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Energy expenditure of bipedal walking is higher than that of quadrupedal walking in Japanese macaques.

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AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 131:33–37 (2006)
Energy Expenditure of Bipedal Walking Is Higher Than
That of Quadrupedal Walking in Japanese Macaques
M. Nakatsukasa,1* E. Hirasaki,2 and N. Ogihara1
1
2
Laboratory of Physical Anthropology, Kyoto University, Kyoto 606-8502, Japan
Laboratory of Biological Anthropology, Osaka University, Osaka 565-0871, Japan
KEY WORDS
locomotor energetics; bipedalism; quadrupedalism, Macaca fuscata;
presteady state; steady state
ABSTRACT
The authors previously compared energetic costs of bipedal and quadrupedal walking in bipedally trained macaques used for traditional Japanese
monkey performances (Nakatsukasa et al. [2004] Am.
J. Phys. Anthropol. 124:248–256). These macaques used
inverted pendulum mechanics during bipedal walking,
which resulted in an efficient exchange of potential and
kinetic energy. Nonetheless, energy expenditure during
bipedal walking was significantly higher than that of
quadrupedal walking. In Nakatsukasa et al. ([2004] Am.
J. Phys. Anthropol. 124:248–256), locomotor costs were
measured before subjects reached a steady state due to
technical limitations. The present investigation reports
sequential changes of energy consumption during 15 min
of walking in two trained macaques, using carbon dioxide production as a proxy of energy consumption, as in
Nakatsukasa et al. ([2004] Am. J. Phys. Anthropol.
124:248–256). Although a limited number of sessions
were conducted, carbon dioxide production was consistently greater during bipedal walking, with the exception of some irregularity during the first minute. Carbon
dioxide production gradually decreased after 1 min, and
both subjects reached a steady state within 10 min.
Energy expenditure during bipedalism relative to quadrupedalism differed between the two subjects. It was
considerably higher (140% of the quadrupedal walking
cost) in one subject who walked with more bent-knee,
bent-hip gaits. This high cost strongly suggests that ordinary macaques, who adopt further bent-knee, bent-hip
gaits, consume a far greater magnitude of energy during
bipedal walking. Am J Phys Anthropol 131:33–37,
2006. V 2006 Wiley-Liss, Inc.
A previous study by the authors revealed that the
energetic cost of bipedal walking was significantly higher
(20–30%) than that of quadrupedal walking in two Japanese macaques (Macaca fuscata) (Nakatsukasa et al.,
2004). This result raised concerns about generalizations
drawn from the findings of Taylor and Rowntree (1973),
who revealed that the energetic costs of bipedal and
quadrupedal walking were identical in two chimpanzees
and two capuchin monkeys. Unfortunately, bipedal locomotor energetic data in nonhuman primates are still
limited to these six individuals of three species.
Nakatsukasa et al. (2004) used Japanese macaques
engaging in traditional bipedal monkey performances
(hereafter, performing macaques). These macaques use
inverted pendulum mechanics during bipedal walking,
which results in an efficient exchange of potential and
kinetic energy (Hirasaki et al., 2004; Ogihara et al.,
2005). Thus, the observed bipedal locomotor costs should
underestimate those of ordinary Japanese macaques.
However, locomotor energetic data of ‘‘untrained’’ Japanese macaques have not been obtained because they are
not suitable for use in energetic experiments.
There were a few technical problems in the previous
study. A major problem concerned the relatively short
experimental duration (2 min on average). This was due
to the difficulty of conducting a sufficient number of
experiments with long duration and changing conditions
(velocity, mode of locomotion). It is generally assumed
that at least several minutes are required for an animal
to reach steady state from the resting state. Energetic
costs during a presteady state are higher than those
during a steady state (Taylor et al., 1982), and do not
provide proper measures for interspecific comparisons.
Nakatsukasa et al. (2004) argued that this did not hamper an evaluation of the relative costs of bipedalism to
quadrupedalism in the same individuals, because the duration of the experimental session did not differ significantly between these locomotor modes. However, the duration of the presteady state in Japanese macaques and
the extent to which presteady-state costs are higher than
those of the steady state remain unknown. This information is important for a comparison of the data in Nakatsukasa et al. (2004) with other data obtained by the
standard method (e.g., Taylor et al., 1982).
We recently succeeded in collecting locomotor energetic
data for a longer duration (15 min) in two performing
macaques, although the number of sessions was limited.
Here, we tested our previous conclusions by investigating sequential changes of energy expenditure during
15-min experimental sessions. In addition, we evaluated
the extent to which differences of bipedal walking
C 2006
V
WILEY-LISS, INC.
C
Grant sponsor: Japanese Society for the Promotion of Science;
Grant sponsor: Biodiversity Research for the 21st Century COE
(A14); Grant number: Biodiversity Research for the 21st Century
COE (A14), Grant-in-Aid 16370104.
*Correspondence to: Masato Nakatsukasa, Laboratory of Physical
Anthropology, Graduate School of Science, Kyoto University, Sakyo,
Kyoto 606-8502, Japan. E-mail: nakatsuk@anthro.zool.kyoto-u.ac.jp
Received 1 July 2005; accepted 9 November 2005.
DOI 10.1002/ajpa.20403
Published online 16 February 2006 in Wiley InterScience
(www.interscience.wiley.com).
0.473
0.436
0.387
0.432
0.354
0.319
0.320
0.331
0.325
0.294
0.300
0.313
0.302
0.297
0.261
0.284
0.340
0.297
4
3
2
1
Standard error calculated from LSR slope of CO2 concentration increase rates (in italics).
Mean of minute averages of CO2 production rates for 3 min after outset of exercise. In parentheses, percent value to steady–state value (bottom row).
Mean of minute averages of CO2 production rates for 3–6 min after outset of exercise. In parentheses, percent value to steady–state value (bottom row).
Mean of minute averages of CO2 production rates for 10–15 min after outset of exercise (steady–state value).
0.011
0.007
0.005
(118.0)
0.007
0.010
0.011
(106.0)
0.006
0.013
0.013
0.006
0.008
0.012
0.013
0.010
0.010
0.420
0.389
0.382
0.397
0.365
0.344
0.359
0.356
0.362
0.352
0.348
0.343
0.325
0.367
0.353
0.309
0.323
0.336
0.013
0.014
0.008
(119.0)
0.013
0.018
0.010
(108.0)
0.007
0.018
0.008
0.007
0.011
0.007
0.011
0.014
0.008
0.555
0.507
0.503
0.521
0.487
0.464
0.473
0.475
0.426
0.436
0.450
0.462
0.443
0.407
0.431
0.453
0.463
0.439
0.010
0.009
0.007
(116.0)
0.007
0.010
0.007
(112.0)
0.007
0.013
0.007
0.008
0.008
0.013
0.007
0.010
0.015
0.404
0.444
0.428
0.426
0.405
0.411
0.410
0.409
0.405
0.397
0.381
0.392
0.354
0.380
0.379
0.354
0.360
0.366
0.014
0.019
0.016
(109.0)
0.007
0.005
0.009
(105.0)
0.020
0.005
0.015
0.011
0.011
0.009
0.010
0.015
0.021
0.428
0.446
0.412
0.429
0.417
0.411
0.410
0.413
0.398
0.383
0.403
0.391
0.389
0.395
0.381
0.399
0.394
0.392
0.081
0.020
0.010
(113.0)
0.028
0.060
0.091
(111.0)
0.015
0.026
0.012
0.050
0.046
0.017
0.023
0.012
0.017
0.413
0.360
0.363
0.379
0.375
0.382
0.357
0.372
0.356
0.319
0.324
0.352
0.376
0.322
0.322
0.330
0.322
0.335
0.021
0.012
0.012
(121.0)
0.013
0.010
0.019
(112.0)
0.010
0.006
0.013
0.012
0.023
0.016
0.033
0.015
0.014
0.465
0.400
0.380
0.415
0.382
0.372
0.388
0.381
0.366
0.331
0.318
0.324
0.347
0.330
0.365
0.356
0.309
0.341
0.455
0.506
0.495
0.485
0.485
0.495
0.439
0.473
0.458
0.457
0.407
0.427
0.407
0.420
0.447
0.417
0.398
0.418
0.482
0.489
0.433
0.468
0.495
0.448
0.444
0.462
0.442
0.433
0.411
0.431
0.419
0.417
0.413
0.416
0.409
0.415
0.048
0.039
0.039
(113.0)
0.014
0.035
0.018
(111.0)
0.028
0.030
0.021
0.078
0.040
0.021
0.057
0.015
0.040
0–1 min
1–2 min
2–3 min
0–3–min mean2
3–4 min
4–5 min
5–6 min
3–6–min mean3
6–7 min
7–8 min
8–9 min
9–10 min
10–11 min
11–12 min
12–13 min
13–14 min
14–15 min
10–15 min mean4
0.022
0.037
0.029
(116.0)
0.055
0.036
0.063
(113.0)
0.047
0.026
0.029
0.051
0.029
0.036
0.049
0.022
0.041
B, 6.1 kg
Biped 1
E1
A, 10.0 kg
Biped 2
E1
A, 10.0 kg
Biped 1
E1
A, 8.2 kg
Quad 2
E1
A, 8.2 kg
Quad 1
E1
A, 8.2 kg
Biped 2
E1
The subjects were two male Japanese macaques, maintained by the Suo Monkey Performance Association
(Kumamoto Prefecture, Japan) and trained for bipedal
performance from age 2 years. The Suo Monkey Performance is regarded as an intangible cultural asset by
the local government, and is operated under municipal
regulations with respect to animal welfare and treatment. Experiments were conducted with the collaboration of the Suo Monkey Performance Association, following the guidelines for animal experimentation of Kyoto
University. Subject A had been used in previous experiments (subject 2 in Nakatsukasa et al., 2004). This macaque was an 8-year-old male as of May 2005. Experiments were conducted in September 2003 and May 2005,
when his body mass was 8.2 kg and 10.0 kg, respectively.
Subject B was a 5-year-old male, and possessed a body
mass of 6.1 kg in May 2005.
The experimental method used was similar to that
outlined previously (Nakatsukasa et al., 2004). The size
of the chamber was reduced by half to detect gas concentration more precisely (1.9 3 0.95 3 1.9 m). The capacity
of the chamber was about 3,330 l, excluding the total
volume of the subject, trainer, and experimental apparatus. The experimental duration was 15 min. If sessions
were consecutive, at least 20 min of resting time were
taken between sessions. The subjects walked on the
treadmill set in the chamber at a velocity of 2.0 km/hr.
They did not show apparent tiring after completing
15 min of walking at this velocity. The trainer accompanied the subject and breathed through a tube connected
to the outside of the chamber.
Carbon dioxide (CO2) concentrations within the chamber were recorded using an infrared gas analyzer (Model
CGT-7000, Shimadzu Corp.) positioned in the chamber.
The recording was started 30 sec after the chamber was
closed, following a wait for the diffusion of the inner air.
The data-collection intervals for experiments conducted
in 2003 and 2005 were 10 sec and 5 sec, respectively.
A least-squares linear regression (LSR) was calculated
between CO2 concentration (ppm) and elapsed time for
data recorded every minute. The LSR slope represented
the average CO2 concentration increase for 1 min. The
standard error of the LSR slope was also calculated. The
slope was converted into a standardized CO2 production
rate (ml/kg/sec) (Table 1). Although this value is not the
same as the true energetic cost given by oxygen consumption, CO2 production can be used as a convenient
measure of energy consumption under the assumption
that the respirator quotient was constant during experiments (Nakatsukasa et al., 2004). It is known empirically that the respirator quotient is in the range of 0.8–
0.9 in animals fed with balanced diets.
The bipedal kinematics of subject A were reported in
Hirasaki et al. (2004) as the third subject among the performing macaque group. Kinematic data of subject B
were taken in October 2004 by a common method using
two CCD camera systems and digitizing software on a
PC, as in Hirasaki et al. (2004).
TABLE 1. Sequential changes of standardized CO2 production (ml/kg/sec) calculated for each minute during 15-min walking
MATERIALS AND METHODS
B, 6.1 kg
Quad 1
E1
B, 6.1 kg
Quad 2
E1
style affected energy expenditure in these Japanese macaques. Bipedal kinematics for one of the two performing
macaques involved a more bent-knee, bent-hip style,
with the trunk more inclined in comparison to the other
experimental subject. The development of inverted pendulum mechanics during bipedal walking was at an incipient stage.
0.020
0.012
0.013
(145.0)
0.007
0.012
0.009
(111.0)
0.010
0.007
0.009
0.011
0.010
0.009
0.007
0.018
0.015
M. NAKATSUKASA ET AL.
A, 8.2 kg
Subject, body mass
Trial
Biped 1
E1
34
LOCOMOTOR ENERGETICS IN JAPANESE MACAQUE
Fig. 1. Sequential changes of standardized CO2 production
(ml/kg/hr) during 15-min walking exercise in subject A in 2003:
two trials for bipedal and quadrupedal walking (thin broken
and dotted lines), and their averages (thick solid lines). CO2
production was calculated for each minute from outset of
exercise.
35
Fig. 2. Sequential changes of standardized CO2 production
(ml/kg/hr) during 15-min walking exercise in subject A in 2005:
two trials for bipedal walking (thin broken and dotted lines),
and their average (thick solid line). CO2 production was calculated for each minute from outset of exercise.
RESULTS
Rates of CO2 production and their standard errors
were measured in two subjects under different conditions, and the results are summarized in Table 1 and
shown in Figures 1–3. Figure 1 represents results
obtained from subject A when he was 6.3 years of age.
Two sessions were recorded for both bipedal and quadrupedal walks. The average of the two sessions in each
locomotor mode is also shown. Values after the first minute (left endpoint) are rather irregular in comparison to
later values. They are rather high in quadrupedal sessions, and low in bipedal sessions. However, CO2 production rates exhibit gradual decreasing tendencies after
the first minute, although some fluctuations are
observed. The values for the bipedal walk are always
higher than corresponding values of the quadrupedal
walk. The decreasing tendency is absent 8 min after the
outset of the experiment, indicating that the subject
reached a steady state. Combining the two sessions, the
grand mean of the last 5 min of data is 0.417 (ml/kg/sec)
for the bipedal walk and 0.338 for the quadrupedal walk,
with the former being 23% higher than the latter.
Figure 2 represents CO2 production rates in subject A
at age 8 years. Only bipedal walk data were recorded.
The observed features are the same as those shown in
Figure 1. In this case, the subject might have reached a
steady state somewhat later, at 10 min after the outset
of the sessions. The grand mean of the last 5 min of data
is 0.379 (ml/kg/sec). Owing to the increased body mass
(þ1.8 kg) from 2003, the locomotor cost of a bipedal walk
per body mass is reduced by 10% (Nakatsukasa et al.,
2004).
Figure 3 represents the CO2 production rate of subject
B during bipedal and quadrupedal walks. Two sessions
were recorded for the quadrupedal walk, and one session
for the bipedal walk. Average values are shown for the
quadrupedal sessions. Although fluctuations exist during
later phases of all sessions, this subject probably reached
a steady state around 8 min after the onset of the
experiment. CO2 production values for the bipedal walk
are always higher than corresponding quadrupedal values. The average of the last 5 min of data is 0.439 (ml/
kg/sec) in the bipedal walk, and 0.317 in the quadrupedal walk. Thus, the bipedal cost at steady state is about
Fig. 3. Sequential changes of standardized CO2 production
(ml/kg/hr) during 15-min walking exercise in subject B: one trial
for bipedal walking, and two for quadrupedal walking (thin
solid, broken, and dotted lines). Thick solid line represents average of quadrupedal data. CO2 production was calculated for
each minute from outset of exercise.
40% higher than that recorded during a quadrupedal
walk.
DISCUSSION
The results of this study support the conclusion in our
previous study (Nakatsukasa et al., 2004). Although CO2
production rates fluctuate irregularly during the first
minute of experiments in some cases (Fig. 1), they
decrease in both bipedal and quadrupedal sessions in a
parallel manner. The irregularity of CO2 production soon
after exercise may be influenced by temporary storing of
CO2 in the blood and muscles. Although the experimental sessions in Nakatsukasa et al. (2004) were generally
short, the present results indicate that energy expenditure during bipedal walking relative to quadrupedal
walking (B/Q ratio) in the first 2 min is more or less similar to that recorded in later stages, though it is not coincident. Clearly, bipedal walking at a steady state in performing macaques needs more energy than quadrupedal
walking at a steady state.
Table 1 compares the presteady state CO2 production
rates with those at steady state (¼ mean of 10–15 min of
data). Average CO2 production rates in the first 3 min
vary from 109–145% of those recorded at steady state,
36
M. NAKATSUKASA ET AL.
with a grand mean of 119%. Many of the experimental
sessions in Nakatsukasa et al. (2004) fall into this range.
Thus, their values are likely to be higher than those at
steady state by 20% on average.
Average CO2 production rates during 3–6 min after
the outset of exercise are constantly higher than rates
recorded at steady state during either bipedal or quadrupedal walking. They are 105–113% (110% on average) of
steady-state values. In all sessions, subjects reached a
steady state within 10 min from the outset of exercise.
However, if practical limitations hamper the continuation of such a long experiment, a compromise may be to
use data covering a period of 3–6 min. Although they
overestimate energy consumption at steady state, B/Q
ratios during this period are similar to those at steady
state (Table 1).
The B/Q ratio at steady state in subject A is 124%.
This ratio is consistent with the findings of previous
experiments (Table 3 of Nakatsukasa et al., 2004), when
the subject was 4–5 years of age. Thus, the relative energetic cost of bipedalism to quadrupedalism did not
change significantly following 3 years’ growth in this
individual. This is concordant with the fact that kinematic features of this individual did not change very
much between 2001 and 2004 (E. Hirasaki, unpublished
data).
The B/Q ratio in subject B at steady state (138%) is
much higher than that recorded for subject A (and subject 1 in Nakatsukasa et al., 2004). Since data for the
bipedal walk were obtained from only one session, and
the two quadrupedal sessions displayed large fluctuations, care must be taken not to rely too heavily on these
data. Nonetheless, this individual apparently consumed
a relatively greater amount of energy during bipedalism
in comparison to other performing macaques.
Figure 4 compares stick diagrams of the bipedal walk
(at ca. 3.5 km/hr) in performing and ordinary macaques.
These stick diagrams were recorded using the method
described in Hirasaki et al. (2004).
Subject B shows a subtle upward convex trajectory of
the hip joint during the support phase (inverted pendulum-like mechanics). His trunk is more inclined, and the
hip and knee joints are less extended, compared to other
performing macaques. Thus, he is not a good biped
among performing macaques. However, his hip and knee
joints are much more extended in comparison to ordinary macaques, and he is still a better biped compared
to ordinary macaques.
It is reasonable to use these bipedal locomotor characteristics to explain the relatively high energetic consumption of bipedal walking. Data for these stick diagrams were obtained from faster bipedal walks, conducted at a velocity of approximately 3.5 km/hr. It
should also be noted that these bipedal walking characteristics, except for hip-joint motions, in both performing
and ordinary macaques are not affected significantly by
walking velocity (Hirasaki et al., 2004). Although hipjoint motion is affected by walking speed, differences
between performing and ordinary subjects are maintained, regardless of walking speed.
According to the biomechanical theoretical analysis of
Ishida (1991), ‘‘untrained’’ macaques consume 50% more
energy during a bipedal walk compared to well-trained
(¼ performing) macaques, while ‘‘moderately trained’’
macaques consume 10% more energy in a similar comparison. Subject B consumes about 10% extra energy
during a bipedal walk when compared to subject A
Fig. 4. Stick diagrams of bipedal walking at velocity of
approximately 3.5 km/hr. a–c: Performing macaques. a, b: Subjects 1 and 2 in Nakatsukasa et al. (2004). c: Subject B. d, e:
Ordinary experimental macaques. Note that subject B walks
with more inclined trunk and less extended hip and knee joints
compared to other performing macaques (a, b). However, hip
and knee joints are still more extended compared to ordinary
macaques.
(138% vs. 124% in B/Q ratio), given that their quadrupedal walking requires a constant amount of energy. This
result is close to the prediction of Ishida (1991).
Although the above calculation is based on a small
number of experiments and data improvement is necessary, it strongly suggests the rather high energy consumption of bipedal walking in ordinary Japanese macaques, who may be categorized as ‘‘untrained’’ or ‘‘bad’’
bipeds (Ogihara et al., 2005). Cercopithecines generally
share similar postcranial skeletal features in relation to
ancestral semiterrestrial habits (Schultz, 1970; Strasser,
1988; Strasser and Delson, 1987; Harrison, 1989; Nakatsukasa, 1994). If the relative cost of a bipedal walk is so
expensive in ordinary Japanese macaques, it is unlikely
that energy expenditures in bipedal and quadrupedal
walking are the same in other cercopithecine monkeys
(and very likely colobine monkeys). Locomotor energetic
experiments on primates are difficult. However, to test
the conclusion of Taylor and Rowntree (1973), comparable data on platyrrhines should be collected.
Our study confirmed that variation of gait features
such as trunk angle, hindlimb joint angles, and trajectory of the center of mass significantly affect the energetic cost of bipedalism in nonhuman primates. This
result was not surprising, as it had already been predicted by inverse dynamic simulation (Yamazaki, 1985;
Ishida, 1991). Nonetheless, experimental data are important to reproduce the predictions of inverse dynamic simulation models. Carey and Crompton (2005), for example,
demonstrated that a usage of emphasized bent-hip, bentknee bipedalism (as in Fig. 4d,e) considerably overloaded
locomotor physiology in modern humans, compared to
LOCOMOTOR ENERGETICS IN JAPANESE MACAQUE
fully upright bipedalism. However, even comparatively
‘‘minor’’ differences of bipedal gait characteristics (Fig.
4a–c) introduce significant influence on the energetic
cost of bipedal locomotion. Improvement of locomotor efficiency allows an animal to relocate the saved energy
for other purposes, such as growth or reproduction, or to
have a wider day range. This result, therefore, suggests
that if early hominids had traveled daily over long distances, their gait characteristics might be an acute target of natural selection.
However, identifying the most proximate target of
selection on the locomotor musculoskeletal system is not
straightforward. Stern (1999) proposed that a reduction
of peak vertical force and an increase of speed were beneficial in Australopithecus afarensis, even if bent-hip,
bent-knee bipedalism sacrificed locomotor energetic efficiency. Susman et al. (1984) argued that the less developed sacro-iliac stabilization mechanism in A. afarensis
could be related to the low peak vertical force in benthip, bent-knee bipedalism (but see opposing argument in
Carey and Crompton, 2005). Steudel-Numbers and Tilkens (2004) presented a similar case, where experiments
clearly indicated that an elongation of the lower limb
significantly contributed to a reduction of bipedal locomotor cost. However, the lower limb of early australopithecines (A. afarensis and A. africanus) stayed comparatively short for at least half a million years. Here also,
another target (or other targets) of selection must be
presumed, such as lateral balance control during the single stance phase and/or climbing capability (SteudelNumbers and Tilkens, 2004). Locomotor energetics is a
very important component for understanding the paleobiology of early hominids and the transition to habitual
bipedality in protohominids. However, it will be most
useful when it is holistically synthesized with information on hominids’ paleoecology and their skeletal functional morphology.
CONCLUSIONS
We confirmed that highly trained bipedal performing
macaques consumed more energy during bipedal locomotion compared to quadrupedal walking through 15-min
exercise experiments. The subjects reached steady state
within 10 min in both bipedal and quadrupedal sessions.
The presteady extra cost was about 20% of the steadystate cost rates in the first 3 min, and about 10% during
3–6 min after the outset of exercise. The energetic cost
of bipedal walking relative to quadrupedal walking was
124% in one subject and 138% in the other. The relatively high cost in the latter subject was related to his
bipedal gait characteristics such as a more inclined
trunk, and less extended knee and hip joints. However,
this subject was an appreciably ‘‘better’’ biped (less benthip, bent-knee) compared to untrained Japanese macaques. Therefore, the relative bipedal energetic cost
should be rather high in untrained macaques. The same
result probably holds true for Old World monkeys in
general.
37
ACKNOWLEDGMENTS
We are very grateful to the trainers of the Suo Monkey
Performance Association for their enormous patience in
carrying out the repetitive experiments required of this
investigation. This study was made possible by the very
generous support received from the Suo Monkey Performance Association.
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Ishida H. 1991. A strategy for long distance walking in the earliest hominids: effect of posture on energy expenditure during
bipedal walking. In: Coppens Y, Senut B, editors. Origine(s)
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