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First published online August 17, 2007
Journal of Experimental Biology 210, 3043-3053 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.009381
Effects of mass and body composition on fasting fuel utilisation in grey seal pups (Halichoerus grypus Fabricius): an experimental study using supplementary feeding
1 NERC Sea Mammal Research Unit, Gatty Marine Laboratory, University of St
Andrews, Fife, KY16 8LB, UK
2 School of Biological Sciences, University of Aberdeen, AB24 2TZ,
UK
* Author for correspondence (e-mail: k.a.bennett{at}dundee.ac.uk)
Accepted 24 June 2007
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Summary |
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Key words: protein depletion, fat reserves, energy use, fuel allocation, energy partitioning, first year survival
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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). Trade-offs
between allocation of resources to fat storage, protein deposition (growth)
and metabolism involve complex and subtle interactions, and can have immediate
effects on the performance and survival of young animals as they become
nutritionally independent. This study tested the hypothesis that fuel use
during fasting in grey seal (Halichoerus grypus Fabricius) pups
depends upon the size and composition of available energy sources. We
investigated whether pups that receive more fuel, and in particular protein,
utilise fat and protein differently while fasting from those that receive
less. We examined the effects of the size of energy reserves at weaning on
variation in fat and protein utilisation during the postweaning fast. We also
supplemented a group of pups with fat and protein (as herring) immediately
after weaning and compared their mass and body composition changes with unfed
pups.
Female grey seals usually produce a single pup each year, which they feed
on high-fat milk during a brief (
18 day), intensive nursing period
(Fedak and Anderson, 1982).
While suckling, pups lay down lean body mass and a substantial blubber layer
(Anderson and Fedak, 1987;
Fedak and Anderson, 1982;
Mellish et al., 1999). The
amount of fat and protein laid down by the pup depends on the degree of
maternal provisioning and the pup's metabolism. Investing more resources to
rear a pup that is heavier and fatter at weaning is costly to females, which
experience reduced pupping success in the year following a large investment
(Pomeroy et al., 1999).
However, there are benefits to the offspring of being larger and in better
condition. Probability of first year survival increases with body mass and
size of energy reserves at weaning in a variety of pinniped species
(Baker and Fowler, 1992;
Biuw, 2003;
Burns, 1999;
Harding et al., 2005;
Hindell, 1991;
Hindell et al., 1999;
Le Boeuf et al., 1994;
McConnell et al., 2002;
McMahon et al., 2000;
McMahon and Burton, 2005),
including grey seals (Hall et al.,
2001; Hall et al.,
2002). Their survival depends not only on the degree of maternal
provisioning, but also on allocation of fat and, perhaps more importantly,
protein, to energetic and developmental needs after weaning.
Grey seal pups are weaned abruptly and undergo a land-based fast of
1–4 weeks before going to sea
(Reilly, 1991). They are
reliant on their endogenous fat and protein reserves while fasting on land,
and during the first days or weeks at sea, until they begin to feed. Pups face
a trade-off between the use of body stores for metabolic fuel and for tissue
structure and function. In addition to its role as the major energy reserve,
fat provides insulation against the cold water temperatures once the animals
go to sea. Protein has a low energy density, but plays a small, significant
role as a metabolic fuel, and is also a major structural and functional
component of tissues (Caloin,
2004). The trade-off between the contribution of fat and protein
to energetic needs and developmental requirements may be linked to body size
and composition at weaning and will influence how long pups can fast before
they succumb to starvation.
The ability of seal pups to undergo an extended land-based postweaning fast
is attributed to their low physical activity levels, metabolic depression,
protein sparing and heavy reliance on their fat reserves to meet energetic
requirements (Nordoy and Blix,
1985; Nordoy et al.,
1990; Reilly,
1991; Worthy and Lavigne,
1987). While some protein use is unavoidable, protein sparing
delays depletion of protein to critical levels, when tissue structure and
function are irreversibly compromised and terminal starvation begins
(Caloin, 2004;
Garrow, 1959;
Garrow et al., 1965). In other
mammals, differences in size and body composition lead to substantial
variability in the proportional contribution of protein and lipid to energy
demands while fasting, and an inverse relationship exists between adiposity
and the contribution of protein to energy expenditure
(Atkinson et al., 1996;
Cherel et al., 1992;
Caloin, 2004;
Dulloo and Jacquet, 1999;
Dunn et al., 1982;
Goodman et al., 1980).
Inter-individual variability in the proportional contribution of fat and
protein to energy expenditure has been demonstrated in some phocids
(Biuw, 2003;
Crocker et al., 1998;
Muelbert et al., 2003;
Noren and Mangel, 2004).
Elephant seal (Mirounga sp.) pups with greater fat reserves at
weaning use fat catabolism to meet proportionally more of their energy costs
than leaner pups (Biuw, 2003:
Noren and Mangel, 2004). The
contribution of protein to metabolism varies as a function of body composition
in lactating adult northern elephant seal (M. angustirostris) females
(Crocker et al., 1998).
Muelbert and colleagues (Muelbert et al.,
2003) have shown that body composition changes in harbour seal
(Phoca vitulina) pups in the first 19 days after weaning are related
to weaning body mass. The largest pups show no change in body composition as
they lose mass, whereas percentage lipid content declines in the lightest pups
(Muelbert et al., 2003).
Previous studies on fasting grey seal pups have shown that more than 90% of
their energy costs are met by fat catabolism
(Nordoy and Blix, 1985;
Nordoy et al., 1990;
Reilly, 1991;
Worthy and Lavigne, 1987).
Variation in fuel allocation between individuals, most notably with regard to
body mass or the size of energy reserves, has not been reported. In the
present study we investigated whether mass and composition at weaning
influences fuel use in fasting grey seal pups. To determine whether providing
extra resources, and particularly protein, can alter subsequent fasting
changes in mass and body composition, we fed a group of pups after weaning,
and compared their energy use and fuel allocation with pups that did not
receive extra food.
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4 days) for 23 animals and within
2 days after weaning for the remaining two pups], the sex of each pup was
recorded and the animal was fitted with a rototag (Dalton ID Systems, Henley
on Thames, Oxon, UK) in the interdigital webbing of one rear flipper. The 23
pups handled in early lactation were captured again at late lactation (age
15 days) to measure changes in mass and body composition during suckling.
Pups were assumed to have weaned when the female was not observed in
attendance for 1 day. All pups were penned in a large (115 mx80 m)
outdoor enclosure within 2 days after weaning and remained there until
reaching 70% of their weaning mass or 30 kg, whichever happened first. On
release, pups were painted with unique large and highly visible symbols on
their backs, and their presence/absence on the colony was noted daily. Pups
present after release were re-weighed every 3 days until the study was ended
(18/12/2001 or 16/12/2002), when most animals had left. Date of departure was
assumed to be the day after the last sighting of the animal and was known for
24 of the 25 study animals. One Fed (see definition below) male was still on
the colony after the researchers had left.
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Effects of supplementary feeding on fuel use were investigated in 19 pups.
Seven (Fed) pups received supplementary food on each of the first 5 days after
penning. On each of these 5 days, after they had been weighed, Fed pups were
given a single meal of 1–1.5 kg defrosted whole herring (Clupea
harengus; Lunar Freezing, Peterhead, UK). Herring was used to deliver
high energy in a single, short meal (7 min per feed), without the need
for stomach intubation, which is needed for feeding milk and can damage the
oesophagus. Herring is readily available and is fed to newly weaned pups
without adverse consequences, as well as juveniles and adults, in the seal
holding facility at the Sea Mammal Research Unit (SMRU), University of St
Andrews. The Control group consisted of 12 pups that experienced a similar
handling frequency to Fed pups. The remaining six pups were handled only three
times during fasting as part of a separate study (Low). They were used to
investigate effects of weaning mass and body composition on fasting fuel use,
but were excluded from direct comparisons with Fed animals due to differences
in handling regime.
Analysis of body composition
Animals were weighed using a Salter spring balance (accuracy ±0.2
kg) each time they were handled (every three days after weaning for the Fed
and Control groups, and three times after weaning for the Low group). Body
composition estimates, using deuterium oxide (2H2O)
dilution as described (Reilly and Fedak,
1990), were performed early and late in lactation for the 23 pups
handled during suckling, on entry into the pen for the remaining two, and
between 14 and 26 days after weaning (mean=21±3.5 days) for all study
pups. Mass, but not body composition, was measured after the end of the
feeding period in Fed pups. Thus all postweaning body composition changes
incorporate the 5-day feeding period, as well as fasting, in Fed pups. After
the animal was weighed, a plasma sample was taken from the extradural vein
into a sterile 10 ml heparin treated vacutainer (Becton Dickinson, Cowley,
Oxon, UK) to measure background 2H2O enrichment in body
fluids [method D (Speakman and Racey,
1987)]. A pre-weighed dose of 3–5 ml
2H2O (99.9%; Sigma-Aldrich Chemicals, Gillingham,
Dorset, UK) was then injected and a second plasma sample was taken 3–4.5
h later to determine 2H2O enrichment after equilibration
with body water compartments (Costa et
al., 1986; Reilly,
1991). After centrifugation, 4x 50 µl aliquots of each
sample were flame-sealed into capillary tubes, and stored at room temperature
until analysis. 2H2O enrichment in parts per million in
two sub-samples of the background and enriched plasma samples and standards
was measured in duplicate in a pyrolysis inlet mass spectrometer (Micromass
isoprime, GV Instruments, Manchester, UK)
(Speakman and Krol, 2005).
Samples were reanalysed if variation between replicates was over 2%. Dilution
space was calculated as described (Krol
and Speakman, 1999). Absolute mass and percentage of each of the
body components (fat, protein, water and ash) were determined from body water
content, using equations derived by comparison of 2H2O
dilution with chemical composition of grey seal carcasses
(Reilly and Fedak, 1990). This
method has been validated and shown to be accurate by comparison to chemical
analysis in Antarctic fur seals (Arctocephalus gazella)
(Arnould et al., 1996a). Energy
available for metabolism was calculated assuming energy densities of 39.33 kJ
g–1 and 17.99 kJ g–1 for fat and protein,
respectively (Schmidt-Nielsen,
1997). Table 1
summarises the number of pups in each group for which mass and body
composition information was available.
Overall daily rate of mass gain during suckling and mass loss during
fasting (kg–1 day–1) and body composition
changes during those periods were calculated for all pups. Increases in mass,
water, energy, fat, protein and ash during suckling were calculated from the
difference between measured values at first and last capture during lactation.
Overall average daily rates of loss of mass, water, energy, fat, protein and
ash during fasting were calculated from the difference between measured values
at first and last capture after weaning, divided by the number of days between
these two captures. As in northern elephant seal pups, 17.8% of their weaning
protein mass was assumed to be utilised during the moult, which begins around
the time of weaning in grey seal pups
(Noren et al., 2003). This
protein was not available for metabolism and was subtracted from weaning
protein mass before protein mass loss, energy content and relative
contribution of fat and protein to energy use were calculated.
The mass, energy, protein, fat and ash contents of the meals for the Fed
group were included in the calculations, as follows. The herring contained
16–18% fat (Lunar Freezing, supplier's unpublished information) and
therefore its composition was assumed to be similar to published values
(Gallivan and Ronald, 1981) of
63% water; 18.5% fat; 15.5% protein. These values were used to calculate the
total energy, water, fat and protein available to the seals by multiplying by
the total amount of fish consumed by each animal (mean=5.87 kg±0.58
kg). The amount of energy required to digest the fish, termed the heat
increment of feeding (HIF), was assumed to be 15.74% of its gross energetic
content, as reported (Gallivan and Ronald,
1981) for a similar sized meal of herring fed to a harp seal
(Pagophilus groenlandicus). Gross energetic content of fat and
protein were assumed to be 39.5 and 23.5 kJ g–1 for this
calculation (Schmidt-Nielsen,
1997). HIF was then subtracted from the energy available for
metabolism. The protein, fat, water and ash mass of the fish was corrected for
an assumed assimilation efficiency of 91%, determined from another experiment
on harp seals fed a herring diet (Lawson
et al., 1997). These values were divided by fast duration and
added to the appropriate value for daily tissue loss already calculated for
each of the Fed animals, i.e. animals assimilated then subsequently lost the
tissue components from the fish. Faecal and urinary losses were not
measured.
Weaning mass and body composition were extrapolated from mass changes
during suckling and/or fasting, assuming animals continued to gain mass at the
same rate between late lactation and weaning, or lose mass at the same rate
from weaning to first capture postweaning. Departure mass and body composition
were extrapolated from fasting loss rates. There is no evidence that healthy
fasting seal pups undergo a change in fuel utilisation towards the end of
fasting (Nordoy et al., 1990;
Houser and Costa, 2003), as
occurs in other fasting animals (Cherel et
al., 1988a; Cherel et al.,
1988b; Cherel et al.,
1988c), unless held in captivity for substantially longer than the
normal fast duration (Nordoy et al.,
1992). Animals were thus assumed to continue to lose body
components at the same rate between last postweaning capture and departure.
The amounts of energy, fat and protein and % of protein at weaning that had
been utilised by departure, daily change in % fat content and % contribution
of fat to daily energy use were calculated.
Statistical analyses
Statistical analyses were performed using MINITAB (Minitab 13.32, Minitab
Inc, 2000) or R (R 1.9.1, R Development Core Team, 2003)
(Ihaka and Gentleman, 1996).
Differences between Control and Fed groups in their pattern of mass loss over
3-day intervals from the start of fasting (i.e. date of natural weaning for
Controls and end of feeding for Fed pups) were investigated using linear
mixed-effects models (LMEs), which incorporate the covariance between
successive repeated-measures data points in the model structure, thus
accounting for individual differences in the response without
over-parameterising the model (Chatfield,
1989; Crawley,
2002). Individual was included as a random term. Group and day
after the end of feeding (i.e. days after weaning for Controls and days after
the end of feeding for Fed pups) were included as fixed effects. The model was
fitted using a maximum likelihood estimate. Differences between Fed and
Control groups in body mass and composition variables at weaning and
departure, and their average rate of loss after weaning were investigated.
Body composition variables were derived from mass and body water measurements,
thus multiple analyses of variance (MANOVAs) were used to determine whether
the effect of group on these two variables was sufficiently different for them
to be examined separately. Where there was a significant group difference in
body water, MANOVA was used to investigate whether the effect was due to
differences in fat or protein content.
The effect of natural variability in body size and composition on fasting
fuel utilisation was examined using information from Control and Low pups for
which body composition estimates were available for both weaning and departure
(Table 1). Fed pups were
excluded from this analysis because they were unlikely to be representative of
naturally fasting pups. We performed forward and backward stepwise linear
regression to investigate the effects of energy reserves at weaning on changes
in mass and body composition parameters. Similar stepwise regression analysis
has been used previously in studies investigating relationships between body
composition parameters and measures of energy expenditure (e.g.
Rosenbaum et al., 1997).
Explanatory variables in the saturated models to explain fasting changes in
fat, protein and total body masses, daily energy use, % change in fat and %
contribution of fat to daily energy use were weaning body mass, absolute fat
and protein masses, energy available for metabolism and % fat. Analysis of
variance (ANOVA) was used to compare the fit between models each time a
parameter was added or removed. Bonferonni corrections were applied
(P<0.008 considered significant) to account for potential
interdependence between dependent variables, since all body composition values
were derived from mass and body water.
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Computational tests to investigate the sensitivity of our findings to
variation in assumed fish composition and digestibility were performed. The
fat content of the fish was varied by ±3% to incorporate a range of
body compositions from mature North Sea herring caught in summer. The assumed
digestibility of the protein component of the herring was also reduced to 65%
(Trumble et al., 2003). These
tests did not produce substantial changes in the calculated rates of tissue or
energy use (data not shown). For example, if only 65% of protein was usable by
the animals, calculated rates of protein loss fell by 10 g per day, and a
1% average increase in the relative contribution of fat to energy use was
observed. Thus our findings are robust to potential errors in energy content
and assimilation efficiency assumptions. Values for mass and body composition
at weaning and departure and average daily changes in mass and body
composition parameters during fasting are shown in
Table 2.
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Impact of body composition on postweaning fuel use
The models that best explained variability in energy use and fuel
allocation parameters in unfed (Control and Low) pups are shown in
Fig. 4. The daily average
fasting rate of mass loss increased as a function of protein mass at weaning,
corrected for protein used in the moult (y=1.51x–0.09;
r2=0.503; F(1,16); P=0.0006).
The daily average fasting rate of energy use
(y=0.02x–10.51; r2=0.553;
F(1,16); P=0.0002) and fat loss
(y=0.0005x–0.27; r2=0.551;
F(1,16); P<0.0001) increased as a function of
the energy available for metabolism at weaning. The apparent positive
relationship between rate of protein loss and protein mass at weaning was not
significant (y=0.018x–0.047;
r2=0.154; F(1,16); P=0.0598).
Interestingly, there was no significant linear relationship between adiposity
at weaning and % contribution of fat to energy use
(y=–0.25x+63.13; r2=0.03;
F(1,16); P=0.510;
Fig. 4C). Although the absolute
fat content decreased markedly in all pups while fasting, percentage body fat
increased significantly by 4.07% from 45.84±3.31% at weaning to
49.93±4.01% at departure (paired t-test:
T(18)=4.95, P<0.0001), and this increase was
negatively correlated with percentage contribution of fat to energy use
(y=–0.02x+1.59; r2=0.790;
F(1,16); P<0.0001).
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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). This study highlights the ability of seal pups to conserve
protein while experiencing large changes in metabolic rate and fat
utilisation. Pups that received extra food would have benefited largely as a
result of greater protein availability, which should extend the amount of time
available in which to find food before death from starvation
(Caloin, 2004;
Garrow, 1959;
Garrow et al., 1965).
Although not comparable with the mass gained during suckling
(1.5–2 kg day–1)
(Anderson and Fedak, 1987;
Bowen et al., 1992;
Fedak and Anderson, 1982;
Mellish et al., 1999;
Pomeroy et al., 1999),
supplementation slowed the rate of mass loss and resulted in a small mass gain
in some of the Fed pups, probably due to the presence of food in the gut,
assimilation of the food into body stores, or both. The fall in mass loss rate
early in the first 10 days after the onset of fasting in both Control and Fed
pups was likely due to a reduction in metabolic rate that has been well
documented in fasting animals (Arnould et
al., 2001; Bowen et al.,
1992; Castellini and Rea,
1992; Cherel et al.,
1988a; Cherel et al.,
1988b; Cherel et al.,
1988c; Dunn et al.,
1982; Hambly and Speakman,
2005; Le Maho et al.,
1981; Nordoy et al.,
1990; Rea and Costa,
1992).
Feeding either stimulated metabolic rate, or prevented a fast-induced
metabolic depression. Although daily energy use after weaning in Fed pups was
extremely variable and values lay within the range of previously reported
fasting values (Reilly, 1991),
the mean value was 17.41 MJ day–1, almost double that of the
Controls. The disparity was even greater if increased energy use only occurred
during and immediately after feeding, instead of throughout the fast as the
average daily energy use values imply. This feeding-stimulated increase in
metabolism may, in part, represent greater energetic costs of digestion than
were accounted for here. We used a high value for HIF (15.74% of the gross
energetic content of the food), which was reported in captive seals fed
similar quantities of herring (1–2 kg) to the amounts used in the
current study (Gallivan and Ronald,
1981). However, animals that have previously been fasting
experience greater metabolic costs of feeding because they must rapidly
reverse the gut atrophy that occurs during starvation
(Boza et al., 1999;
Dunel-Erb et al., 2001;
Karasov and Diamond, 1983;
Secor et al., 1994).
Feeding-induced increases in metabolic rate after starvation or fasting tend
to be relatively modest in endotherms because their basal metabolic rate is
already high. A 20% increase in metabolic rate, which is seen in adult harbor
seals and northern elephant seal pups refed after fasting
(Markussen et al., 1992;
Rea and Costa, 1992) is not
sufficient to account for the doubling in energy use in Fed pups compared to
Controls. Although Fed pups had only recently weaned, in addition to the
normal costs of digestion, it is likely that they required some restructuring
and upregulation of the digestive machinery and other organs involved in
nutrient processing to cope with the sudden presence of solid food in their
previously empty gut and the novel diet not normally encountered so soon after
weaning. The specific causes and timing of the alterations to metabolism could
not be identified, partly because the direct effects of feeding on body
composition were not known. The immediate effects of feeding after weaning on
energy expenditure and longer term postabsorptive effects on body composition
changes and energy use need to be teased apart to better understand the
processes observed here. If a similar increase in metabolic rate occurs at the
onset of feeding at sea, it could have important consequences for continued
survival of pups if their intake does not match their increased
expenditure.
This study highlighted the degree of variability in the use of fat and
protein by fasting grey seal pups. Lean mass at weaning was positively
correlated with mass loss and protein loss rates, although this latter
relationship was not significant. Those pups with greater energy reserves at
weaning had higher rates of energy use and fat loss than smaller, leaner
animals. These effects are likely to be largely due to the increase in
metabolic costs that occurs with increasing body size
(Kleiber, 1975).
Our results do not show that fuel use in fasting grey seal pups shifts
towards a greater proportional contribution of fat to energy use with
increasing adiposity at weaning. This is at odds with findings from a wide
range of other species. Fasting humans
(Dulloo and Jacquet, 1999),
rats (Cherel et al., 1992;
Dunn et al., 1982;
Goodman et al., 1980), brown
bears (Ursus arctos) (Hilderbrand
et al., 2000), Gentoo (Pycoscelis papua) and king
penguins (Aptenodytes patagonicus)
(Cherel et al., 1993) and
Svalbard ptarmigan (Lagopus mutus hyperboreaus)
(Lindgard et al., 1992)
allocate fat and protein to meet energetic costs based on their available
reserves; fatter animals show a greater proportional contribution of fat to
energy expenditure and spare protein more effectively. Fasting marine mammals,
including elephant seal pups, subantarctic fur seal pups (A.
tropicalis) and polar bears (U. maritimus), also show an
increase in energy expenditure, contribution of fat to energy costs and the
loss of fat tissue as a function of the size of initial fat reserves
(Atkinson et al., 1996;
Beauplet et al., 2003;
Biuw, 2003;
Carlini et al., 2001;
Noren et al., 2003;
Noren and Mangel, 2004). The
proportional contribution of fat to energetic costs in grey seal pups is not a
simple function of body fatness, and may depend on other factors such as
activity levels and the requirements for fat and protein in developmental
processes. Those studies on other phocid species that have demonstrated a
significant positive relationship between adiposity and reliance on fat to
meet energetic costs have included larger numbers of animals with much larger
ranges of sizes and body compositions than used here and previously in grey
seals (Biuw, 2003;
Carlini et al., 2001;
Muelbert et al., 2003;
Noren et al., 2003;
Noren and Mangel, 2004). The
proportional contribution of fat to total energetic costs in grey seals
clearly requires further investigation with a much larger sample of animals,
including pups at the extremes of lean and fat body compositions.
In previous studies, fat contributed 94% to energy expenditure in wild and
captive grey seal pups (Nordoy and Blix,
1985; Nordoy and Blix,
1991; Reilly,
1991; Worthy and Lavigne,
1987). Once the protein used for hair growth was removed from the
calculations the values of proportional contribution of fat breakdown to
energy use seen here (86–99%) were comparable with, but more variable
than, these and other previously reported values in fasting seals. These
previous studies employed methods that quantified protein losses more
directly, such as measurement of urinary nitrogen production, and urea
turnover and computed tomography (Adams and
Costa, 1993; Houser and Costa,
2001; Nordoy and Blix,
1985; Pernia et al.,
1980). The use of deuterium dilution to estimate body composition
can overestimate protein catabolism if water is lost to inexchangeable pools,
during, for example, de novo lipogenesis
(Speakman, 1997). This is
unlikely to be the cause of the lower values of proportional contribution of
fat to energy use seen here. Some of the variability in the proportional
contribution of fat to energy use may be due to individual differences in the
percentage of weaning protein reserves utilised in replacing lost pelage,
which may in turn relate to the timing of the moult with respect to weaning.
The single value of 17.8% of weaning protein mass used in hair regeneration
(Noren et al., 2003) is likely
to be a particularly high estimate for grey seals, which, unlike northern
elephant seals, do not lose the underlying epidermis as well as the lanugo
during moulting, and thus do not need to regenerate skin as well as hair. If a
similar correction for protein incorporated into new hair growth was applied
to the data from a previous study on fuel use in grey seal pups
(Reilly, 1991), which used
otherwise apparently identical methods to those used here, the proportional
contribution of fat to energy use in those seals would be much greater than
94%. The discrepancy in protein use and proportional contribution of fat to
energy use between our study and this previous work
(Reilly, 1991) cannot be
accounted for by methodology and the reasons for the difference between
studies remains unclear.
Although healthy pups have substantial fat reserves, fat cannot be
completely depleted because it is vital for both metabolic fuel and insulation
at sea (Worthy and Lavigne,
1987). There is a remarkable degree of consistency in relative fat
content between weaning and departure in the grey seal pups studied here,
similar to findings in other phocid seal pups
(Biuw, 2003;
Carlini et al., 2001;
Muelbert and Bowen, 1993;
Nordoy and Blix, 1985;
Noren et al., 2003;
Rea and Costa, 1992). The
mechanisms underlying this consistency in relative fat content, despite
considerable mass loss, are likely to be consequences of a stochastic
metabolic process observed in most animals in which fat is used and mobilised
relative to its availability. However, these processes must require some
degree of regulation, perhaps by hormones involved in fuel use in other
animals (Mercer and Speakman,
2001).
The size of fuel reserves at weaning, the rate at which those reserves are
used and the amount of fat and protein depletion that can be tolerated, place
immediate constraints on how long grey seal pups can survive before they begin
to feed independently. Variability in fuel use and allocation can have a
substantial impact on the ability to fast for extended periods
(Biuw, 2003;
Caloin, 2004;
Cherel et al., 1992;
Dulloo and Jacquet, 1999;
Dunn et al., 1982;
Goodman et al., 1980). The
size of protein reserves and the rate at which they are used are often more
limiting to survival than fat stores, especially in fat or obese animals
(Caloin, 2004;
Cherel, et al., 1992). Animals
can starve to death while they possess substantial fat stores if body protein
reaches levels at which tissue function and integrity is compromised
(Cherel et al., 1992). This
occurs when body protein stores are reduced by 30–50% in children
(Garrow et al., 1965) and dogs
(Garrow, 1959), irrespective
of remaining fat depots. We predicted how long the pups in this study could
survive after departure before their fat and protein reserves were depleted,
assuming that death occurred when fat was depleted by 70–95%
(Caloin, 2004), and protein
mass was reduced by 30–50% of weaning values
(Garrow, 1959;
Garrow et al., 1965).
As in similar studies on southern elephant seal pups
(Biuw, 2003;
McConnell et al., 2002), it
was assumed that pups did not begin to feed and continued to lose tissue
components at the same rate at sea as they had on land. Intuitively one might
anticipate that expenditure would be greater at sea because of elevated
activity, in which case the pups would die sooner than estimated. However,
empirical data from adult seals indicates that at sea metabolic rates may
actually be lower than when they are on land
(Arnould et al., 1996b). This
is supported by captive studies in grey seals that have shown lower metabolic
rates in diving animals versus those resting at the surface
(Sparling and Fedak, 2004),
and would make the survival estimates conservative if this also applies to
pups.
Similar to other studies on seals, we found that animals are predicted to
die from protein depletion earlier than from fat depletion (77.6±59.2
vs 80.8±70.2 days after weaning), even assuming animals can
tolerate as much as 50% protein loss and only 70% fat loss. Our predictions
thus suggest that protein stores, rather than fat reserves, limit survival
time in fasting grey seal pups. From these predictions we estimated that wild
pups may be able to fast for an average of 6 weeks before protein is depleted.
This contrasts with earlier studies on captive pups that suggested wild pups
may be able to fast for only 3–4 weeks in total
(Nordoy and Blix, 1985). If
grey seal pups could only withstand a 30% reduction in protein, similar to
that tolerated by terrestrial animals
(Cherel et al., 1992;
Garrow, 1959;
Garrow et al., 1965), 25% of
the animals in the current study would have starved to death before they left
the colony. However, none of the pups died while fasting on land and several
of those that had approached or reached what would normally be considered
critical protein levels while ashore survived for several months at sea (SMRU,
unpublished). This suggests that grey seal pups, like southern elephant seal
pups (Biuw, 2003), have a
greater tolerance to protein depletion than reported in terrestrial mammals.
If pups can tolerate a 60% reduction in their protein reserves that can be
mobilised for metabolism, they have an average of 36 days in which to find
food before their protein reserves become critically depleted. For grey seal
pups in the North Sea, this should be adequate time in which to locate food
and learn to forage, given their travel rates and dispersion on leaving the
colony (K.A.B., unpublished) and their proximity to feeding grounds, such as
the Marr Bank and Wee Bankie. These predictions suggest that higher rates of
protein use while fasting could compromise the ability of the pups to survive
at sea by reducing the time available to find food before the onset of
terminal starvation, especially during years when prey is scarce or hard to
catch.
This study suggests a possible mechanism through which female grey seals
that invest more in their offspring enhance their pups' first year survival
probability. Pups that were given extra food spared protein reserves at the
expense of fat stores and had more protein available to them. Pups that
receive more milk from their mother may thus have a greater time margin in
which to find food after leaving the natal colony before they deplete their
protein reserves. The observed increased chance of first year survival in pups
that wean heavier and fatter (Hall et al.,
2001; Hall et al.,
2002) may be due to greater protein stores and increased protein
sparing capacity, rather than greater fat reserves, per se. Seal milk
and herring have similar protein contents (12% vs 15.5%,
respectively), and the amounts of protein received by the pups in this study
were similar to or slightly lower than those that they may have received from
extra provisioning over the same period by their mother. However, there are
clear differences between feeding on high fat liquid milk and lower fat solid
fish, despite similar relative protein contents of these two foods, and pups
may assimilate and utilise them in radically different ways. The effects of
extra maternal provisioning on body composition and fasting fuel use need
further attention.
In summary, there is considerable variation in daily energy use and the contribution of fat to energy use in fasting grey seal pups, but this is not a function of percentage body fat. Feeding immediately after weaning seems to stimulate energy use, but the causes and possible consequences for wild pups are unclear. While this study leaves some important questions unanswered, it serves to highlight directions for further work. Additional studies are needed to investigate both the effect of initial adiposity on the proportional contribution of fat to energy use, and the effects of body fuel dynamics during fasting on first year survival of grey seals. Supplements of both protein and fat prior to fasting, separately and in combination, would help to clarify the regulatory mechanisms and information flow underlying decisions about energy partitioning in fasting pups. More studies like this are required if we are to integrate ecological and physiological studies of resource allocation, understand the role of environmental variability in shaping maternal strategies and life histories, and create models that allow us to link resource availability to population trajectories.
List of abbreviations
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Acknowledgments |
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References |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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Adams, S. H. and Costa, D. P. (1993). Water conservation and protein metabolism in northern elephant seal pups during the postweaning fast. J. Comp. Physiol. B 163,367 -373.[Medline]
Anderson, S. S. and Fedak, M. A. (1987). Grey seal energetics: females invest more in male offspring. J. Zool. 211,667 -679.
Arnould, J. P. Y., Boyd, I. L. and Speakman, J. R. (1996a). Measuring the body composition of Antarctic fur seals (Arctocephalus gazella): validation of hydrogen isotope dilution. Physiol. Zool. 69,93 -116.
Arnould, J. P. Y., Boyd, I. L. and Speakman, J. R. (1996b). The relationship between foraging behaviour and energy expenditure in Antarctic fur seals. J. Zool. 239,769 -782.
Arnould, J. P. Y., Green, J. A. and Rawlins, D. R. (2001). Fasting metabolism in Antarctic fur seal (Arctocephalus gazella) pups. Comp. Biochem. Physiol. 129A,829 -841.
Atkinson, S. N., Nelson, R. A. and Ramsay, M. A. (1996). Changes in the body composition of fasting polar bears (Ursus maritimus): the effect of relative fatness on protein conservation. Physiol. Zool. 69,304 -316.
Baker, J. D. and Fowler, C. W. (1992). Pup weight and survival of northern fur seals Callorhinus ursinus. J. Zool. 227,231 -238.
Beauplet, G., Guinet, C. and Arnould, J. P. Y. (2003). Body composition changes, metabolic fuel use, and energy expenditure in subantarctic fur seal (Arctocephalus tropicalis) pups at Amsterdam Island. Physiol. Biochem. Zool. 76,262 -270.[CrossRef][Medline]
Biuw, M. (2003). Variations in body composition and energy utilisation by southern elephant seal pups over the first year. PhD thesis, University of St Andrews, UK.
Boggs, C. L. (1992). Resource allocation: exploring connections between foraging and life history. Funct. Ecol. 6,508 -518.[CrossRef]
Bowen, W. D., Stobo, W. T. and Smith, S. J. (1992). Mass changes of gray seal, Halichoerus grypus, pups on Sable Island – differential maternal investment reconsidered. J. Zool. 227,607 -622.
Boza, J. J., Moerrez, D., Vuichoud, J., Jarret, A. R.,
Gaudaud-de-Weck, D., Fritsche, R., Donnet, A., Schiffrin, E. J., Perruisseau,
G. and Ballevne, O. (1999). Food deprivation and refeeding
influence growth, nutrition and functional recovery of rats. J.
Nutr. 129,1340
-1346.
Burns, J. M. (1999). The development of diving behaviour in juvenile Weddell seals: pushing physiological limits in order to survive. Can. J. Zool. 77,737 -747.
Caloin, M. (2004). Modeling of lipid and protein depletion during total starvation. Am. J. Physiol. 287,E790 -E798.[CrossRef]
Carlini, A. R., Marquez, M. E. I., Ramdohr, S., Bornemann, H., Panarello, H. O. and Daneri, G. A. (2001). Postweaning duration and body composition changes in southern elephant seal (Mirounga leonina) pups at King George Island. Physiol. Biochem. Zool. 74,531 -540.[CrossRef][Medline]
Castellini, M. A. and Rea, L. D. (1992). The biochemistry of natural fasting at its limits. Experientia 48,575 -582.[CrossRef][Medline]
Chatfield, C. (1989). The Analysis of Time Series: An Introduction. London, New York: Chapman & Hall.
Cherel, Y., Robin, J. P. and Le Maho, Y. (1988a). Physiology and biochemistry of long-term fasting in birds. Can. J. Zool. 66,159 -166.
Cherel, Y., Robin, J. P., Walch, O., Karmann, H., Netchitailo, P. and Le Maho, Y. (1988b). Fasting in king penguin. I. Hormonal and metabolic changes during breeding. Am. J. Physiol. 254,R170 -R177.[Medline]
Cherel, Y., Leloup, J. and Le Maho, Y. (1988c). Fasting in king penguin. II. Hormonal and metabolic changes during molt. Am. J. Physiol. 254,R178 -R184.[Medline]
Cherel, Y., Robin, J. P., Heitz, A., Calgari, C. and Le Maho, Y. (1992). Relationships between lipid availability and protein utilization during prolonged fasting. J. Comp. Physiol. B 162,305 -313.[Medline]
Cherel, Y., Freby. F., Gilles, J. and Robin, J. P. (1993). Comparative fuel metabolism in Gentoo and king penguins – adaptation to brief versus prolonged fasting. Polar Biol. 13,263 -269.
Costa, D. P., Le Boeuf, B. J., Huntley, A. C. and Ortiz, C. L. (1986). The energetics of lactation in the northern elephant seal, Mirounga angustirostris. J. Zool. 209,21 -33.
Crawley, M. J. (2002). Mixed effects models. In Statistical Computing: An Introduction to Data Analysis Using S-Plus, pp. 669-707. Chichester: John Wiley.
Crocker, D. E., Webb, P. M., Costa, D. P. and Le Boeuf, B. J. (1998). Protein catabolism and renal function in lactating northern elephant seals. Physiol. Zool. 71,485 -491.[Medline]
Dulloo, A. G. and Jacquet, J. (1999). The control of partitioning between protein and fat during human starvation: its internal determinants and biological significance. Br. J. Nutr. 82,339 -356.[Medline]
Dunel-Erb, S., Chevalier, C., Laurent, P., Bach, A., Decrock, F. and Le Maho, Y. (2001). Restoration of the jejunal muscosa in rats refed after prolonged fasting. Comp. Biochem. Physiol. 129A,933 -947.
Dunn, M. A., Houtz, S. K. and Hartsook, E. W.
(1982). Effects of fasting on muscle protein turnover, the
composition of weight loss, and energy balance of obese and nonobese Zucker
rats. J. Nutr. 112,1862
-1875.
Fedak, M. A. and Anderson, S. S. (1982). The energetics of lactation: accurate measurements from a large wild mammal, the grey seal (Halichoerus grypus). J. Zool. 198,473 -479.
Gallivan, G. J. and Ronald, K. (1981). Apparant specific dynamic action in the harp seal (Phoca groenlandica). Comp. Biochem. Physiol. 69A,579 -581.
Garrow, J. S. (1959). The effect of protein depletion on the distribution of protein synthesis in the dog. J. Clin. Invest. 38,1241 -1250.[Medline]
Garrow, J. S., Fletcher, K. and Halliday, D. (1965). Body composition in severe infantile malnutrition. J. Clin. Invest. 44,417 -425.[Medline]
Goodman, M. N., Larsen, P. R., Kaplan, M. M., Aoki, T. T., Young, V. R. and Ruderman, N. B. (1980). Starvation in the rat. II. Effect of age and obesity on protein sparing and fuel metabolism. Am. J. Physiol. 239,E277 -E286.[Medline]
Hall, A. J., McConnell, B. J. and Barker, R. J. (2001). Factors affecting first-year survival in grey seals and their implications for life history strategy. J. Anim. Ecol. 70,138 -149.[CrossRef]
Hall, A. J., McConnell, B. J. and Barker, R. J. (2002). The effect of total immunoglobulin levels, mass and condition on the first-year survival of grey seal pups. Funct. Ecol. 16,462 -474.[CrossRef]
Hambly, C. and Speakman, J. R. (2005). Contribution of different mechanisms to compensation for energy restriction in the mouse. Obes. Res. 13,1548 -1557.[Medline]
Harding, K. C., Fujiwara, M., Axberg, Y. and Harkonen, T. (2005). Mass-dependent energetics and survival in harbour seal pups. Funct. Ecol. 19,129 -135.[CrossRef]
Hilderbrand, G. V., Schwartz, C. G., Robbins, C. T. and Hanley, T. A. (2000). Effect of hibernation and reproductive status on body mass and condition of coastal brown bears. J. Wildl. Manage. 64,178 -183.[CrossRef]
Hindell, M. A. (1991). Some life history parameters of a declining population of southern elephant seals. J. Anim. Ecol. 60,119 -134.[CrossRef]
Hindell, M. A., McConnell, B. J., Fedak, M. A., Slip, D. J., Burton, H. R., Reijnders, P. J. H. and McMahon, C. R. (1999). Environmental and physiological determinants of successful foraging by naive southern elephant seal pups during their first trip to sea. Can. J. Zool. 77,1807 -1821.
Houser, D. S. and Costa, D. P. (2001). Protein catabolism in suckling and fasting northern elephant seal pups (Mirounga angustirostris). J. Comp. Physiol. B 171,635 -642.[CrossRef][Medline]
Houser, D. S. and Costa, D. P. (2003). Entrance into stage III fasting by starveling northern elephant seal pups. Mar. Mamm. Sci. 19,186 -197.[CrossRef]
Ihaka, R. and Gentleman, R. (1996). R: a language for data analysis and graphics. J. Comput. Graph. Stat. 5,299 -314.[CrossRef]
Karasov, W. H. and Diamond, J. M. (1983). Adaptive regulation of sugar and amino acid transport by vertebrate intestine. Am. J. Physiol. 245,G443 -G462.[Medline]
Kleiber, M. (1975). The Fire of Life: An Introduction to Animal Energetics. Huntington, New York: R. E. Krieger.
Krol, E. and Speakman, J. R. (1999). Isotope dilution spaces of mice injected simultaneously with deuterium, tritium and oxygen-18. J. Exp. Biol. 202,2839 -2849.[Abstract]
Lawson, J. W., Miller, E. H. and Noseworthy, E. (1997). Variation in assimilation efficiency and digestive efficiency of captive harp seals (Phoca groenlandica) on different diets. Can. J. Zool. 75,1285 -1291.
Le Boeuf, B. J., Morris, P. and Reiter, J. (1994). Juvenile survivorship of northern elephant seals. In Elephant Seals: Population Ecology, Behavior and Physiology (ed. B. J. Le Boeuf and R. M. Laws), pp.121 -136. Berkeley, Los Angeles, London: University of California Press.
Le Maho, Y., Vu Van Kha, H., Koubi, H., Dewasmes, G., Girard, J., Ferre, P. and Cagnard, M. (1981). Body composition, energy expenditure, and plasma metabolites in long-term fasting geese. Am. J. Physiol. 241,E342 -E354.[Medline]
Lindgard, K., Stokkan, K. A., Le Maho, Y. and Groscolas, R. (1992). Protein utilisation during starvation in fat and lean Svalbard ptarmigan, Lagopus mutus hyperboreus. J. Comp. Physiol. B 162,607 -613.[CrossRef]
Markussen, N. H., Ryg, M. and Oritsland, N. A. (1992). Metabolic rate and body composition of harbor seals, Phoca vitulina, during starvation and refeeding. Can. J. Zool. 70,220 -224.
McConnell, B. J., Fedak, M., Burton, H. R., Engelhard, G. H. and Reijnders, P. J. H. (2002). Movements and foraging areas of naive, recently weaned southern elephant seal pups. J. Anim. Ecol. 71,65 -78.[CrossRef]
McMahon, C. R. and Burton, H. R. (2005). Climate change and seal survival: evidence for environmentally mediated changes in elephant seal, Mirounga leonina, pup survival. Proc. R. Soc. Lond. B Biol. Sci. 272,923 -928.[Medline]
McMahon, C. R., Burton, H. R. and Bester, M. N. (2000). Weaning mass and the future survival of juvenile southern elephant seals, Mirounga leonina, at Macquarie Island. Antarct. Sci. 12,149 -153.
Mellish, J. A. E., Iverson, S. J. and Bowen, W. D. (1999). Variation in milk production and lactation performance in grey seals and consequences for pup growth and weaning characteristics. Physiol. Biochem. Zool. 72,677 -690.[CrossRef][Medline]
Mercer, J. G. and Speakman, J. R. (2001). Hypothalamic neuropeptide mechanisms for regulating energy balance: from rodent models to human obesity. Neurosci. Biobehav. Rev. 25,101 -116.[CrossRef][Medline]
Muelbert, M. M. C. and Bowen, W. D. (1993). Duration of lactation and postweaning changes in mass and body composition of harbor seal, Phoca vitulina, pups. Can. J. Zool. 71,1405 -1414.
Muelbert, M. M. C., Bowen, W. D. and Iverson, S. J. (2003). Weaning mass affects changes in body composition and food intake in harbour seal pups during the first month of independence. Physiol. Biochem. Zool. 76,418 -427.[CrossRef][Medline]
Nordoy, E. S. and Blix, A. S. (1985). Energy sources in fasting grey seal pups evaluated with computed tomography. Am. J. Physiol. 249,R471 -R476.[Medline]
Nordoy, E. S. and Blix, A. S. (1991). Glucose and ketone body turnover in fasting grey seal pups. Acta Physiol. Scand. 141,565 -571.[Medline]
Nordoy, E. S., Ingebretsen, O. C. and Blix, A. S. (1990). Depressed metabolism and low protein catabolism in fasting grey seal pups. Acta Physiol. Scand. 139,361 -369.[Medline]
Nordoy, E. S., Stijfhoorn, D. E., Raheim, A. and Blix, A. S. (1992). Water flux and early signs of entrance into phase III of fasting in grey seal pups. Acta. Physiol. Scand. 144,477 -482.[Medline]
Noren, D. P. and Mangel, M. (2004). Energy reserve allocation in fasting northern elephant seal pups: inter-relationships between body condition and fasting duration. Funct. Ecol. 18,233 -242.[CrossRef]
Noren, D. P., Crocker, D. E., Williams, T. E. and Costa, D. P. (2003). Energy reserve utilisation in northern elephant seal pups (Mirounga angustirostris) pups during the postweaning fast: size does matter. J. Comp. Physiol. B 173,443 -454.[CrossRef][Medline]
Pernia, S. D., Hill, A. and Ortiz, C. L. (1980). Urea turnover during prolonged fasting in the northern elephant seal. Comp. Biochem. Physiol. 65B,731 -734.[CrossRef]
Pomeroy, P. P., Fedak, M. A., Rothery, P. and Anderson, S. (1999). Consequences of maternal size for reproductive expenditure and pupping success of grey seals at North Rona, Scotland. J. Appl. Ecol. 68,235 -253.[CrossRef]
Rea, L. D. and Costa, D. P. (1992). Changes in standard metabolism during long-term fasting in northern elephant seal pups (Mirounga angustirostris). Physiol. Zool. 65, 97-111.
Reilly, J. J. (1991). Adaptations to prolonged fasting in free-living, weaned gray seal pups. Am. J. Physiol. 260,R267 -R272.[Medline]
Reilly, J. J. and Fedak, M. A. (1990).
Measurement of the body composition of living gray seals by hydrogen isotope
dilution. J. Appl. Physiol.
69,885
-891.
Rosenbaum, M., Nicolson, M., Hirsch, J., Murphy, E., Chu, F. and
Leibel, R. L. (1997). Effects of weight change on plasma
leptin concentrations and energy expenditure. J. Clin. Endocrinol.
Metab. 82,3647
-3654.
Schmidt-Nielsen, K. (1997). Animal Physiology: Adaptation and Environment. Cambridge: Cambridge University Press.
Secor, S. M., Stein, E. D. and Diamond, J. (1994). Rapid upregulation of snake intestine in response to feeding: a new model of intestinal adaptation. Am. J. Physiol. 266,G695 -G705.[Medline]
Sparling, C. E. and Fedak, M. (2004). Metabolic
rates of captive grey seals during voluntary diving. J. Exp.
Biol. 207,1615
-1624.
Speakman, J. R. (1997). Doubly Labelled Water Theory and Practice. London: Chapman & Hall.
Speakman, J. R. and Krol, E. (2005). Validation of the doubly-labelled water method in a small mammal. Physiol. Biochem. Zool. 78,650 -667.[CrossRef][Medline]
Speakman, J. R. and Racey, P. A. (1987). The equilibrium concentration of O-18 in body-water – implications for the accuracy of the doubly-labeled water technique and a potential new method of measuring RQ in free-living animals. J. Theor. Biol. 127, 79-95.[CrossRef]
Trumble, S. J., Barboza, P. S. and Castellinin, M. A. (2003). Digestive constraints on an aquatic carnivore: effects of feeding frequency and prey composition on harbor seals. J. Comp. Physiol. B 173,501 -509.[CrossRef][Medline]
Worthy, G. A. J. and Lavigne, D. M. (1987). Mass loss, metabolic rate, and energy utilisation by harp and gray seal pups during the postweaning fast. Physiol. Zool. 60,352 -364.
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