© 2015. Published by The Company of Biologists Ltd | Journal of Experimental Biology (2015) 218, 3319-3329 doi:10.1242/jeb.125757
RESEARCH ARTICLE
Rapid maturation of the muscle biochemistry that supports diving
in Pacific walruses (Odobenus rosmarus divergens)
ABSTRACT
Physiological constraints dictate animals’ ability to exploit habitats. For
marine mammals, it is important to quantify physiological limits that
influence diving and their ability to alter foraging behaviors. We
characterized age-specific dive limits of walruses by measuring
anaerobic (acid-buffering capacity) and aerobic (myoglobin content)
capacities of the muscles that power hind (longissimus dorsi) and fore
(supraspinatus) flipper propulsion. Mean buffering capacities were
similar across muscles and age classes (a fetus, five neonatal calves, a
3 month old and 20 adults), ranging from 41.31 to 54.14 slykes and
42.00 to 46.93 slykes in the longissimus and supraspinatus,
respectively. Mean myoglobin in the fetus and neonatal calves fell
within a narrow range (longissimus: 0.92–1.68 g 100 g−1 wet muscle
mass; supraspinatus: 0.88–1.64 g 100 g−1 wet muscle mass). By
3 months post-partum, myoglobin in the longissimus increased by 79%,
but levels in the supraspinatus remained unaltered. From 3 months
post-partum to adulthood, myoglobin increased by an additional 26%
in the longissimus and increased by 126% in the supraspinatus;
myoglobin remained greater in the longissimus compared with
the supraspinatus. Walruses are unique among marine mammals
because they are born with a mature muscle acid-buffering capacity and
attain mature myoglobin content early in life. Despite rapid physiological
development, small body size limits the diving capacity of immature
walruses and extreme sexual dimorphism reduces the diving capacity
of adult females compared with adult males. Thus, free-ranging
immature walruses likely exhibit the shortest foraging dives while
adult males are capable of the longest foraging dives.
KEY WORDS: Odobenid, Myoglobin, Acid buffering, Ontogeny,
Aerobic dive limit, Arctic
INTRODUCTION
With continued human perturbation to the environment, it is
becoming increasingly important to predict the ability of animals to
adapt to changes in their environment. For example, the rate of change
within Arctic ecosystems, including changes in sea ice cover, sea
level, water temperature and ocean currents (Rothrock et al., 1999;
Parkinson and Cavalieri, 2002; Comiso and Parkinson, 2004; Walsh,
2008), is exceeding trends recorded over the past several millennia
(Root et al., 2003; Overpeck et al., 2005; Walsh, 2008). As long-lived
species with slow reproductive rates, Arctic marine mammals are
poorly equipped to respond to sudden alterations in climate (Moore
1
Institute of Marine Science, University of California, Santa Cruz, Center for Ocean
2
Health, 100 Shaffer Road, Santa Cruz, CA 95060, USA. US Geological Survey,
Alaska Science Center, 4210 University Drive, Anchorage, AK 99508, USA.
3
Department of Biological Sciences, University of Alaska, Anchorage, CPSB 202C,
3101 Science Circle, Anchorage, AK 99508, USA.
*Author for correspondence (snoren@biology.ucsc.edu)
Received 22 May 2015; Accepted 24 August 2015
and Huntington, 2008). Small population sizes following centuries of
commercial harvest and a reliance on specific sea ice conditions
further heighten the sensitivity of these species to environmental
perturbations (O’Corry-Crowe, 2008).
Pacific walruses [Odobenus rosmarus divergens (Illiger 1811);
Fig. 1] may be vulnerable to the effects of global climate change
(Laidre et al., 2008). They are highly specialized shallow benthic
foragers and consume a wide range of invertebrates, primarily
feeding on clams, snails and marine worms (Fay, 1982; Sheffield
and Grebmeier, 2009). Recently, walruses have altered their
foraging behaviors and distribution patterns in the Chukchi Sea
(Jay et al., 2012) in response to receding summer sea ice cover over
the continental shelf. For example, adult females and their young
have increased their use of terrestrial haul-outs when the sea ice is
over deep waters (Kavry et al., 2008; Jay et al., 2012). The number
of walruses converging on coastal haul-outs is expected to increase
as summer sea ice continues to decline (Jay et al., 2011), but the
ability of localized food supplies in these coastal regions to support
large numbers of walruses over the long term is unknown
(Ovsyanikov et al., 2008). Thus, these changes in behavior could
affect the ability of walruses to meet daily energetic requirements
(Noren et al., 2012).
Quantifying the physiological capacities of animals improves
the ability of scientists and managers to determine the range of
environmental conditions under which an animal can persist without
declines in fitness (Wikelski and Cooke, 2006). For marine mammals,
the importance of physiological constraints is evident during
foraging, as their diving behaviors, and hence habitat utilization
patterns, are defined by their breath-hold limits (Costa et al., 2001). To
maximize aerobic submergence times, adult diving mammals have
larger mass-specific body oxygen stores compared with terrestrial
mammals as a result of elevated blood volume, high hemoglobin
levels and greater muscle myoglobin content (Lenfant et al., 1970;
Snyder, 1983; Castellini and Somero, 1981; Kooyman, 1989; for
review, see Butler and Jones, 1997). Compared with adults, neonatal
and juvenile marine mammals have low oxygen storage capacities in
the blood and muscle (Bryden and Lim, 1969; Ronald et al., 1969;
Geraci, 1971; Lane et al., 1972; Kodama et al., 1977; Thorson, 1993;
Lewis et al., 2001; Noren et al., 2001, 2002, 2005, 2014; Burns et al.,
2005, 2007; Richmond et al., 2005, 2006; Fowler et al., 2007; Weise
and Costa, 2007; Verrier et al., 2011) while the cost of growth and
small body size result in comparatively high mass-specific oxygen
utilization rates (Kleiber, 1975). Ultimately, the ratio of oxygen stores
to the rate at which these stores are depleted defines aerobic dive limit
(ADL) (Kooyman, 1989) and hence dictates the foraging strategies of
marine mammals (Costa et al., 2001).
Odobenids [Pacific (O. r. divergens) and Atlantic (Odobenus
rosmarus rosmarus) walruses] are among the largest pinniped
species, which theoretically grants them an intrinsic advantage for
deep diving because of their relatively low mass-specific oxygen
consumption rates (e.g. reviewed in Noren and Williams, 2000).
3319
Journal of Experimental Biology
Shawn R. Noren1, *, Chadwick V. Jay2, Jennifer M. Burns3 and Anthony S. Fischbach2
Fig. 1. Female Pacific walrus and calf. Photo credit: Sarah Sonsthagen, US
Geological Survey.
However, a lack of data on the blood and muscle biochemistry that
supports breath holding in this species makes it difficult to predict
how walruses might alter foraging behaviors in response to the
recent changes in the Arctic marine ecosystem. Little effort has been
given in the past to examine the postnatal development of oxygen
stores in walruses. Obtaining the requisite samples for such a study
is difficult because of the remote Arctic distribution of walruses and
the limited number of immature walruses that are taken in Alaskan
Native subsistence harvests. Indeed, a previous report on the average
muscle oxygen store (myoglobin) in free-ranging Pacific walruses
was based on a low sample size composed of three 4–6 week old
animals and two adults (Lenfant et al., 1970), and because these data
were pooled, the influence of ontogeny was not considered.
In this study, we examined the ontogeny of the muscle
biochemistry that supports diving in Pacific walruses to better
understand their physiological breath-hold capacity, which partly
determines their potential to alter foraging behaviors, as might be
required in response to the rapid changes in the Arctic ecosystem.
Specifically, we measured anaerobic capacity (acid-buffering
capacity) and aerobic capacity (myoglobin content) in a fetus,
five neonatal calves, an immature 3 month old and 20 adults.
With knowledge of age-specific blood oxygen stores (S.R.N.,
unpublished), muscle oxygen stores (myoglobin; present study) and
body size (Fay, 1982), we calculated age-specific aerobic dive limits
(cADL; Kooyman, 1989). We compared our cADL estimates for
adult males with recorded diving behaviors of free-ranging adult male
walruses in the summer in Bristol Bay (Jay et al., 2001); this revealed
that these animals appear to be diving within their physiological
aerobic dive capacity. Similar comparisons for other sex and age
classes of walruses would be useful, because diving at or near
physiological capacity may reduce an animal’s behavioral plasticity to
alter foraging behaviors when prey becomes limited or more difficult
to access (Costa et al., 2001). This can have marked consequences for
a population, including the disproportionate starvation of immature
animals (Trillmich and Limberger, 1985; DeLong et al., 1991). Thus,
understanding age and sex-specific cADLs can be helpful in
predicting a species’ vulnerability to environmental change before
declines in vital rates occur (Williams et al., 2011).
RESULTS
Ontogeny of muscle biochemistry
The acid-buffering capacity (muscle anaerobic capacity) of the
longissimus dorsi appeared to be similar to that of the supraspinatus
3320
Journal of Experimental Biology (2015) 218, 3319-3329 doi:10.1242/jeb.125757
within the single fetus and within the single 3 month old walrus. The
acid-buffering capacities of these two muscle groups were also
similar within neonatal calves (t=−0.0580, d.f.=4, P=0.957) and
within adults (t=−0.261, d.f.=8, P=0.800). In addition, muscle acidbuffering capacities were similar between neonatal calves and adults
for both the longissimus dorsi (t=0.0145, d.f.=23, P=0.886) and
supraspinatus (t=0.00242, d.f.=12, P=0.998). These results indicate
that buffering capacity does not vary across locomotor muscles and
that the buffering capacity of these muscles is mature at birth
(Table 1, Fig. 2A).
Myoglobin content (muscle aerobic capacity) varied across
locomotor muscle groups. The myoglobin content of the
longissimus dorsi was similar to that of the supraspinatus within
the single fetus and within neonatal calves (t=0.448, d.f.=4,
P=0.678). However, soon after birth, the myoglobin content of
the longissimus dorsi surpassed that of the supraspinatus, as
evidenced by the single 3 month old walrus, which had twice as
much myoglobin in the longissimus dorsi than in the supraspinatus.
Even into adulthood, the myoglobin level of the longissimus dorsi
remained greater than that of the supraspinatus (t=4.808, d.f.=6,
P=0.0003), with myoglobin levels in the longissimus dorsi
representing 1.13× the levels in the supraspinatus.
Myoglobin content also varied within muscle groups throughout
ontogeny. Myoglobin content in neonatal calves was significantly
lower than that in adults within both the longissimus dorsi
(t=−7.968, d.f.=23, P<0.001) and the supraspinatus (t=−5.933,
d.f.=10, P<0.001; Fig. 2B, Table 1). However, myoglobin content
matured quickly in the longissimus dorsi. By 3 months post-partum,
the myoglobin content of the longissimus dorsi of the single
3 month old walrus was 79% of the levels found in the longissimus
dorsi of adults.
Table 1. Biochemistry of locomotor muscles in Pacific walruses
Age class (N)
Mean±s.e.m.
Acid-buffering capacity (slykes)
Longissimus dorsi
Fetus (1)
42.06 (102%)
Calf (5)
41.86±2.89 (101%)
Immature (1)
54.14 (131%)
Mature adult (20)
41.30±1.76
Supraspinatus
Fetus (1)
46.93 (112%)
Calf (5)
42.01±2.25 (100%)
Immature (1)
45.38 (108%)
Mature adult (9)
42.00±1.95
Mb content (g 100 g−1 wet muscle)
Longissimus dorsi
Fetus (1)
0.92 (24%)
Calf (5)
1.68±0.11‡ (44%)
Immature (1)
3.01 (79%)
Mature adult (20)
3.80±0.13*
Supraspinatus
Fetus (1)
0.88 (26%)
Calf (5)
1.64±0.19§ (49%)
Immature (1)
1.48 (44%)
Mature adult (7)
3.35±0.208*
Minimum
Maximum
–
34.93
–
29.54
–
47.78
–
59.19
–
36.54
–
32.90
–
49.33
–
49.33
–
1.32
–
3.00
–
1.98
–
4.71
–
1.05
–
2.70
–
2.18
–
4.14
Values in parentheses are the percentage of adult levels. At P<0.05, no
differences in acid-buffering capacity were found across the two muscle
groups, while myoglobin (Mb) content across the two muscle groups only
differed within the adult age class (denoted by *). At P<0.05, no differences in
acid-buffering capacity were found across age classes, while Mb content in
longissimus dorsi and supraspinatus of neonatal calves was significantly lower
than that of the respective muscle in adults (denoted by ‡ and §, respectively).
A sample size of 1 precluded comparisons within and among the fetus and
immature age classes. See Results for statistics.
Journal of Experimental Biology
RESEARCH ARTICLE
RESEARCH ARTICLE
Journal of Experimental Biology (2015) 218, 3319-3329 doi:10.1242/jeb.125757
the muscle in walruses (Fig. 3). This result concurs with previous
investigations (Noren, 2004a; Lestyk et al., 2009; Burns et al., 2010;
Noren et al., 2014a).
Supraspinatus
Longissimus dorsi
Average supraspinatus
Average longissimus dorsi
55
A
Total body oxygen stores and cADLs
45
40
35
105%
20
84%
25
102%
30
112%
(1) (1)
(5) (5)
(1) (1)
(9) (20)
Fetus
Calf
Immature
Adult
15
10
5
4.0
B
3.5
3.0
2.5
60
2.0
A
50
(5) (5)
(1) (1)
88%
(1) (1)
(7) (20)
0
Fetus
Calf
Immature
Age class
Adult
Fig. 2. Muscle acid-buffering capacity and myoglobin content in Pacific
walruses throughout ontogeny. (A) Muscle acid-buffering capacity
(anaerobic capacity) and (B) myoglobin (Mb) content (aerobic capacity). The
proportion of the muscle biochemistry in the supraspinatus in relation to the
longissimus dorsi is shown by percentages in the longissimus dorsi bars for
each age class. The number in parentheses within each bar represents the
number of individuals analyzed for that muscle and age class. See Results and
Table 1 for statistics.
Even though the developmental trajectory of buffering capacity
and myoglobin were different, some still might argue that the
postnatal development of myoglobin drives changes in buffering
capacity throughout maturation because myoglobin contributes to
the buffering capacity of muscle. Similar to Castellini and Somero
(1981) and Noren (2004), we analyzed the relationship between
myoglobin content and the ratio of buffering capacity to myoglobin
content in each of the muscle groups. Acid-buffering capacity of
the muscle was constant across ontogeny; thus, as myoglobin
content increased during development, the proportion of acidbuffering capacity (AB) in relation to myoglobin content ([Mb])
decreased in both the longissimus dorsi (AB[Mb]−1=60.59−0.47Mb,
d.f.=26, F=150.4584, P<0.0001) and supraspinatus (AB
[Mb]−1=75.35−0.59Mb, d.f.=13, F=112.9958, P<0.0001). These
relationships demonstrate that muscles with low myoglobin
content had an extremely high ratio of buffering capacity to
myoglobin content compared with myoglobin-rich muscle,
implying that myglobin is not the predominate buffering agent of
40
30
20
10
0
–10
0.5
60
1.0
1.5
2.0
2.5
3.0
3.5
4.0
4.5
5.0
B
50
40
30
20
10
0
–10
1
2
Mb content (g 100
3
g–1
4
wet muscle)
Fig. 3. Relationship between myoglobin content and acid-buffering
capacity to myoglobin content ratio in Pacific walruses throughout
ontogeny. Data are given in relation to myoglobin content in the longissimus
dorsi (A) and supraspinatus (B). Each point represents the mean for an
individual specimen that was analyzed in triplicate. Age classes are as follows:
fetus (white), calf (light gray), immature (3 month old; dark gray) and adult
(black). Males and females are denoted by triangles and circles, respectively.
The star represents an animal of unknown sex. See Results for statistics.
3321
Journal of Experimental Biology
0.5
96%
1.0
49%
1.5
96%
Mb content (g 100 g–1 wet muscle mass)
0
Because the 3 month old specimen had already achieved 79% of
adult myoglobin content in the longissimus dorsi, we assumed that
walruses ≥1 year old had achieved adult myoglobin content in the
longissimus dorsi for the purpose of calculating body oxygen stores.
As myoglobin content in the longissimus dorsi increased with
maturity from 0 to 1 year post-partum, calculated total mass-specific
oxygen stores in the muscle increased from approximately 7.54 ml
O2 kg−1 in neonatal calves, to 14.31 ml O2 kg−1 in the 3 month old
walrus, to 21.58–22.26 ml O2 kg−1 in adults (Fig. 4). The massspecific oxygen storage capacity in the lung and blood remained
relatively constant throughout ontogeny because we assumed that
mass-specific lung capacity did not change with age and the oxygen
storage capacity in the blood (hemoglobin levels) remained constant
throughout growth (S.R.N., unpublished). Thus, as the oxygen
storage capacity of the muscle increased with maturity, the relative
contribution of the muscle oxygen store to total body oxygen stores
(lung, blood and muscle) increased (Fig. 4).
Acid-buffering capacity:Mb content (slykes 100 g wet muscle g–1 Mb)
Acid-buffering capacity (slykes)
50
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Journal of Experimental Biology (2015) 218, 3319-3329 doi:10.1242/jeb.125757
60
50
40
30
55%
46%
40%
39%
21%
33%
43%
44%
24%
20%
17%
17%
Calf
Immature
Mature F
Mature M
20
10
0
Age class
Fig. 4. Mass-specific oxygen stores of neonatal calves, immature
(3 month old) and adult Pacific walruses. Total mass-specific oxygen stores
are represented by the bars, which are divided into three colors to represent
each of the oxygen compartments: lung, muscle and blood. M, male; F, female.
Percentages within the bars show the proportion of the total oxygen stores that
each compartment represents. See Materials and methods for the
assumptions made for these calculations and the references that support
these assumptions.
DISCUSSION
Although there have been several recent investigations on the
ontogeny of myoglobin in pinnipeds ( phocids and otariids;
Table 2), the development of the muscle biochemistry that
A
16
Increasing muscle oxygen stores and decreasing mass-specific
metabolic rate associated with increasing body size resulted in an
increase in cADL with age (Fig. 5). As breath-hold capacity
increases with maturity, walruses are afforded more submergence
25.0
Bottom time (min)
Male cADL (2⫻Kleiber MR)
Female cADL (2⫻Kleiber MR)
Male cADL (hypometabolism)
Female cADL (hypometabolism)
Male cADL (FMR)
Female cADL (FMR)
14
10
8
16
14
12
10
8
6
4
2
0
6
4
2
Bottom
time (min)
0
0
2
4
6
8
10
12
14
16
22.5
20.0
17.5
15.0
360
320
280
240
200
160
120
80
40
B
12.5
16
10.0
0
1
2
3
4
5 6
7
8
9 10 11 12 13 14 15 16
Age (years)
Fig. 5. Calculated aerobic dive limit (cADL) in relation to age for female
and male Pacific walruses. Three metabolic rates (MRs) were assumed. The
dotted lines represent cADLs based on hypometabolism [2× Kleiber MR (BMR;
Kleiber, 1975) reduced by 39%], which likely overestimates physiological
limits. The solid lines represent cADLs based on a MR equivalent to 2× Kleiber
MR, which has been shown to provide the best estimate of the diving capacity
of mature marine mammals. The dashed lines represent cADLs based on field
MR (FMR, equivalent to 6× Kleiber MR), which likely underestimates the diving
capacity of mature marine mammals, but could approximate those of immature
marine mammals, which typically have elevated metabolism. See Materials
and methods for the assumptions made for these calculations and the
references that support these assumptions.
3322
10
8
16
14
12
10
8
6
4
2
0
6
4
2
0
s)
0
12
(year
2.5
Bottom time (min)
5.0
360
320
280
240
200
160
120
80
40
Dive depth (m)
Fig. 6. Theoretical bottom time in relation to age and dive depth for Pacific
walruses. (A) Female, (B) male. An ascent and descent swim speed of
0.8 m s−1 and a calculated aerobic dive time based on a diving metabolism of
2× Kleiber MR were assumed to set the limits for bottom time at each depth.
See Materials and methods for the assumptions made for these calculations
and the references that support these assumptions.
Journal of Experimental Biology
14
7.5
Age
cADL (min)
12
s)
70
time. For example, when assuming a diving metabolism of 2×
Kleiber metabolic rate, a neonatal walrus can only dive aerobically
for approximately 5 min, while a weaned 2–3 year old walrus can
dive for 9–11 min. This compares with an ADL of 13–15 min for a
fully grown adult female and male walrus, which affords deeper
depth dives and longer bottom times to search for prey along the
seafloor. Nonetheless, there is a trade-off between dive depth and
bottom time (Fig. 6). For example, a newly weaned 2 year old
walrus has 8–9 min of bottom time at a depth of 41 m, 6–7 min
of bottom time at 80 m, 5–6 min of bottom time at 102 m, and only
1–2 min of bottom time at 200 m. This compares with 12–13 min,
10–11 min, 9–11 min and 5–6 min of bottom time for fully grown
adult female and male walruses diving down to the same depths.
(year
Lung oxygen
Muscle oxygen
Blood oxygen
Age
Mass-specific oxygen stores (ml kg–1)
80
RESEARCH ARTICLE
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Table 2. Ontogeny of myoglobin content in pinnipeds, with species listed by family according to increasing duration of dependency period
Age class
Time into DP (%)
Mb content (g 100 g−1 muscle)
Neonate (<7 days)
Nursing pup
Weaned pup
Yearling
Adult female and male (NS different)
11
50
100
1.56 (41%)
2.28 (60%)
2.72 (71%)
4.22 (110%)
3.83
Hooded seal, Cystophora cristata
DP=35 days
Burns et al., 2007
Neonate (<2 days)
Nursing (2–4 days)
PWF (5–14 days)
Yearling (12 months)
Adult female
3
9
29
100
3.15 (33%)
2.93 (31%)
2.99 (32%)
7.65 (81%)
9.48
Gray seal, Halichoerus grypus
DP=45 days
Noren et al., 2005
Newborn (3 days)
Weaned (17 days)
Mid-PWF (29 days)
Late PWF (41 days)
Yearling (1 year)
Adult female (>6 years)
7
38
64
91
100
1.7 (43%)
2.1 (53%)
2.5 (63%)
2.7 (68%)
3.2 (80%)
4.0
Harp seal, Pagophilus groenlandicus
DP=47 days
Burns et al., 2007
Neonate (<2 days)
Nursing (3–12 days)
PWF (14–28 days)
Yearling (10–11 months)
Adult female
2
17
45
100
2.1 (24%)
2.67 (31%)
3.42 (40%)
5.7 (66%)
8.6
Northern elephant seal, Mirounga angustirostris
DP=90 days
Thorson, 1993
Nursing pup (<2 weeks)
Weaned (4 weeks)
Mid-PWF (8 weeks)
Late PWF (12 weeks)
Juveniles (8–10 months)
Adult female (3–12 years)
Sub-adult male (3–6 years)
Adult male (8–12 years)
8
31
62
93
100
2.9 (43%)
3.4 (51%)
4.5 (67%)
5.1 (76%)
5.7 (85%)
6.7
6.1
6.4
California sea lion, Zalophus californianus
DP=8 months
Weise and Costa, 2007
5 months
9 months
Small juvenile (1.5–2.5 years)
Large juvenile
Adult female
Sub-adult males
Adult males
42
75
100
2.36 (53%)
2.41 (55%)
2.95 (67%)
3.1 (70%)
4.42
2.93
3.52
Australian sea lion, Neophoca cinerea
DP=12 months
Fowler et al., 2007
6 months
14.5 months
22.6 months
3 years
Adult female
26
100
0.8 (30%)
1.3 (48%)
1.6 (59%)
2.2 (81%)
2.7
Australian fur seal, Arctocephalus pusillus doriferus
DP=12 months
D. P. Costa (University of California, Santa Cruz,
USA), personal communication
1 month
5 months
7 months
9 months
Adult female
8
42
58
75
0.38 (9%)
0.63 (15%)
0.89 (21%)
0.96 (23%)
4.16
Steller sea lion, Eumetopias jubatus
DP=12 months
Richmond, 2004
1 month
5 months
9 months
19 months
21 months
29 months
Adult female
Adult males
8
42
75
100
0.57 (20%)
1.29 (45%)
2.02 (70%)
2.46 (86%)
2.73 (95%)
3.11 (108%)
2.87
4.9
Species and Mb reference
Phocids
Harbor seal, Phoca vitulina
DP=28 days
Burns et al., 2005
Dependency period (DP) is the nursing interval plus any post-weaning fast (PWF). When a range for nursing durations is provided, the shortest period was
assumed.
Values in parentheses are the percentage of the levels in adult females.
Harbor seal: 4 week nursing (Reeves et al., 1992), swim and dive at birth (Greaves et al., 2005); hooded seal: 4 day nursing (Reeves et al., 1992), 1 month PWF
(Bowen et al., 1987); gray seal: 17 day nursing, 4 week PWF (Reeves et al., 1992); harp seal: 12 day nursing (Reeves et al., 1992), 5–6 week PWF (Sivertsen,
1941); Northern elephant seal: 28 day nursing, 2 months PWF (Reeves et al., 1992); California sea lion: 4–8 months, but frequently >1 year nursing (Reeves et al.,
1992); Australian sea lion: at least 1 year nursing (Reeves et al., 1992); Australian fur seal: at least 1 year, rarely 2 year nursing (Reeves et al., 1992); Steller sea
lion: 1, 2 and sometimes 3 year nursing (Reeves et al., 1992).
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Otariids
supports the diving breath hold in odobenids (the third group of
pinnipeds) has received little attention. Unlike the majority of
newborn pinnipeds, which primarily remain on land until they are
weaned from their mother’s milk, walruses enter the water within
about 2 days of birth (Fay, 1982) and are often observed nursing
underwater (Reeves et al., 1992). Walruses also have one of
the longest nursing and maternal dependency periods among
pinnipeds, as they have been observed nursing up to 35 months
post-partum and are attended by their mothers for 2–3 years
(Fay, 1982; Kovacs and Lavigne, 1992). Thus, odobenids have a
unique life history pattern among pinnipeds in that they are
precocial swimmers at birth despite having a prolonged maternal
dependency period. Typically, pinniped pups demonstrate a tight
correlation between the timing of muscle maturation and
independence from their mother, marking the time that the pup
must enter the water to forage (Table 2). Unlike the majority of
other pinnipeds, entry into the water and maternal independence
are uncoupled in the walrus. This provides a novel system to
explore the mechanism(s) that might drive the maturation of
these biochemical properties as well as to consider the ecological
consequences for mothers that must forage alongside
physiologically constrained offspring.
Development of muscle acid buffering
To date, the full developmental trajectory of the acid-buffering
capacity of the locomotor muscle has only been investigated in four
marine mammals [cetaceans: Tursiops truncatus (Noren, 2004) and
Phocoena phocoena (Noren et al., 2014a); pinnipeds: Cystophora
cristata and Pagophilus groenlandicus (Lestyk et al., 2009; Burns
et al., 2010)]. Based on this limited dataset, maturation of acidbuffering capacity in marine mammals appears to be protracted.
Adult levels are attained after the age of weaning, at least 25 days
post-partum in pinnipeds (Lestyk et al., 2009) to greater than
1.5 years post-partum in cetaceans (Noren, 2004; Noren et al.,
2014a). Of the species measured to date, Pacific walruses are unique
in that they are born with a mature locomotor muscle acid-buffering
capacity (Table 1, Fig. 2A).
For pinnipeds, it appears as though impending exposure to
exercise and hypoxia in the aquatic environment was the
evolutionary driver that promoted locomotor muscle readiness, as
is the case for the walrus that must follow its mother into the
water soon after birth. Oddly, results from studies on cetacean
species do not align with this assumption. The postnatal maturation
period required for cetaceans (dolphins, porpoises and whales) to
attain adult levels of muscle acid-buffering capacity is prolonged,
despite cetacean neonates being born directly into the ocean
(Noren, 2004; Noren et al., 2014a). The disparate results between
walruses and cetaceans may be associated with differences in
buffering capacity among adults. The acid-buffering capacity of
the longissimus dorsi is substantially lower in adult walruses
(41.30 slykes; Table 1) compared with adult cetaceans (63.70–
94.50 slykes; Noren, 2004) and other adult pinnipeds (81.1–
81.9 slykes; Burns et al., 2010). In addition, the acid-buffering
capacity of walrus muscle is low even when compared with the acidbuffering capacity of the locomotor muscle of adult terrestrial
mammals (49.70–66.9 slykes; Castellini and Somero, 1981).
Thus, postnatal development of this biochemical property may
not be necessary in walruses because the acid-buffering capacity of
their locomotor muscle is not above levels found in terrestrial
mammals. It is possible that the swim speeds needed to capture
sedentary or slow-moving benthic prey, and the short dive
durations (Jay et al., 2001) for their large body size, have not
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Journal of Experimental Biology (2015) 218, 3319-3329 doi:10.1242/jeb.125757
asserted adequate evolutionary pressure for increasing muscle acidbuffering capacity in walruses. Indeed, marine mammal species
that exhibit fast swim speeds and/or prolonged dive durations
typically have increased muscle acid-buffering capacity (Noren,
2004; Velten, 2012).
Development of muscle myoglobin
The demands of exercise and hypoxia have also been thought to
promote the maturation of myoglobin content (Morrison et al., 1966;
Stephenson et al., 1989; MacArthur, 1990; Saunders and Fedde,
1991; Kanatous et al., 2009; Geiseler et al., 2013). Detailed studies
of the ontogeny of myoglobin content in aquatic birds (Haggblom
et al., 1988), penguins (Weber et al., 1974; Ponganis et al., 1999;
Noren et al., 2001) and pinnipeds (Thorson, 1993; Noren et al.,
2005; Burns et al., 2005, 2007; Richmond, 2004; Fowler et al., 2007;
Weise and Costa, 2007) have demonstrated that myoglobin content
increased as immature animals increased exposure to physical
activity, thermal demands and hypoxia (Noren et al., 2001). Yet,
these factors fail to explain patterns observed for cetaceans, which
have prolonged myoglobin maturation periods despite facing the
demands of activity, thermoregulation and hypoxia immediately at
birth (for review, see Noren et al., 2014a). Interestingly, speciesspecific myoglobin maturation durations among odontocetes were
correlated with the calf nursing interval, suggesting that the rate of
muscle maturation evolved to match maternal dependency periods
(Noren et al., 2014a).
Closer examination of myoglobin maturation patterns across
pinnipeds demonstrates a delineation between phocids (true seals)
and otariids (fur seals and sea lions). Phocids typically show more
rapid development of myoglobin content compared with otariids
(Table 2). The rapid muscle development of phocids was attributed
to early entry into water and short maternal dependency periods
(nursing plus any post-weaning fast: 28–90 days; Table 2) compared
with the protracted muscle development of otariids, thought to be a
consequence of late entry into water and prolonged maternal
dependency periods (>1 year; Table 2; Burns et al., 2004; Noren
et al., 2005; Richmond et al., 2006). However, in these two groups,
pup independence is generally coupled with entry into the water,
making it difficult to discern which of the two factors (maternal
dependency period versus exposure to hypoxia) has driven the
pattern of muscle development over evolutionary time.
Walruses demonstrate rapid myoglobin maturation (Table 1,
Fig. 2B). Within a short period after birth, the myoglobin content of
the longissimus dorsi of walrus neonatal calves represents 44%
of adult levels, and by 3 months post-partum, the levels are 79% of
adult levels. This is one of the most rapid developmental trajectories
of myoglobin among pinnipeds, and is similar to the developmental
patterns observed for all phocid species studied to date (see Table 2
for review). The results from walruses suggest that perhaps exposure
to physical activity, thermal demands or hypoxia, but not the
duration of the maternal dependency period, may define speciesspecific myoglobin maturation patterns among pinnipeds.
Alternative hypotheses have suggested that inter-specific variation
in myoglobin maturation across species is due to differences in early
growth rates (Burns et al., 2004) and iron availability associated
with fasting (Burns and Hammill, 2008). Yet, despite dolphins and
walruses both having prolonged growth phases and being
consistently nursed, the myoglobin maturation period of dolphins
is prolonged (Noren et al., 2001) while that of walruses is truncated
( present study). Additional research is required to elucidate the
mechanism(s) behind the disparate maturation patterns found across
aquatic air-breathing animals.
Journal of Experimental Biology
RESEARCH ARTICLE
Limited samples from females precluded us from analyzing sexspecific differences in the developmental trajectory of myoglobin in
Pacific walruses. Of the studies that have examined myoglobin
maturation in pinnipeds, only four have examined sex-specific
differences (Table 2). Within phocids, adult myoglobin levels are
similar across males and females for both the harbor seal (Phoca
vitulina) and the sexually dimorphic Northern elephant seal
(Mirounga angustirostris). In contrast, sex-specific differences in
myoglobin levels were identified in two sexually dimorphic otariid
species (Table 2), but the results were inconsistent. For Stellar sea
lions (Eumetopias jubatus), adult males had 1.7× more myoglobin
per gram of muscle then female conspecifics, but in California sea
lions (Zalophus californianus), adult females had 1.3× more
myoglobin per gram of muscle than male conspecifics. It is likely
that sex-specific foraging behaviors and the magnitude of sexual
dimorphism work in concert to influence the magnitude of the
onboard muscle oxygen store.
Diving capacity of walruses
Walruses are among the largest pinniped species, which should
grant them an intrinsic advantage for prolonged breath holds
because the oxygen-carrying capacity of the body increases with
body mass by the power of 1.0 while metabolic rate increases with
body mass by the power of 0.75 (Kleiber, 1975). However, contrary
to this expectation, the recorded maximum dive duration of adult
male Pacific walruses is only 15.2 min (Jay et al., 2001) and they
are rarely observed to venture into water deeper than 80 m (Vibe,
1950; Fay, 1982). It is likely that Pacific walruses are
physiologically capable of deeper dives; indeed, Fay and Burns
(1988) observed that Pacific walruses killed on ice floes over deep
water (102–117 m) had fresh, undigested infaunal and epifaunal
mollusks and crustaceans in their stomachs. Therefore, it is likely
that walruses choose to stay in shallow water because their preferred
prey is most abundant on the shallow continental shelf (Fay and
Burns, 1988; Born et al., 2005; Jay et al., 2012). This behavior is in
agreement with the idea that it is advantageous for benthivores to
stay in shallow water to minimize transit time to the benthos and
thereby maximize foraging time on the seafloor, because foraging
time decreases with water depth (Costa and Gales, 2003).
To determine whether the physiology of Pacific walruses can
support longer dive durations and deeper dive depths than
previously observed, we calculated ADL. Based on calculations
that assumed a diving metabolism of 2× Kleiber metabolic rate, a
mature 830 kg female and 1200 kg male can dive aerobically for up
to 13.3 and 14.8 min, respectively (Fig. 5). When ascending and
descending at a speed typical of diving walruses (0.8 m s−1; Gjertz
et al., 2001; Jay et al., 2001), females and males can achieve
maximum dive depths of 319 and 355 m, respectively (Fig. 6). This
calculation is supported by observations of dive depths of >250 m in
Atlantic walruses (Born et al., 2005). However, deep dives such as
these would not allow walruses much time to search for prey on the
seafloor (Fig. 6). Thus, there is a trade-off between achieving greater
foraging depths and having adequate submergence time to locate,
handle and consume benthic prey (Fig. 6). Indeed, the median
foraging depth recorded for four adult male Pacific walruses in
Bristol Bay, AK, USA, was 41 m, and at this depth they spent a
median of 5–6 min along the benthos (Jay et al., 2001). According
to the calculated ADL determined in this study for a 1200 kg mature
adult male walrus, 13 min of bottom time would be permitted at this
depth (Fig. 6). This suggests that these walruses had considerable
flexibility to alter behavior, in terms of increasing search time along
the benthos.
Journal of Experimental Biology (2015) 218, 3319-3329 doi:10.1242/jeb.125757
Assuming that myoglobin levels are similar between males and
females, as found for the sexually dimorphic Northern elephant seal
(Thorson, 1993), the greater body size of male walruses could
provide males with an advantage over females when competing for
food in resource-limited habitats. This could occur when males and
females overlap in distribution during the winter breeding season
(Fay, 1982). Indeed, differences in diving behavior have been
documented in other sexually dimorphic pinnipeds, such as gray
(Halichoerus grypus; Beck et al., 2003) and southern elephant
(Mirounga leonina; McIntyre et al., 2010) seals, which supports the
hypothesis that body size is a limiting factor on the physiological
capacity for diving. For elephant seals, sex-specific differences in
dive depths were associated with differences in prey selection
between males and females; this could be a mechanism to minimize
inter-sexual competition (McIntyre et al., 2010). For walruses, there
is no evidence of resource partitioning between sexes when they
overlap in distribution during the winter. Thus, inter-sexual
competition could be problematic for female walruses if prey
availability decreases in the Arctic because the typical response of
pinnipeds to low prey availability is to increase dive duration
(Feldkamp et al., 1989; Crocker et al., 2006; Melin et al., 2008), and
breeding females will be most at risk as they have elevated energetic
requirements because of the costs of lactation (Noren et al., 2012,
2014b). In addition, maternal foraging behaviors could be
constrained by the physiological limits of their progeny. Although
neonatal calves accompany their mothers almost all the time,
including during foraging trips (Kovacs and Lavigne, 1992;
Charrier et al., 2010), there is no evidence that calves dive
alongside their mothers all the way to the seafloor. If neonatal calves
must accompany their mothers to the seafloor, maternal foraging
behaviors would be constrained by the physiological limits of their
calves, which only have a cADL of 4.7 min. This limit provides
3 min of bottom time for a 41 m dive and only 1 min of bottom
time for an 80 m dive. However, this cADL does support the
submergence times required for the calf to nurse underwater, which
last from about 0.5 to 2.0 min (Miller and Boness, 1983).
Even by the age of weaning (approximately 2 years post-partum;
Fay, 1982), the dive capacity of immature walruses is considerably
lower than that of adults (Fig. 6). Differences in diving ability at this
stage are associated with differences in body size because, by
weaning, the blood oxygen stores of walruses are mature (S.R.N.,
unpublished), and it is likely that the muscle oxygen store is mature,
based on the observation that the longissimus dorsi myoglobin
content was already 79% of adult levels by 3 months post-partum
(Table 1). Although there are no published accounts of the diving
behaviors of immature walruses for comparison, studies on other
pinnipeds have demonstrated shorter dive durations, shallower dive
depths and a lower percentage of time spent diving by immature
seals compared with adult conspecifics (Thorson, 1993; Lydersen
and Hammill, 1993; Lydersen et al., 1994; Horning and Trillmich,
1997; McCafferty et al., 1998; Bowen et al., 1999; Burns, 1999;
Greaves et al., 2005; Fowler et al., 2006; Rehberg and Burns, 2008).
The dive durations of immature pinnipeds increase with age as body
size increases (Kooyman et al., 1983; Lydersen and Hammill, 1993;
Thorson, 1993; Lydersen et al., 1994; Burns and Castellini, 1996;
Burns et al., 1998; Horning and Trillmich, 1997; Burns et al., 1998;
Costa et al., 1998; McCafferty et al., 1998; Burns, 1999), and these
ontogenetic differences in diving capacity have been associated
with inter-age class variations in diet (Bowen et al., 1999; Field
et al., 2007; Jeglinski et al., 2012). Yet, unlike these other pinnipeds,
independent juvenile walruses consume the same prey as adults, as
suggested by their stomach contents (Fay, 1982), which could result
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RESEARCH ARTICLE
in competition between juvenile and adult walruses for food if prey
became limited. The lower breath-hold capacity and inexperience
with foraging of independent immature walruses could result in
their being disproportionately affected during periods of low prey
availability, as has been observed in other immature pinnipeds
(Trillmich and Limberger, 1985; DeLong et al., 1991).
Conclusions
Among the pinniped species with prolonged maternal dependency
periods, Pacific walruses demonstrate the most rapid maturation of
the muscle biochemistry that supports breath hold while diving.
Pacific walruses are unique in that they are born with a mature
locomotor muscle acid-buffering capacity; however, the level
attained by adulthood is low compared with that of adult
cetaceans and other pinnipeds. The development of myoglobin in
walruses is one of the most rapid among pinnipeds. Additional
research is required to elucidate the mechanism(s) behind the
disparate muscle maturation patterns found across aquatic airbreathing animals. As for other young marine mammals, the smaller
body size of independent immature walruses could make them
competitively disadvantaged, in terms of diving capacity, in a
resource-limited environment. In addition, the extreme sexual
dimorphism in this species grants adult males a competitive
advantage over adult females in terms of breath-hold capacity to
support prey exploitation.
MATERIALS AND METHODS
Specimen and muscle collection
Muscle samples were collected during subsistence harvest of free-ranging
Pacific walruses. Neonatal calves (N=5; 1 male, 3 females, 1 unknown) were
harvested from Gambell, AK, USA. Sexually mature adults were harvested
from Gambell, AK, USA (N=2; 1 male, 1 female) and at Round Island, AK,
USA (N=18 males). The US Geological Survey’s Walrus Research program
is permitted to collect and possess biological samples from walruses
salvaged from the Alaska Native subsistence harvest under the US Fish and
Wildlife Service Marine Mammal Scientific Research permit no.
MA801652-7, which is issued by the Office of Management Authority
under regulation 50 CFR 18.31 of the US statute 16 USC 1371 (a) (1).
Additional samples were taken from a full-term, stillborn male fetus at Six
Flags Discovery Kingdom (Vallejo, CA, USA) and an orphaned immature
male walrus (3 months old) that was euthanized at the Alaska SeaLife Center
(Seward, AK, USA). Collection and laboratory protocols were approved
under University of California, Santa Cruz (UCSC) IACUC NORES1306.
Whole carcasses were typically sampled immediately after death. Whole
carcasses that could not be sampled immediately were frozen in a ‘snow
cellar’, with sampling occurring up to a maximum of 12 days postmortem.
A minimum of 10 g of muscle was sampled from the major swimming
muscle (longissimus dorsi). This muscle was sampled at a location
approximately three finger widths above the hip; the same site was
sampled from phocids, which also use hind flipper propulsion (Noren et al.,
2005). For a subset of walruses (the fetus, all neonatal calves, the immature
walrus and 9 adults), the supraspinatus was also sampled. This site was also
sampled from otariids that use front flipper propulsion (Weise and Costa,
2007). Muscle samples from the subsistence hunts were kept chilled with ice
and snow in a cooler and put into a −7°C freezer, shipped frozen to UCSC,
and stored in a −20°C freezer until muscle biochemical analyses were
performed within 6 months of collection. Muscle samples obtained from the
fetus and immature animal were immediately frozen in a −20°C freezer after
sampling.
Muscle biochemical analyses
To examine a component of the anaerobic capacity of the muscle, we
explored the ability of the muscle to buffer against lactic acid. The muscle
buffering capacity (β) due to non-bicarbonate buffers was determined using
procedures of Castellini and Somero (1981), adapted by Noren (2004).
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Briefly, thawed muscle samples (approximately 0.5 g) were minced in 10 ml
normal saline solution (0.9% NaCl), and sonicated (Sonifier Cell Disrupter
Model 450, Branson Ultrasonics Corporation, Danbury, CT, USA) for 3 min
on ice. Samples were maintained at 37°C by immersion of the test flask in a
warm water bath and titrated with 0.2 N NaOH. Buffering capacity was
measured in slykes (µmoles of base required to raise the pH of 1 g wet
muscle mass by one pH unit, over the range of pH 6.0 to pH 7.0). Changes
in pH were measured using an Accumet basic pH mV−1 °C−1 meter
(AB15+, Fisher Scientific) with an Accumet liquid-filled, glass-body
single-junction combination pH Ag/AgCl Electrode (13-620-285, Fisher
Scientific) and separate ATC probe (13-620-19, Fisher Scientific). All
samples were run in triplicate with a muscle sample obtained from a harbor
porpoise (Phocoena phocoena), which served as a control because the acidbuffering capacity of this specimen was determined previously (Noren et al.,
2014a).
To examine the oxygen storage capacity in the muscle, myoglobin
content ([Mb], reported in g Mb 100 g−1 wet muscle) was determined using
the procedure of Reynafarje (1963), which was adapted for marine
mammals by Noren and Williams (2000). Approximately 0.5 g of thawed
muscle was minced in a low ionic strength buffer (40 mmol l−1 phosphate,
pH 6.6) and then sonicated (Sonifier Cell Disrupter Model 450, Branson
Ultrasonics Corporation) for 2–3 min on ice. The buffer to tissue ratio was
19.25 ml buffer g−1 wet tissue. The samples were centrifuged at −4°C and
28,000 g for 50 min (Sorvall RC–5C Plus superspeed refrigerated
centrifuge, DuPont Instruments). The clear supernatant was extracted
and then bubbled at room temperature with pure CO for approximately
8 min. To ensure a complete reduction, 0.02 g of sodium dithionite was
added. The absorbance of each sample was read at room temperature at 538
and 568 nm on a spectrophotometer (Shimadzu UV – visible recording
spectrophotometer UV-160, Shimadzu Corporation, Kyoto, Japan). All
samples were run in triplicate with a muscle sample obtained from a harbor
porpoise (P. phocoeana), which served as a control because the myoglobin
content of this specimen was determined previously (Noren et al., 2014a).
Modeling breath-hold limits
The cADL was determined by dividing calculated total body oxygen stores
by estimates of diving metabolic rate following methods described in
Kooyman (1989). The cADL can accurately predict experimentally
determined ADL (Kooyman, 1989; Kooyman and Ponganis, 1998) when
estimates of body oxygen stores and metabolic rate are reliable (Ponganis
et al., 1997). We maximized the reliability of our calculations by utilizing
species-specific oxygen store data and we considered a range of metabolic
rates. Details regarding the assumptions that were made for these
calculations are provided below.
The calculations for the oxygen storage capacity of the blood (in l) are as
follows:
Arterial O2 ¼ ð0:33 BV M Þ ð½Hb] 0.00134Þ
ð0:95 0:20 saturationÞ;
Venous O2 ¼ ð0:67 BV MÞ ð½Hb 0:00134Þ
ð0:95 saturation
ð0:05 0:95 saturationÞÞ;
ð1Þ
ð2Þ
where 0.33 and 0.67 are the estimated proportions of arterial and venous
blood, respectively (Lenfant et al., 1970), the blood volume (BV) of
walruses is 0.106 l blood kg−1 (Lenfant et al., 1970), and age-specific body
mass (M) was derived from fig. 21 in Fay (1982), which provided a graphical
representation of age versus measurements of body mass from free-ranging
Pacific walruses. S.R.N. (unpublished) found that the hemoglobin content
([Hb]) of walruses did not vary with age; therefore, the average value of
163.5 g Hb l−1 of blood was used in the calculation for all age classes. The
oxygen-binding capacity of hemoglobin is 1.34 ml O2 g−1 Hb (Kooyman,
1989), and the proportion of saturation and depletion of arterial and venous
oxygen reserves is described in detail in Ponganis (2011).
The calculation for the oxygen storage capacity of the muscle (in l) is as
follows:
Muscle O2 ¼ ð½Mb] 0.00134ÞMm ;
ð3Þ
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Journal of Experimental Biology (2015) 218, 3319-3329 doi:10.1242/jeb.125757
Lung O2 ¼ M TLV 0:50 0:15,
ð4Þ
where body mass (M) was determined as described above and total
lung volume (TLV) is 0.116 l kg−1 based on the measured lung capacity of
three, 4–6 week old walrus calves (Lenfant et al., 1970). We assumed
that the diving lung volume for a pinniped is 50% of TLV, with
15% representing the oxygen concentration of the air in the lungs
which is extracted during the dive (Ponganis, 2011). Because of a lack of
data on how lung volume changes with age in walruses, we assumed a
constant mass-specific lung volume as in previous pinniped studies (e.g.
Noren et al., 2005). Future research on how lung volume changes with
maturation is warranted as recent studies on other marine mammals show
opposite trends; the mass-specific lung volume of sea otters (Enhydra lutra;
Thometz et al., 2015) decreases with maturation while the mass-specific
lung volumes of two phocid species increase with age (J.M.B.,
unpublished).
Across air-breathing, diving vertebrates, including three Odontocete
species (Noren et al., 2012), shallow-diving emperor penguins (Aptenodytes
forsteri; Ponganis et al., 2010) and freely diving Weddell seals
(Leptonychotes weddelli; Castellini et al., 1992; Ponganis et al., 1993),
a cADL that assumed an oxygen consumption rate of 2× Kleiber basal
metabolic rate (BMR; Kleiber, 1975) best approximated experimentally
determined ADLs. Based on these findings, we assumed a metabolic rate of
2× BMR for our cADLs to estimate age-specific bottom times at various
dive depths, where BMR (in l O2 min−1) is 0.0101×M0.75 (Kleiber, 1975).
To calculate bottom time during a dive, we assumed a swim speed of
0.8 m s−1, which is the typical ascent and descent swim speed of foraging
adult walruses (Gjertz et al., 2001; Jay et al., 2001). Thus, total bottom time
at each depth was calculated according to:
Bottom time ¼ cADL
ð0:8 2dÞ;
ð5Þ
−1
where cADL is the calculated ADL (in s), 0.8 is swim speed (in m s ) and d
is the depth of the dive (in m).
Without empirical data on the age-specific diving metabolism of
walruses, additional cADLs were determined, to provide a range for the
diving capacities of walruses. cADLs that assumed hypometabolism during
submergence were considered based on the observation that California sea
lions (Zalophus californianus) can reduce resting-surface metabolic rates of
2× BMR by 39% when sedentarily submerged (Hurley and Costa, 2001).
This assumption likely overestimates cADLs as free-ranging diving
walruses are not sedentary. cADLs that assumed oxygen consumption
rates during activity were also determined by using the average field
metabolic rate (FMR) of two adult male Atlantic walruses (approximately
6× BMR; Acquarone et al., 2006). However, the use of a FMR could
underestimate cADLs because marine mammals use oxygen-conserving
strategies while diving, such as the dive response (Scholander, 1963) and
stroke and glide propulsion (Williams et al., 2000). Nonetheless, an elevated
diving metabolism may be the best model for the diving capacity of
immature walruses as Weddell seals (Leptonychotes weddelli) demonstrate a
strong developmental effect on diving metabolism, where experimentally
determined ADLs for pup, yearling and adult Weddell seals were best
approximated by cADLs that assumed a diving metabolism of 4× BMR,
2× BMR and 1× BMR, respectively (for review of these data, see Schreer
et al., 2001).
Statistics
Limited samples from females precluded us from analyzing sex-specific
differences in the developmental trajectory of muscle biochemistry within
Pacific walruses. Thus, samples across males and females were combined
for statistical analyses. Differences in muscle acid-buffering capacity and
myoglobin content between the supraspinatus and longissimus dorsi within
the neonatal calf and adult age classes were tested using paired Student’s
t-tests. Differences in muscle acid-buffering capacity and myoglobin
content of the supraspinatus and longissimus dorsi between the neonatal
calf and adult age classes were tested using Student’s t-tests. Only one fetus
and one immature 3 month old walrus were sampled, so they were not
included in the statistical comparisons. The interaction between the ratio of
acid-buffering capacity to myoglobin content with age-related changes in
myoglobin content in the supraspinatus and longissimus dorsi was
examined with non-linear regression analyses. All statistical analyses were
conducted using SigmaStat 3.5 (Systat Software, Inc.).
Acknowledgements
We thank the Alaska Native hunters in Gambell and hunters from villages near
Round Island for generously providing muscle samples from the walruses they
harvested; J. Garlich-Miller (US Fish and Wildlife Service, Marine Mammals
Management, Anchorage) and S. Rice (Alaska Department of Fish and Game,
stationed at Round Island) provided assistance in securing some of these samples.
We thank Six Flags Discovery Kingdom (Vallejo, CA, USA) for providing muscle
samples from the stillborn walrus calf. We thank the veterinary and husbandry staff at
Alaska SeaLife Center (Seward, AK, USA) for providing muscle samples from the
euthanized orphaned 3 month old walrus. We thank D. Pabst, and T. Williams and
her laboratory for providing reviews of previous versions of this manuscript. Any use
of trade names is for descriptive purposes only and does not imply endorsement by
the federal government.
Competing interests
The authors declare no competing or financial interests.
Author contributions
S.R.N. developed the approach, performed the majority of the laboratory and data
analyses, and prepared the manuscript. C.V.J. secured funding for the study and
helped prepare the manuscript. J.M.B. performed the laboratory analyses for the
adult muscle samples. A.S.F. went into the field in Alaska to secure the muscle
samples from the neonatal calves and adult walruses.
Funding
This work was funded by the US Geological Survey’s (USGS) Ecosystem Mission
Area, Wildlife Program.
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where [Mb] is myoglobin content in g per 100 g wet muscle mass, 1.34 ml
O2 g−1 Mb is the oxygen-binding capacity of Mb (Kooyman, 1989) and Mm
is total muscle mass in the body. Total muscle mass was calculated as
Mm=0.2410×M1.084 (equation derived from dissection of various aged male
and female Atlantic walruses; Knutsen and Born, 1994), where M was
determined as described above. Observations of swimming walruses
suggest that hind flipper propulsion is the dominant form of locomotion
(Fay, 1985); thus, following the approach of studies of phocids, which also
use hind flipper propulsion, age-specific Mb levels from the longissimus
dorsi were assumed for all muscle groups for the purpose of calculating total
muscle oxygen storage capacity (i.e. Noren et al., 2005). Although the
approach of assuming one myoglobin level across all muscle groups is
consistent with previous studies that have calculated the muscle oxygen
stores of marine mammals, this approach can potentially overestimate the
muscle oxygen store as non-swimming muscles of marine mammals
generally have lower myoglobin levels (e.g. Polasek and Davis, 2001;
Lestyk et al., 2009).
The calculation for the oxygen storage capacity of the lung (in l) is as
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