samples from the inner
adipose layer of the polar bears, a space-filling model
for the bear samples was built by the program SIMCA
(Soft Independent Modeling of Class Analogies) (Wold
1976; Wold 1978; Ugland and Massart 1996), available
in the SIRIUS software package. The model was based
on 2 PCs of the inner adipose tissue samples. The dis-
tance of all other samples to this model was then
computed as residual standard deviation, RSD.
RESULTS
The FA composition in the adipose tissue of polar
bears and the blubber of the 3 species of phocid seals
had a typically marine pattern, with relatively high
amounts of monounsaturated and polyunsaturated n3
fatty acids (Table 1, Fig. 1). There were large differ-
ences among individuals within each species, resulting
in high SDs of the mean values.
The inner and middle layer of polar bear adipose tissue
had a similar FA composition, while the outer layer dif-
fered from the other 2 layers. However, only 10 of the 28
FAs occurred in significantly different amounts in the in-
ner compared to the outer layer (p < 0.01, Table 1). The
FAs with the largest differences were 22:1n11 and
22:1n9, with higher values in the inner layer, and 20:4n6
with lower values in the inner layer. In addition, 20:1n9
30
20
10
0
Fig. 1.
Ursus maritimus, Phoca hispida, Phoca groenlandica
and
Erignathus barbatus
. Average relative amounts of the fatty acids
(from left to right for each fatty acid) in the inner adipose tissue layer of polar bears (2 × 18 samples), and in the blubber of ringed
seals (90 samples from 10 animals), harp seals (195 samples from 10 animals), and bearded seals (125 samples from 9 animals).
Values are means + SD
 278
Mar Ecol Prog Ser 265: 275–282, 2003
Table 1.
Ursus maritimus, Phoca hispida, Phoca groenlandica
and
Erignathus barbatus
. Average amounts of fatty acids, as per-
centage of sum ± SD, in the inner, middle and outer blubber layer of polar bears, based on 2 parallel samples from each layer from
each of 18 bears, in the blubber of ringed seals, based on 90 samples from 10 individuals, in the blubber of harp seals, based on
195 samples from 10 individuals, and in the blubber of bearded seals, based on 125 samples from 9 individuals. The inequality
signs are given for the fatty acids where there is a significant difference, p < 0.01, between the inner and outer blubber layer
of polar bears
Fatty acid
Polar bear
Ringed seal
Harp seal
Bearded seal
Inner
Middle
Outer
14:0
2.7 ± 0.5
2.8 ± 0.5
2.5 ± 0.5
4.2 ± 0.8
5 ± 1
3.2 ± 0.7
14:1n5
0.5 ± 0.2
0.5 ± 0.1
0.6 ± 0.2
1.1 ± 0.5
1.4 ± 0.8
0.7 ± 0.4
iso-15:0
0.25 ± 0.05
0.26 ± 0.05
0.25 ± 0.05
0.23 ± 0.03
0.22 ± 0.04
0.21 ± 0.05
a.iso-15:0
0.11 ± 0.03
0.11± 0.02 > 0.09 ± 0.02
0.09 ± 0.02
0.08 ± 0.01
0.2 ± 0.4
15:0
0.21 ± 0.06
0.21 ± 0.05 < 0.29 ± 0.06
0.28 ± 0.04
0.24 ± 0.05
0.35 ± 0.05
16:0
5 ± 1
6 ± 1
<
7 ± 1
8 ± 3
9 ± 3
9 ± 2
16:1n7
8 ± 3
8 ± 3
10 ± 3
19 ± 3
15 ± 3
17 ± 4
16:2n6
0.3 ± 0.1
0.3 ± 0.1
0.31 ± 0.09
0.7 ± 0.1
0.54 ± 0.06
0.5 ± 0.1
18:0
2.2 ± 0.4
2.3 ± 0.4
<
2.5 ± 0.5
0.8 ± 0.3
0.8 ± 0.3
1.7 ± 0.7
18:1n9
34 ± 5
34 ± 5
35 ± 5
19 ± 5
23 ± 4
17 ± 5
18:1n7
4.7 ± 0.7
4.8 ± 0.7
<
5.4 ± 0.9
5 ± 1
5.3 ± 0.8
6 ± 1
18:1n5
0.47 ± 0.08
0.47 ± 0.09
0.50 ± 0.08
0.6 ± 0.2
0.40 ± 0.09
0.7 ± 0.1
18:2n6
1.8 ± 0.2
1.8 ± 0.2
1.8 ± 0.2
1.2 ± 0.2
1.9 ± 0.2
1.5 ± 0.4
18:3n3
0.48 ± 0.08
0.48 ± 0.08
0.48 ± 0.09
0.7 ± 0.1
0.9 ± 0.1
0.6 ± 0.1
18:4n3
0.5 ± 0.3
0.5 ± 0.3
0.4 ± 0.3
2.1 ± 0.6
3.6 ± 0.9
1.5 ± 0.5
20:0
0.11 ± 0.02
0.11 ± 0.03
0.12 ± 0.05
0.04 ± 0.01
0.05 ± 0.02
0.07 ± 0.03
20:1n9
19 ± 3
18 ± 3
>
15 ± 3
9 ± 3
7 ± 3
10 ± 5
20:2n6
0.32 ± 0.05
0.33 ± 0.05
0.31 ± 0.04
0.20 ± 0.04
0.28 ± 0.06
0.5 ± 0.2
20:4n6
0.21 ± 0.05
0.21 ± 0.04 <
0.4 ± 0.2
0.37 ± 0.08
0.31 ± 0.04
0.8 ± 0.2
20:3n3
0.06 ± 0.02
0.06 ± 0.02
0.05 ± 0.02
0.08 ± 0.01
0.09 ± 0.02
0.11 ± 0.04
20:4n3
0.4 ± 0.1
0.4 ± 0.1
<
0.6 ± 0.2
0.53 ± 0.06
0.62 ± 0.07
0.8 ± 0.2
20:5n3
1 ± 1
1.2 ± 0.9
2 ± 1
9 ± 1
8 ± 2
7 ± 2
22:1n11
2.9 ± 0.8
2.8 ± 0.7
>
1.6 ± 0.7
2 ± 1
2 ± 1
2 ± 2
22:1n9
0.9 ± 0.2
0.9 ± 0.2
>
0.6 ± 0.2
0.4 ± 0.2
0.5 ± 0.3
0.6 ± 0.4
21:5n3
0.33 ± 0.07
0.34 ± 0.06
0.30 ± 0.07
0.49 ± 0.07
0.56 ± 0.05
0.6 ± 0.1
22:5n3
6 ± 1
6 ± 1
5.3 ± 0.9
4.9 ± 0.7
4.3 ± 0.8
4.3 ± 0.9
22:6n3
6 ± 1
7 ± 1
7 ± 1
9 ± 2
9 ± 2
12 ± 2
24:1n9
0.14 ± 0.04
0.14 ± 0.04
0.2 ± 0.1
0.06 ± 0.03
0.11 ± 0.08
0.18 ± 0.09
had significantly higher values in the inner layer, while
the saturated acids 15:0, 16:0 and 18:0, the monounsat-
urated acid 18:1n7, and the polyunsaturated acid 20:4n3
had lower values in the inner layer.
When all 28 FAs were included in a PCA, the distinc-
tion between the inner and outer layer was incomplete
(Fig. 2). This was largely due to variation between indi-
vidual polar bears, which was the largest source of
variation in the data (46% of the total), and therefore
manifested along PC 1. The differences between layers
were responsible for 18% of the total variation in the
data, evident along PC 2 (Fig. 2).
When a PCA was carried out on the basis of the 10 FAs
that were significantly different between layers (Table
1), the layer difference became the most prominent dif-
ference, accounting for 61% of the total variation (Fig. 3).
The samples from the inner and outer layers were then
completely separated, except for Bears 4 and 9. Clear ex-
ceptions were evident compared to the other 16 bears,
which may have been caused by inadequate sample
handling (see ‘Materials and methods’).
Differences among the individual bears were also
apparent in the PC-plot in Fig. 3, i.e. along the second
PC. The plot shows that the individual variance in the
inner and outer adipose tissue was correlated because
each bear was positioned approximately in the same
position along PC 2.
The values for the FA compositions of the blubber for
the 3 seal species are presented in Table 1 and Fig. 1.
These are average values for all determinations within
a species, i.e. 90 separate samples from 10 ringed seals,
195 samples from 10 harp seals and 125 samples from
9 bearded seals. The variances around the means are
due to individual differences between animals and
differences between the blubber layers.
The FA composition of the blubber of each seal spe-
cies was significantly different from that of the inner
adipose tissue of the polar bears (p < 0.01) for all
28 FAs. These differences were larger than the differ-
ences among the 3 seal species, or the differences
between the polar bear adipose tissue layers. In the
case of 14 of the FAs, the relative amounts in the
Grahl-Nielsen et al.: Fatty acid composition of polar bear adipose tissue
279
PC 2
18%
20:4n6
This RSD plot only shows the difference
between the inner bear adipose tissue and the
other samples, not the direction of the differ-
ences, which is shown in the PC plot in Fig 2. The
outer bear adipose tissue lies in one direction
from the inner adipose tissue, while the seal
blubber samples lay perpendicular to this. This
indicates that different FAs were responsible for
the differences in the 2 cases.
24:1n9
3
11
17
14
bearded seal
12
20:0
15:0
8
10
7
18:0
13
1
3
15
18:1n7
5
16:0
20:5n3
3
4
ringed seal
11
18
2
14
14
11
16
16:1 n7
9
18
20:4n3
13
1
7
7
5
5
18:1n5
1
9
9
18
4
4
6
8
17
6
6
10
2
8
harp seal
16
12
13
15
17
10
16
2
12
15
a.iso-15:0
14:1n5
16:2n6
18:3n3
DISCUSSION
20:1n9
14:0
22:5n3
22:6n3
21:5n3
20:3n3
22:1n9
18:4n3
Polar bears are opportunistic feeders and they
do occasionally forage on white whales
Delphi-
napterus leucas
, walrus
Odobenus rosmarus
,
birds, eggs, reindeer
Rangifer tarandus platy-
rhynchus
and other carrion (Calvert & Stirling
1990, Smith & Sjare 1990, Stempniewicz 1993, De-
rocher et al. 2000), in addition to phocid seals,
which make up the bulk of their diet (Smith 1980,
Stirling & Øritsland 1995). The diet of polar bears is
obtained principally from the marine food web
(Ramsay & Hobson 1991, Hobson & Stirling 1997).
In the Barents Sea — Svalbard area — the diet is
found to consist almost exclusively of ringed,
bearded and harp seals (Lønø 1970, Derocher et al.
2002). Harp seals have only been found in the diet
during the summer season (April/May to October).
Since the bears in the present project were sam-
pled in mid-April, we assume they had eaten
mostly ringed and bearded seals during the 6 mo
prior to sampling.
Since seals from the 3 species were not avail-
able in the area during the time we were sam-
22:1n11
PC 1
46%
Fig. 2.
Ursus maritimus, Phoca hispida, Phoca groenlandica
and
Erig-
nathus barbatus
. Principal component biplot of adipose tissue sam-
ples from 18 polar bears based on all 28 variables, i.e. fatty acids. Cir-
cles represent samples from the inner tissue, diamonds the middle
tissue, and squares the outer tissue. Each symbol represents the aver-
age of 2 parallel samples. The individual identification number of the
bear is given within the symbols. The location of the fatty acids in the
plot indicates their importance for the spread of the samples, with
those farthest from the origin along a PC having a higher importance
for that PC, and consequently for the position of the samples in the
plot. (The 4 fatty acids with the lowest importance, i.e. situated close
to the origin, are not shown). Samples of blubber from the 3 prey spe-
cies (90 samples from 9 ringed seals, 195 samples from 10 harp seals
and 125 samples from 9 bearded seals) are projected onto the PC plot
of the polar bear samples, without being included in the principal
component analysis. They are located within the 3 ovals
blubber of all 3 seal species were higher than the rela-
tive amounts in polar bears. In 8 FAs, the relative
amounts in the blubber of all 3 seal species were lower
than the amounts in polar bears. Only 5 FAs in polar
bears had relative amounts that fell within the range of
relative values among the 3 seal species. The seal sam-
ples appear on one side of the bear samples in the PC
plot (Fig. 2). Only a few of the samples of bearded seals
overlapped with the bear samples.
A space-filling model of the inner adipose tissue sam-
ples from the bears was computed. This box model was
based on 2 significant PCs. The outer limit of the model,
i.e. the RSD, at the 99% level was 0.69. All samples with
a RSD above this value are significantly different from
the model of the inner adipose tissue (Fig. 4). Only one of
the samples from the inner adipose tissue was an outlier,
according to the model. Most of the samples from the
middle adipose tissue layer were inside the model, re-
flecting the similarity between the 2 innermost layers.
Almost all samples from the outer layer were outside the
model. All the seal samples were significantly different
from the inner layer of the bear adipose tissue.
PC 2
15%
22:1
n
11
16
16
15:0
20:4
n
6
18
16:0
11
22:1
n
9
8
18
11
7
8
10
3
18:0
20:4
n
3
1
a.iso-15:0
1
6
20:1
n
9
2
14
6
2
3
10
7
17
18:1
n
7
9
12
14
9
4
5
12
5
17
15
4
15
13
13
PC 1
61%
Fig. 3.
Ursus maritimus.
Principal component biplot of inner
and outer adipose tissue samples of 18 polar bears based on
the 10 fatty acids with significant differences between the lay-
ers (see Table 1). Circles represent samples from the inner tis-
sue and squares the outer tissue. Each symbol represents the
average of 2 parallel samples. Individual identification
numbers of each bear is given in the symbols
PC 2
18%
20:4n6
24:1n9
3
111
117
bearded seal
114
112
20:0
15:0
8
110
7
18:0
113
1
3
115
5
16:0
20:5n3
18:1n7
3
4
ringed seal
11
118
2
114
11
116
16:1 n7
14
20:4n3
9
18
1
7
7
5
18:1n5
13
5
1
9
9
18
4
4
6
8
17
6
6
10
2
8
harp seal
16
13
15
17
10
12
16
2
12
15
16:2n6
a.iso-15:0
14:1n5
18:3n3
14:0
20:1n9
22:5n3
22:6n3
20:3n3
21:5n3
18:4n3
22:1n9
PC 1
46%
22:1n11
PC 2
15%
22:1
n
11
22:1
n
11
116
116
15:0
15:0
20:4
n
6
20:4
n
6
118
16:0
16:0
111
22:1
n
9
22:1
n
9
8
118
111
7
8
20:4
n
3
20:4
n
3
110
18:0
18:0
3
1
a.iso-15:0
a.iso-15:0
1
6
2
114
2
3
110
20:1
n
9
20:1
n
9
6
7
18:1
n
7
18:1
n
7
117
114
9
112
9
4
5
17
112
5
4
115
115
113
113
PC 1
61%
Â
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