482
Development of the Blood and Muscle Oxygen Stores in Gray Seals
(Halichoerus grypus): Implications for Juvenile Diving Capacity
and the Necessity of a Terrestrial Postweaning Fast
Shawn R. Noren1,*
Sara J. Iverson2
Daryl J. Boness1
1
Conservation and Research Center, National Zoological
Park, Smithsonian Institution, Washington, DC 20008;
2
Department of Biology, Dalhousie University, Halifax, Nova
Scotia B3H 4J1, Canada
Accepted 11/5/2004; Electronically published 5/24/2005
ABSTRACT
To successfully transition from nursing to foraging, phocid seal
pups must develop adequate diving physiology within the limited time between birth and their first independent foraging
trip to sea. We studied the postpartum development of oxygen
stores in gray seals (Halichoerus grypus, n p 40 ) to better understand the ontogeny of diving capacity in phocids. Hemoglobin (Hb), hematocrit (Hct), blood volume (BV), and myoglobin (Mb) levels in newborn (3 d postpartum [DPP]) and
newly weaned (17 Ⳳ 0.4 DPP) pups were among the lowest
measured across age classes. During the pups’ terrestrial postweaning fast (PWF), Hb, Hct, mass-specific BV, and Mb increased by 28%, 21%, 13%, and 29%, respectively, resulting in
a 35% increase in total body mass-specific oxygen stores and
a 23% increase in calculated aerobic dive limit (CADL). Although Hb and Hct levels at the end of the PWF were nearly
identical to those of yearlings, total body mass-specific oxygen
stores and CADL of weaned pups departing for sea were only
66%–67% and 32%–62%, respectively, of those for yearlings
and adult females. The PWF represents an integral component
of the physiological development of diving capacity in phocids;
however, newly independent phocids still appear to have limited
diving capabilities at the onset of foraging.
1
Corresponding author. Present address: National Marine Fisheries Service,
Southwest Fisheries Science Center, 8604 La Jolla Shores Drive, La Jolla, California 92038; e-mail: shawn.noren@noaa.gov.
Physiological and Biochemical Zoology 78(4):482–490. 2005. 䉷 2005 by The
University of Chicago. All rights reserved. 1522-2152/2005/7804-4088$15.00
Introduction
Aquatic lung-breathing animals routinely experience prolonged
periods of apnea while diving. During these periods, aerobic
metabolic processes are supported by the use of onboard oxygen
stores. The blood oxygen storage capacity in vertebrates is dependent on hemoglobin content and blood volume (Snyder
1983), while the muscle oxygen storage capacity is dependent
on myoglobin content and muscle mass (Kooyman 1989). As
a result, adult diving endotherms (i.e., aquatic and marine
mammals and birds) have evolved elevated levels of blood hemoglobin (Lenfant et al. 1970; Snyder 1983; Kooyman 1989),
blood volume (Snyder 1983; Kooyman 1989), and muscle myoglobin (Castellini and Somero 1981; Kooyman 1989), which
confer greater mass-specific oxygen storage capacities than their
terrestrial relatives (Snyder 1983; Kooyman 1989). Within marine mammals, species that dive the deepest and for the longest
durations have the greatest oxygen carrying capacity in both
the blood (Ridgway and Johnston 1966; Lenfant et al. 1970;
Hedrick et al. 1986; Hedrick and Duffield 1991) and muscle
(Castellini and Somero 1981; Noren and Williams 2000).
Although adult marine endotherms have elevated oxygen
stores, recent studies suggest that neonates and juveniles have
relatively low oxygen storage capacity in both the blood and
muscle (Thorson 1993; Thorson and Le Boeuf 1994; Ponganis
et al. 1999; Burns et al. 2000; Noren et al. 2001, 2002; Noren
2002; Clark 2004; Richmond 2004). In terrestrial and semiaquatic endotherms, the oxygen storage capacity in the blood
and muscle also changes throughout life. For example, the hemoglobin content in humans (Rothstein 1993) and sheep (Potocnik and Wintour 1996) increases with age throughout development. Meanwhile, increases in myoglobin content in
diving ducks, muskrats, geese, and voles have been associated
with factors such as development, increased exposure to hypoxia, increased physical activity, and increased thermal demands
(Morrison et al. 1966; Stephenson et al. 1989; MacArthur 1990;
Saunders and Fedde 1991; MacArthur et al. 2001).
Neonatal pinnipeds (seals, fur seals, and sea lions) must shift
from a terrestrial environment to an amphibious existence soon
after birth or weaning. This transition requires that pups are
physically prepared for the demands of swimming, diving, and
foraging in the ocean in a relatively short period of time. In
most true seals (family: Phocidae), the nursing period is short
(4–50 d), and weaning occurs abruptly when the mother re-
Development of Blood and Muscle Oxygen Stores in Gray Seals
turns to sea to forage and leaves her pup on the beach (Oftedal
et al. 1987). The abandoned pups then undergo a terrestrial
postweaning fast (PWF) of weeks or months during which they
must rely on the large reserves of fat acquired during the suckling period (Bowen 1991). Although the adaptive significance
of this PWF is not well understood, it may serve to prolong
the pups’ time on land so that the pups attain critical physiological traits for future diving and foraging at sea (Thorson
1993; Thorson and Le Boeuf 1994).
Only a few studies have simultaneously measured the development of the blood and muscle oxygen stores in phocids
or other marine mammals (northern elephant seal [Mirounga
angustirostris]: Thorson 1993; Thorson and Le Boeuf 1994;
hooded seal [Cystophora cristata]: Burns et al. 2000; bottlenose
dolphin [Tursiops truncatus]: Noren et al. 2001, 2002; Noren
2002; harbor seal [Phoca vitulina]: Clark 2004; and Steller sea
lion [Eumetopias jubatos]: Richmond 2004). Understanding the
phylogenetic constraints and ecological correlates of the development of these physiological traits requires data on a wide
number of species. The gray seal (Halichoerus grypus) is a largebodied phocid seal whose pups nurse for only about 16 d
(Boness et al. 1995) and undergo a 3–4-wk PWF (Davies 1949;
Coulson and Hickling 1964). Only limited data are available
on the oxygen stores of gray seals (Scholander 1940; Lapennas
and Reeves 1982; Reed et al. 1994a, 1994b), and the development of these stores in this species is virtually unknown (Greenwood et al. 1971). Thus, gray seals serve as an excellent model
for the study of oxygen storage development during the PWF.
Our aim was to examine the development of the blood and
muscle oxygen stores in gray seals from birth through the PWF,
to compare these stores to those measured in yearlings and
adults, and to investigate how this postnatal development may
impact early diving capacity.
Material and Methods
Field Sampling
The Smithsonian’s Conservation and Research Center Institutional Animal Care and Use Committee approved all experimental protocols used in this research. Fieldwork was conducted on gray seals (Halichoerus grypus) at Sable Island, Nova
Scotia, Canada (43⬚55⬘N, 60⬚00⬘W) during the pupping season,
December 2002 to February 2003. Ten yearlings and 10 motherpup pairs at 3 d postpartum (DPP) were captured and sampled
once for morphological and physiological measurements.
Eighty additional mother-pup pairs were marked at parturition
with dye, given a uniquely numbered hind flipper tag (Jumbo
Rototag), and subsequently observed until the pups were
weaned. Ten of these known-age pups (nursing period: 14–19
d; mean: 16.6 Ⳳ 0.5 d) were captured at weaning and held in
a pen for longitudinal studies, during which they were sampled
at 0, 12, and 24 d postweaning (DPW) and subsequently
released.
483
At each sampling, seals were manually captured and sedated
with a dose of diazepam, which was administered via the extradural vein at a dose of approximately 0.23, 0.23, 0.29, and
0.13 mg kg⫺1 for newborns (3 DPP), weaned pups, yearlings,
and adult females, respectively. Dorsal body length, axillary
girth, and body mass were measured for each individual. Samples were then taken for measurement of blood hemoglobin
(Hb) content, hematocrit (Hct), plasma volume (PV), blood
volume (BV), and muscle myoglobin (Mb) content as follows.
An initial 10-mL blood sample (T p 0 min) was taken from
the extradural vein into a heparinized tube (Vacutainer Brand)
followed by an intravenous injection (2.5–4.5 mL) of Evans
blue dye at a concentration of 4, 10, and 25 mg mL⫺1 for
newborns (3 DPP), weaned pups and yearlings, and adults,
respectively. Sequential 10-mL blood samples were taken as
described above at 10, 20, and 30 min postinjection. A muscle
sample was taken from the primary locomotor muscle, longissimus dorsi, at a location above the hip. The biopsy site was
first shaved, cleansed with betadine, and infiltrated subcutaneously with 2.5 mL lidocaine containing epinephrine (Astra
Pharmaceuticals). A 1-cm incision was made with a no. 11
scalpel blade through which a 6-mm sterile biopsy punch (Miltex) was inserted and a muscle sample was taken from beneath
the blubber layer. Muscle biopsy samples averaged about 50
mg. All blood and muscle samples were immediately placed on
ice and transported to the field lab several hours later.
Laboratory Analyses
At the field lab, duplicate 10-mL aliquots of whole blood from
each T p 0 min tube were added to aluminum-foil-wrapped
cryovials containing 2.5 mL of Drabkins Solution (Total Hemoglobin Sigma Kit 525A). The samples were stored in the
dark at room temperature until analysis (within 2 mo of collection). Hb was determined from these samples using the cyanmethemoglobin technique following methods described in the
total hemoglobin kit (Sigma Kit 525A) and adapted for phocid
seals (Thorson 1993; Thorson and Le Boeuf 1994).
Hct was also determined from the T p 0 min samples using
the microcentrifuge method. A small amount of blood from
each sample was collected in duplicate into microhematocrit
tubes and spun at 13,460 g/11,500 rpm for 5 min in a MicroMB Microhematocrit/Microcentrifuge (ThermoIEC). Percent
packed cell volume was then determined from a microcapillary
reader (ThermoIEC).
The remaining blood from the 0-, 10-, 20-, and 30-min blood
samples were spun at 1,000 rpm for 25 min in a desktop centrifuge. The supernatant from each sample was pipetted into
a 15-mL tube (Corning) and frozen at 0⬚C in the field for
several weeks and at ⫺20⬚C thereafter. PV and BV were then
determined from these samples (within 2 mo) using the Evans
blue-dye method as described by Swan and Nelson (1971) and
adapted by El-Sayed et al. (1995).
484 S. R. Noren, S. J. Iverson, and D. J. Boness
Muscle samples were frozen at 0⬚C in the field for several
weeks and at ⫺80⬚C thereafter. Mb content of the muscle was
determined within 5 mo of collection following the methods
of Reynafarje (1963) as described in detail by Noren and Williams (2000) and Noren et al. (2001).
Mean Corpuscular Hemoglobin Content (MCHC) and Aerobic
Dive Limit (ADL) Calculations
To determine the average concentration of hemoglobin in a
red blood cell, mean corpuscular hemoglobin content (MCHC)
was calculated for each seal according to the equation
MCHC p (Hb Hct⫺1) # 100.
(1)
To estimate the absolute maximum dive duration supported
by aerobic processes for each seal, the calculated aerobic dive
limit (CADL) was determined by dividing the calculated total
body oxygen store by metabolic rate following the methods
described by Kooyman (1989). The total body oxygen store for
each seal was determined by inputting their mass-specific Hb,
BV, and Mb into the following equations adapted for phocids:
blood oxygen store p arterial O2 ⫹ venous O2 ,
(2)
arterial O2 p (0.33 # BV # m)
# (Hb # 1.34 mL O2 g Hb⫺1),
(3)
venous O2 p (0.66 # BV # m)
# (Hb # 1.34 mL O2 g Hb⫺1),
(4)
where 0.33 and 0.66 are the estimated proportions of arterial
and venous blood, respectively (Lenfant et al. 1970), 1.34 mL
O2 g Hb⫺1 is the oxygen binding capacity of Hb (Kooyman
1989), m is body mass (kg), and arterial and venous saturation
is 0.20 to 0.95 and 0.15 to 0.90 for equations (3) and (4),
respectively (Kooyman 1989.
muscle oxygen store p (Mb # 1.34 mL O2 g Mb⫺1)
# (m # p),
(5)
where 1.34 mL O2 g Mb⫺1 is the oxygen binding capacity of
Mb (Kooyman 1989) and p is the estimated proportion of
muscle mass in the body. We assumed that muscle mass represented 18.5% and 30.0% of total body mass for pups and
yearlings/adults, respectively, according to previous data for
phocid seals (Kooyman et al. 1983; Burns et al. 2000). The lung
oxygen store was assumed to represent a constant proportion
of body mass at 4.1 mL O2 kg⫺1, based on the estimated diving
lung volume (27.3 mL kg⫺1; Kooyman et al. 1970) and the
expected proportion of oxygen extracted from the air in the
lungs (0.15; Kooyman 1989).
It was beyond the scope of this study to measure the diving
metabolic rate of gray seals. Thus, the CADL of the adult age
class was based on the average metabolic rate measured for
adult diving gray seals (5.2 mL O2 min⫺1 kg⫺1; Reed et al.
1994a), which approximated 1.9 times that of Kleiber (1975).
This metabolic rate is similar to that which was used to calculate
ADLs of adult elephant seals (Thorson 1993; Thorson and Le
Boeuf 1994). Immature seals have relatively higher metabolic
rates than adults because of additional costs associated with
growth and development. Thus, because the resting metabolic
rate of juvenile gray seals does not approach predicted values
for adults until sometime after the first year of life (Boily and
Lavigne 1997), the CADL of all immature seals was based on
the mean daily energy expenditure measured for gray seal pups
during the PWF (Reilly 1991), which approximated 2.7 times
that of Kleiber (1975).
Statistics
To assess developmental changes throughout the PWF, we used
one-way repeated-measures ANOVA in combination with Tukey all pairwise comparisons tests to compare oxygen storage
parameters (Hb, Hct, MCHC, PV, BV, and Mb), mass-specific
oxygen stores (blood, muscle, and total body), and CADLs
across the longitudinally sampled weaned pups (at 0, 12, and
24 DPW). We used one-way ANOVA in combination with
Tukey all pairwise comparison tests to compare these same
parameters across population age classes. These comparisons
included the independent cross-sectional sampled animals (3DPP neonates, yearlings, and adults) and only one data point
from the longitudinally sampled weaned pups (24 DPW). Values are reported as mean Ⳳ SE. Findings were considered significant when P ! 0.05.
Results
In pups sampled longitudinally, Hb content and Hct significantly increased on average by 28% and 21% over the PWF,
respectively, while MCHC did not change (Fig. 1; Table 1).
These results suggest that the number of red blood cells in the
blood increased while the oxygen carrying capacity of individual
red blood cells stayed consistent. As a result, mass-specific BV
increased significantly throughout the PWF from 11.6% of
body mass at weaning to 13.1% of body mass by 24 DPW (Fig.
2; Table 1). The consistent value for mass-specific PV over the
PWF (Fig. 2; Table 1) provides evidence that the increased Hb
and Hct at the end of the PWF were associated with the addition
of red blood cells into the circulation and were not a result of
dehydration. Although mean muscle Mb content appeared to
increase on average by 29% over the PWF, the differences were
not significant, a result potentially associated with the limited
Development of Blood and Muscle Oxygen Stores in Gray Seals
485
in Hb content, Hct, mass-specific PV and BV, and muscle Mb
content (Table 1). For example, adult mass-specific BV and Mb
are 1.3–1.8 and 1.9–2.4 times, respectively, those for newborn
and weaned pups. Meanwhile, MCHC remained relatively constant among groups (Table 1).
As a result of the ontogenetic changes in the blood and
muscle, the oxygen storage of the body increased with a concomitant increase in CADL throughout development. The
mass-specific oxygen stores in the blood and in total differed
significantly across longitudinal sample points and increased
by 46% and 35%, respectively, over the PWF (Table 2). Although mass-specific muscle oxygen stores appeared to increase
by 33% over the PWF, the differences were not significant, a
result potentially associated with the limited number of Mb
samples for the 0-DPW sample point (Table 2). Cross-sectional
comparisons across the population demonstrated that there
were also significant differences among age classes in massspecific oxygen stores in blood, in muscle, and in total (Table
2). Furthermore, assuming that the oxygen storage in the lung
represented a constant proportion of body mass, the relative
contribution of each of the three storage sources to total oxygen
stores changed ontogenetically. The relative contribution from
both lung and blood storage appeared to decline from birth
through adulthood (Table 2). In contrast, the relative contribution of muscle oxygen storage appeared to increase from
12% of total stores at birth to 16% near the end of the PWF
and to 21%–27% in yearlings and adults (Table 2). Concurrent
with the increasing total body mass-specific oxygen stores during the PWF and thereafter, CADL increased significantly by
23% during the PWF and increased across age classes such that
the CADL of adult females is 4.0, 3.1, and 2.0 times that of
newborn pups (3 DPP), weaned pups (24 DPW), and yearlings,
respectively (Table 2). Furthermore, the actual aerobic dive limits of young foraging gray seals may be even lower than the
CADLs presented here. The CADLs of the immature gray seals
were calculated with a metabolic rate measured from fasting
gray seal pups (Reilly 1991). The fast undoubtedly elicited depressed metabolic rates. Once the pups initiate foraging, a rapid
increase in metabolic rate is expected, which would further
decrease aerobic dive limit.
Figure 1. a, Hemoglobin content (Hb), b, hematocrit (Hct), and c,
mean corpuscular hemoglobin content (MCHC) of gray seals during
the postweaning fast (PWF) plotted as days postpartum (DPP). Each
unique symbol represents an individual pup that was studied longitudinally during the PWF. Hb and Hct increased significantly during
the PWF (F2, 29 p 31.505, P ! 0.001 and F2, 29 p 8.192, P p 0.003, respectively), but MCHC did not change (F2, 29 p 1.716, P p 0.208;
repeated-measures ANOVA).
number of samples for the 0-DPW sample point (Fig. 3; Table
1).
Cross-sectional comparisons across the population demonstrated that there were significant differences among age classes
Discussion
Of the marine mammal groups, phocid seals have the greatest
mass-specific oxygen storage capacity (Kooyman 1989). In this
family, the blood is the most important oxygen store (generally
about 65%), followed by the muscle (30%) and the lung (5%;
Kooyman 1989). Oxygen storage in the lungs is negligible due
to lung collapse at 40 m (Kooyman et al. 1972; Falke et al.
1985), which prevents gas exchange with the circulatory system
(Kooyman et al. 1970). Like other adult phocids, adult gray
seals have high mass-specific Hb, Hct, BV, and Mb levels (Reed
et al. 1994a, 1994b; Table 1) and similar contributions to total
486 S. R. Noren, S. J. Iverson, and D. J. Boness
Table 1: Blood and muscle properties of gray seal age classes
Class (age)
Newborn (3 DPP)
0 DPW (17 Ⳳ .4 pp)
12 DPW (29 Ⳳ .5 DPP)
24 DPW (41 Ⳳ .5 DPP)
Yearling (1 yr)
Adult (16 yr)
Hb (g dL⫺1)
19.9
18.7
21.7
24.0
23.0
19.6
Ⳳ
Ⳳ
Ⳳ
Ⳳ
Ⳳ
Ⳳ
a
.3
.5d,e
.6e,f
.5b,d,f,g
.9b
1.5a,c
Hct (%)
45.4
42.8
49.8
51.8
50.4
44.9
Ⳳ
Ⳳ
Ⳳ
Ⳳ
Ⳳ
Ⳳ
.6a
1.6d,e
1.5f
1.3b,f,g
1.9
2.1a
MCHC
(g dL⫺1)
43.9
44.2
43.7
46.5
45.8
43.1
Ⳳ
Ⳳ
Ⳳ
Ⳳ
Ⳳ
Ⳳ
.5
1.8
.5
.9
.7
1.8
PV
(mL kg⫺1)
86.6
65.8
60.6
63.2
99.3
116.6
Ⳳ
Ⳳ
Ⳳ
Ⳳ
Ⳳ
Ⳳ
8.9b
1.8
1.8
1.7b,c
3.2a
9.5a,g
BV
(mL kg⫺1)
159.0
115.7
121.8
131.2
201.5
213.2
Ⳳ
Ⳳ
Ⳳ
Ⳳ
Ⳳ
Ⳳ
17.3b
4.1e
4.9
2.6b,c,f
7.5a
17.2a,g
Mb
(g 100 g
muscle⫺1)
1.7
2.1
2.5
2.7
3.2
4.0
Ⳳ
Ⳳ
Ⳳ
Ⳳ
Ⳳ
Ⳳ
.1b,c
.3
.3
.3b
.3g
.3a,g
Note. Values are mean Ⳳ SE for hemoglobin (Hb), hematocrit (Hct), mean corpuscular hemoglobin content (MCHC), mass-specific plasma volume (PV),
mass-specific blood volume (BV), and muscle myoglobin (Mb) content. Ten seals were sampled in each age class, except for newborn Mb (n p 9 ) and 0 d
postweaning (DPW) Mb (n p 7 ). DPP p days postpartum. Significant differences across postweaning (PW) longitudinal sample points (repeated-measures
ANOVA on PW longitudinal samples: 0, 12, and 24 DPW) were found for Hb (F2,29 p 31.505 , P ! 0.001 ), Hct (F2,29 p 8.192 , P p 0.003 ), and mass-specific
BV (F2,29 p 5.965, P p 0.010). Mass-specific PV (F2,29 p 3.274 , P p 0.061 ) and MCHC (F2,29 p 1.716 , P p 0.208 ) values did not differ significantly across
longitudinal sample points during the fast. Although the mean value for Mb appeared to increase throughout the PW period, the differences across sample
points were not significant (F2,26 p 0.764 , P p 0.483 ). Significant differences across population age classes (ANOVA on cross-sectional population samples:
newborn, 24-DPW pup, yearling, and adult) were found for Hb (F3,39 p 6.044, P p 0.002), Hct (F3,39 p 4.769, P p 0.007), mass-specific PV (F3,39 p
11.111, P ! 0.001), mass-specific BV (F3,39 p 8.773 , P ! 0.001 ), and Mb (F3,38 p 11.477 , P ! 0.001 ). MCHC (F3,39 p 2.041 , P p 0.125 ) values did not differ
significantly across population age classes.
a
Values different than 24 DPW (Tukey all pairwise comparison across cross-sectional population samples at P ! 0.05).
b
Values different than adult (Tukey all pairwise comparison across cross-sectional population samples at P ! 0.05).
c
Values different than yearling (Tukey all pairwise comparison across cross-sectional population samples at P ! 0.05).
d
Values different than 12 DPW (Tukey all pairwise comparison across postweaning longitudinal samples at P ! 0.05).
e
Values different than 24 DPW (Tukey all pairwise comparison across postweaning longitudinal samples at P ! 0.05).
f
Values different than 0 DPW (Tukey all pairwise comparison across postweaning longitudinal samples at P ! 0.05).
g
Values different than newborn (Tukey all pairwise comparison across cross-sectional population samples at P ! 0.05).
oxygen stores from blood (66%), muscle (27%), and lung (7%;
Table 2). These large oxygen stores correspond with extended
periods of time at sea, characterized by continuous diving and
a high percentage of time spent submerged (Reed et al. 1994a;
Beck et al. 2003). Adult female gray seals dive along the bottom
of the ocean and are capable of reaching depths greater than
250 m for maximum durations of 22 min (Beck et al. 2003).
In contrast to adult values, gray seal pups had relatively
underdeveloped oxygen stores in the blood and muscle both
at birth (3 DPP) and at weaning (at about 17 DPP; Table 1).
This resulted in low total oxygen stores (Table 2). With such
a brief nursing period, the pups were still very young at weaning
and thus perhaps necessitate a PWF to develop the physiology
that supports diving. The total mass-specific oxygen stores of
the pups increased by 35% after 24 d of the PWF (at about 41
DPP) and were 66% of those in yearlings. At the end of the
PWF, gray seal pups at Sable Island depart to sea to forage for
the first time at approximately 40 DPP (S. R. Noren, unpublished observation). Although the dive capacity of the pups
increased during the PWF, it was still limited compared to that
of yearlings and adults.
These results are consistent with other immature phocids
studied to date, which have low mass-specific BV and Mb levels
at the onset of independent foraging (Thorson 1993; Thorson
and Le Boeuf 1994; Burns et al. 2000; Clark 2004; this study).
Yet there is undoubtedly a minimum level of oxygen stores
required at independence to ensure that immature phocids are
competent predators. Consequently phocids with short postpartum terrestrial periods (nursing and PWF periods combined) should obtain enhanced mass-specific levels of Hb, BV,
Figure 2. Mass-specific plasma volume (PV) and blood volume (BV)
of gray seal pups during the postweaning fast (PWF) plotted as days
postpartum (DPP). Each unique symbol represents an individual pup
that was studied longitudinally during the PWF. Mass-specific PV (gray
symbols) did not change (F2, 29 p 3.274 , P p 0.061), while mass-specific
BV (black symbols) increased significantly during the PWF (F2, 29 p
5.965, P p 0.010; repeated-measures ANOVA).
Development of Blood and Muscle Oxygen Stores in Gray Seals
and Mb earlier in life than those with longer postpartum terrestrial periods. For instance, northern elephant seal pups have
low mass-specific Hb, BV, and Mb at birth and throughout
their 28-d nursing period (Thorson 1993; Thorson and Le
Boeuf 1994). After weaning, elephant seal pups experience a
terrestrial PWF period (Le Boeuf et al. 1972), which lasts on
average for 65 d (Noren et al. 2003). During this time, Hb, BV,
and Mb levels increase with a concomitant rise in total massspecific oxygen stores (Thorson 1993; Thorson and Le Boeuf
1994). At the end of the PWF, 90-d-old elephant seal pups have
mature Hb levels and 68% and 76% of adult mass-specific BV
and Mb, respectively (Thorson 1993; Thorson and Le Boeuf
1994). In comparison, gray seal pups at Sable Island are less
than half the age of elephant seal pups at the onset of independent foraging (40 DPP on average; S. R. Noren, unpublished
observation). At this stage, gray seal pups have mature Hb levels
and 62% and 68% of adult mass-specific BV and Mb, respectively (Table 1). Furthermore, at the time of departure, both
northern elephant and gray seal pups exhibit a similar percentage of the total mass-specific oxygen store found in their
adult female counterpart, 73% (Thorson 1993; Thorson and
Le Boeuf 1994) and 67% (Table 2), respectively. Interestingly,
bottlenose dolphins and Steller sea lions, which both have protracted nursing periods of 11 yr, demonstrate a similar level of
physiological development at weaning, approximately 81% (calculated from Noren et al. 2002) and 83% (Richmond 2004) of
adult total mass-specific oxygen stores, respectively. The convergence of obtaining greater than half of adult mass-specific
oxygen storage capacity across phylogenetically diverse species
at the onset of independence at sea, regardless of days postpartum, suggests that this oxygen store level may have an adap-
Figure 3. Myoglobin content (Mb) of the longissimus dorsi of gray seals
during the postweaning fast (PWF) plotted as days postpartum (DPP).
Each unique symbol represents an individual pup that was studied
longitudinally during the PWF. Although Mb appeared to increase
during the PWF, the differences across sample points were not significant (F2, 26 p 0.764, P p 0.483; repeated-measures ANOVA).
487
tive advantage for preliminary diving and hence foraging activity. Nevertheless, the reduced oxygen stores of immature
marine mammals invariably set the limits to their diving behavior and probably shape their prey selection.
CADLs provide an estimation of how physiological development influences the diving capabilities for individuals. The
increase in oxygen storage capacity from birth to the end of
the PWF in gray seals was associated with a 27% increase in
CADL. Although this stage of development corresponds with
the pups’ first foraging trip at sea, continued physiological development and growth promote an additional 50% and 61%
increase in total oxygen stores and CADL, respectively, during
the pups’ first year at sea (Table 2). However, while the majority
of oxygen store development was complete by the end of the
first year of life (i.e., similar to that of adults), yearlings still
had significantly lower CADLs than adult females. Adult females had a 95% greater CADL than yearlings (Table 2). This
change in CADL is attributed to the attainment of a mature
metabolic rate after the first year of life (Boily and Lavigne
1997) and an increase in body size, with the associated relative
decrease in mass-specific oxygen utilization rates (Kooyman et
al. 1983).
The dramatic differences between the CADLs of weaned pups
heading out to sea, yearlings, and adults suggest that actual
dive times should differ across these segments of the population. The CADL (11.9Ⳳ0.9 min) determined for adult female
gray seals in this study and in a previous study (9.6 min; Reed
et al. 1994a) are consistent with the diving behavior of freeranging adult gray seals, where 90% of dives were less than 8
min (Beck et al. 2003). Although there are no published accounts of the diving behavior of free-ranging immature gray
seals for comparison, direct measurements of the diving behavior of other juvenile phocid seals (northern elephant: Thorson 1993; Thorson and Le Boeuf 1994; ringed [Phoca hispida]:
Lydersen and Hamill 1993; bearded [Erignathus barbatus]:
Lydersen et al. 1994; harbor: Bowen et al. 1999; and Weddell
[Leptonychotes weddellii]: Burns 1999) confirm that juvenile
phocids have shorter dive durations, dive to shallower depths,
and dive for a lower percentage of time compared to adults.
Differences in diving ability are also probably associated with
interage class variation in diet. Hence, prey choice may serve
as an indication of the reduced diving capability of juvenile
gray seals. Gray seals less than a year old demonstrate a greater
reliance on silver hake and consume less deep-dwelling squid
than that demonstrated for gray seals 1 yr old or older (Bowen
et al. 1993).
In summary, the results of our study confirm that across
phylogenetically diverse groups of marine mammals, postnatal
development is required to attain mature mass-specific Hb, Hct,
BV, and Mb levels. Like previous studies (Thorson 1993; Thorson and Le Boeuf 1994; Noren 2002; Clark 2004; Richmond
2004), our study demonstrates that the development of Mb is
protracted in comparison to the development of Hb. Further-
488 S. R. Noren, S. J. Iverson, and D. J. Boness
Table 2: Body mass, estimated mass-specific oxygen stores, and calculated aerobic dive limits (CADL) of gray seal age classes
Class (age)
Newborn (3 DPP)
0 DPW (17 Ⳳ .4
DPP)
12 DPW (29 Ⳳ .5
DPP)
24 DPW (41 Ⳳ .5
DPP)
Yearling (1 yr)
Adult (16 yr)
Mass (kg)
Blood O2 (mL kg⫺1)
Muscle O2 (mL kg⫺1)
a
a,b
Total O2
(mL kg⫺1)
CADL (min)
a,b
21.3 Ⳳ .8
52.8 Ⳳ 1.4
30.8 Ⳳ 3.6 (76 Ⳳ 2%)
21.0 Ⳳ .9c,d (70 Ⳳ 2%)
4.2 Ⳳ .1 (12 Ⳳ 1%)
5.1 Ⳳ .7 (17 Ⳳ 2%)
38.1 Ⳳ 3.7
30.7 Ⳳ 1.1c,d
3.0 Ⳳ .3a,b
3.1 Ⳳ .1d
44.4 Ⳳ 1.3
25.8 Ⳳ 1.5d,e (72 Ⳳ 3%)
6.1 Ⳳ .8 (17 Ⳳ 2%)
36.1 Ⳳ 1.2d,e
3.4 Ⳳ .1d
40.1 Ⳳ 1.3
30.6 Ⳳ .9a,c,e (74 Ⳳ 2%)
6.8 Ⳳ .7a,b (16 Ⳳ 2%)
41.4 Ⳳ .8a,b,c,e
3.8 Ⳳ .1a,b,c,e
51.6 Ⳳ 2.7
191.5 Ⳳ 6.0
45.3 Ⳳ 2.6f,g (72 Ⳳ 2%)
41.5 Ⳳ 4.5 (66 Ⳳ 4%)
12.9 Ⳳ 1.3f,g (21 Ⳳ 2%)
15.9 Ⳳ 1.3f,g (27 Ⳳ 4%)
62.3 Ⳳ 3.2f,g
61.4 Ⳳ 4.6f,g
6.1 Ⳳ .3b,f,g
11.9 Ⳳ .9a,f,g
Note. Values are mean Ⳳ SE. Calculations were based on 10 seals in each age class, except for newborn muscle O2, total O2, and CADL (n p 9 ), and for 0-d
postweaning (DPW) muscle O2, total O2, and CADL (n p 7 ). DPP p days postpartum. Lung O2 was assumed to represent a constant proportion of body mass
(at 4.1 mL kg⫺1; see text) and was added to blood and muscle O2 stores to compute total O2 and percentage contributions (shown in parentheses). Significant
differences across postweaning (PW) longitudinal sample points (repeated-measures ANOVA on PW longitudinal samples: 0, 12, and 24 DPW) were found for
mass-specific oxygen stores in the blood (F2,29 p 20.998 , P ! 0.001 ) and in total (F2,26 p 19.816 , P ! 0.001 ) and for CADL (F2,26 p 11.292 , P p 0.001). Although
the mean value for mass-specific oxygen stores in the muscle appeared to increase throughout the PW period, the differences across sample points were not
significant (F2,26 p 0.763 , P p 0.483 ). Significant differences across population age classes (ANOVA on cross-sectional population samples: newborn, 24-DPW
pup, yearling, and adult) were found for mass-specific oxygen stores in the blood (F3,39 p 5.594 , P p 0.003 ), muscle (F3,38 p 28.301 , P ! 0.001), and in total
(F3,38 p 14.525, P ! 0.001) and for CADL (F3,38 p 63.539, P ! 0.001).
a
Values different than yearling (Tukey all pairwise comparison across cross-sectional population samples at P ! 0.05).
b
Values different than adult (Tukey all pairwise comparison across cross-sectional population samples at P ! 0.05).
c
Values different than 12 DPW (Tukey all pairwise comparison across postweaning longitudinal samples at P ! 0.05).
d
Values different than 24 DPW (Tukey all pairwise comparison across postweaning longitudinal samples at P ! 0.05).
e
Values different than 0 DPW (Tukey all pairwise comparison across postweaning longitudinal samples at P ! 0.05).
f
Values different than newborn (Tukey all pairwise comparison across cross-sectional population samples at P ! 0.05).
g
Values different than 24 DPW (Tukey all pairwise comparison across cross-sectional population samples at P ! 0.05).
more, we provide evidence that the terrestrial PWF of phocid
seals represents an important physiological development period. Despite differences in nursing and PWF durations, immature marine mammals appear to have converged onto developing a similar proportion of adult oxygen stores at the onset
of independence at sea. This indicates that the rate of oxygen
store development matches the rate at which independence is
attained, and it suggests that there may be a critical level of
development for successful diving and, by inference, foraging.
Regardless, limited mass-specific oxygen reserves in combination with small body size constrain the diving capacity of juvenile marine mammals. Differences in diving capabilities
throughout maturation may result in partitioning of prey resources, perhaps minimizing competition between age classes.
To the extent that diving capacity limits access to prey, the
ontogenetic changes described in this study may explain observed losses in juvenile pinnipeds during periods of limited
prey availability (DeLong et al. 1991).
Acknowledgments
We thank W. D. Bowen for providing logistical support during
fieldwork and for helpful comments on earlier versions of the
manuscript. We also thank the many people that provided assistance in the field (particularly S. Budge, J. McMillan, and S.
Tucker) and S. Lang and M. Jakubasz for organizing supplies
for the field and laboratory. D. Costa, O. Oftedal, and L. Ortiz
provided space in their laboratories for the analysis of the blood
and muscle samples. We thank Thermo IEC for the generous
loan of a Micro-MB Microhematocrit/Microcentrifuge and microcapillary reader. Funding was provided by a Smithsonian
Institution postdoctoral fellowship to S.R.N., Friends of the
National Zoo Christensen Fund grant to the National Zoo’s
Conservation and Research Center, the Natural Sciences and
Engineering Research Council, Canada, and the Department of
Fisheries and Oceans, Canada.
Literature Cited
Beck C.A., W.D. Bowen, and S.J. Iverson. 2003. Sex differences
in the diving behaviour of a size dimorphic capital breeder:
the grey seal. Anim Behav 6:777–789.
Boily P. and D.M. Lavigne. 1997. Developmental and seasonal
changes in resting metabolic rates of captive female grey seals.
Can J Zool 75:1781–1789.
Boness D.J., W.D. Bowen, and S.J. Iverson. 1995. Does male
harassment of females contribute to reproductive synchrony
in the grey seal by affecting maternal performance? Behav
Ecol Sociobiol 36:1–10.
Bowen W.D. 1991. Behavioural ecology of pinniped neonates.
Development of Blood and Muscle Oxygen Stores in Gray Seals
Pp. 66–127 in D. Renouf, ed. Behaviour of Pinnipeds. Chapman & Hall, Cambridge.
Bowen W.D., D.J. Boness, and S.J. Iverson. 1999. Diving behavior of lactating harbour seals and their pups during maternal foraging trips. Can J Zool 77:978–988.
Bowen W.D., J.W. Lawson, and B. Beck. 1993. Seasonal and
geographic variation in the species composition and size of
prey consumed by grey seals (Halichoerus grypus) on the
Scotian shelf. Can J Fish Aquat Sci 50:1768–1778.
Burns J.M. 1999. The development of diving behavior in juvenile Weddell seals: pushing physiological limits in order to
survive. Can J Zool 77:737–747.
Burns J.M., A.S. Blix, and L.P. Folkow. 2000. Physiological constraint and diving ability: a test in hooded seals, Cystophora
cristata. FASEB (Fed Am Soc Exp Biol) J 14:A440.
Castellini M.A. and G.N. Somero. 1981. Buffering capacity of
vertebrate muscle: correlations with potentials for anaerobic
function. J Comp Physiol 143:191–198.
Clark C. 2004. Tracking Changes: Postnatal Blood and Muscle
Oxygen Store Development in Harbor Seals (Phoca vitulina).
MS thesis. University of Alaska, Anchorage.
Coulson J.C. and G. Hickling. 1964. The breeding biology of
the grey seal, Halichoerus grypus (Fab.), on the Farne Islands,
Northumberland. J Anim Ecol 33:485–512.
Davies J.L. 1949. Observations of the grey seal (Halichoerus
grypus) at Ramsey Island, Pembrokeshire. Proc Zool Soc
Lond 119:673–692.
DeLong R.L., G.A. Antonelis, C.W. Oliver, B.S. Stewart, M.C.
Lowry, and P.K. Yochem. 1991. Effects of the 1982–83 El
Niño on several population parameters and diet of California
sea lions on the California Channel Islands. Pp. 166–172 in
F. Trillmich and K.A. Ono, eds. Pinnipeds and El Niño: Responses to Environmental Stress. Springer, Berlin.
El-Sayed H., S.R. Goodall, and R. Hainsworth. 1995. Reevaluation of the Evans blue dye dilution method of plasma
volume measurement. Clin Lab Haematol 17:189–194.
Falke K.J., R.D. Hill, J. Qvist, R.C. Schneider, M. Guppy, G.C.
Liggins, P.W. Hochachka, R.E. Elliott, and W.M. Zapol. 1985.
Seal lung collapse during free diving: evidence from arterial
nitrogen tensions. Science 229:556–558.
Greenwood A.G., S.H. Ridgway, and R.J. Harrison. 1971. Blood
values in young grey seals. J Am Vet Med Assoc 159:571–
574.
Hedrick M.S. and D.A. Duffield. 1991. Haematology and rheological characteristics of blood in seven marine mammal
species: physiological implications for diving behaviour. J
Zool (Lond) 225:273–283.
Hedrick M.S., D.A. Duffield, and L.H. Cornell. 1986. Blood
viscosity and optimal hematocrit in a deep-diving mammal,
the northern elephant seal (Mirounga angustirostris). Can J
Zool 64:2081–2085.
Kleiber M. 1975. The Fire of Life: An Introduction to Animal
Energetics. Krieger, New York.
489
Kooyman G.L. 1989. Diverse Divers: Physiology and Behaviour.
Springer, Berlin.
Kooyman G.L., M.A. Castellini, R.W. Davis, and R.A. Maue.
1983. Aerobic diving limits of immature Weddell seals. J
Comp Physiol 151:171–174.
Kooyman G.L., D.D. Hammond, and J.P. Schroeder. 1970.
Bronchograms and tracheograms of seals under pressure.
Science 169:82–84.
Kooyman G.L., J.P. Schroeder, D.M. Denison, D.D. Hammond,
J.J. Wright, and W.P. Bergman. 1972. Blood nitrogen tensions
of seals during simulated deep dives. Am J Physiol 223:1016–
1020.
Lapennas G.N. and R.B. Reeves. 1982. Respiratory properties
of blood of the gray seal Halichoerus-grypus. J Comp Physiol
B 149:49–56.
Le Boeuf B.J., R.J. Whiting, and R.F. Gantt. 1972. Perinatal
behavior of northern elephant seal females and their young.
Behaviour 43:121–156.
Lenfant C.K., K. Johansen, and J.D. Torrance. 1970. Gas transport and oxygen storage capacity in some pinnipeds and the
sea otter. Respir Physiol 9:277–286.
Lydersen C. and M.O. Hamill. 1993. Diving in ringed seal
(Phoca hispida) pups during the nursing period. Can J Zool
71:1178–1182.
Lydersen C., M.O. Hamill, and K.M. Kovacs. 1994. Diving activity in nursing bearded seal (Erignathus barbatus) pups.
Can J Zool 72:96–103.
MacArthur R.A. 1990. Seasonal changes in the oxygen storage
capacity and aerobic dive limits of the muskrat (Ondatra
zibethicus). J Comp Physiol B 160:593–599.
MacArthur R.A., M.M. Humphries, G.A. Fines, and K.L. Campbell. 2001. Body oxygen stores, aerobic dive limits, and the
diving abilities of juvenile and adult muskrats (Ondatra zibethicus). Physiol Biochem Zool 74:178–190.
Morrison P., M. Rosenmann, and J.A. Sealander. 1966. Seasonal
variation of myoglobin in the northern red-backed vole. Am
J Physiol 211:1305–1308.
Noren D.P., D.E Crocker, T.M. Williams, and D.P. Costa. 2003.
Energy reserve utilization in northern elephant seal (Mirounga angustirostris) pups during the postweaning fast: size
does matter. J Comp Physiol B 173:443–454.
Noren S.R. 2002. The Ontogeny of Diving in Bottlenose Dolphins (Tursiops truncatus). PhD diss. University of California,
Santa Cruz.
Noren S.R., G. Lacave, R.S. Wells, and T.M. Williams. 2002.
The development of blood oxygen stores in bottlenose dolphins (Tursiops truncatus): implications for diving capacity.
J Zool (Lond) 258:105–113.
Noren S.R. and T.M. Williams. 2000. Body size and skeletal
muscle myoglobin of cetaceans: adaptations for maximizing
dive duration. Comp Biochem Physiol A 126:181–191.
Noren S.R., T.M. Williams, D.A. Pabst, B. McLellan, and J.
Dearolf. 2001. The development of diving in marine endo-
490 S. R. Noren, S. J. Iverson, and D. J. Boness
therms: preparing the skeletal muscles of dolphins, penguins,
and seals for activity during submergence. J Comp Physiol
B 171:127–134.
Oftedal O.T., D.J. Boness, and R.A. Tedman. 1987. The behavior, physiology, and anatomy of lactation in the pinnipedia.
Pp. 175–245 in H.H. Genoways, ed. Current Mammalogy.
Vol. 1. Plenum, New York.
Ponganis P.J., L.N. Starke, M. Horning, and G.L. Kooyman.
1999. Development of diving capacity in emperor penguins.
J Exp Biol 202:781–786.
Potocnik S.J. and E.M. Wintour. 1996. Development of the
spleen as a red blood cell reservoir in lambs. Reprod Fertil
Dev 8:311–315.
Reed J.Z., P.J. Bulter, and M.A. Fedak. 1994a. The metabolic
characteristics of the locomotory muscles of (Halichoerus
grypus), harbour seals (Phoca vitulina), and Antarctic fur
seals (Arctocephalus gazella). J Exp Biol 194:33–46.
Reed J.Z., C. Chambers, M.A. Fedak, and P.J. Butler. 1994b.
Gas exchange of captive freely diving grey seals (Halichoerus
grypus). J Exp Biol 191:1–18.
Reilly J.J. 1991. Adaptations to prolonged fasting in free-living
weaned grey seal pups. Am J Physiol 260:R267–R272.
Reynafarje B. 1963. Simplified method for the determination
of myoglobin. J Lab Clin Med 61:138–145.
Richmond J. 2004. Ontogeny of Total Body Oxygen Stores and
Aerobic Dive Potential in the Steller Sea Lion (Eumetopias
jubatus). MS thesis. University of Alaska, Anchorage.
Ridgway S.H. and D.G. Johnston. 1966. Blood oxygen and
ecology of porpoises. Science 151:456–457.
Rothstein G. 1993. Origin and development of the blood and
blood-forming tissues. Pp. 41–78 in G.R. Lee, T.C. Bithel, J.
Foerster, J.W. Athens, and J.N. Lukens, eds. Wintrobe’s Clinical Hematology. 9th ed. Lea and Febiger, Malvern, PA.
Saunders D.K. and M.R. Fedde. 1991. Physical conditioning:
effect on the myoglobin concentration in skeletal and cardiac
muscle of bar-headed geese. Comp Biochem Physiol A 100:
349–352.
Scholander P.F. 1940. Experimental investigation on the respiratory function in diving mammals and birds. Hvalrad Skr
22:1–131.
Snyder G.K. 1983. Respiratory adaptations in diving mammals.
Respir Physiol 54:269–294.
Stephenson R., D.L. Turner, and P.J. Butler. 1989. The relationship between diving activity and oxygen storage capacity
in the tufted duck (Aythya fuligula). J Exp Biol 141:265–275.
Swan H. and A.W. Nelson. 1971. Blood volume measurement:
concepts and technology. J Cardiovasc Surg 12:389–401.
Thorson P.H. 1993. Development of Diving in the Northern
Elephant Seal. PhD diss. Univeristy of California, Santa Cruz.
Thorson P.H. and B.J. Le Boeuf. 1994. Developmental aspects
of diving in northern elephant seal pups. Pp. 271–289 in B.J.
Le Boeuf and R.M. Laws, eds. Elephant Seals: Population
Ecology, Behavior, and Physiology. University of California
Press, Berkeley.