Effect of dietary fatty acid composition on depot fat and exercise performance in a migrating songbird, the red-eyed vireo
1 Department of Natural Resources Science, University of Rhode Island,
Kingston, RI, USA
2 Department of Genetic Medicine, Weill Cornell Medical College, New York,
NY, USA
3 Center of Marine Biotechnology, University of Maryland, Baltimore, MD,
USA
4 Division of Biological Sciences, University of Montana, Missoula, MT,
USA
* Author for correspondence (e-mail: bjpierce2{at}yahoo.com)
Accepted 10 January 2005
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Summary |
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Key words: red-eyed vireo, Vireo olivaceous, lipid, unsaturated fatty acid, migration, fatty acid composition, metabolic rate
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Introduction |
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Reliance on fatty acid oxidation to fuel high-intensity endurance exercise
in birds is remarkable because, unlike hydrophilic fuel substrates such as
glucose, transport of fatty acids requires the action of soluble protein
carriers at every step of fatty acid transport. Accordingly, the capacity of
these carriers must be significantly upregulated during intense exercise to
ensure adequate fatty acid transport, but few studies have focused on this
type of physiological modulation
(Guglielmo et al., 2002a;
McWilliams et al., 2004
).
There are many possible ways that migratory birds could augment fatty acid
utilization capacity during exercise, including mechanisms associated with
adipose sources, circulatory pathways, myocyte uptake and intracellular
disposal (esterification and oxidation;
Roberts et al., 1996
; Weber et
al.,
1996a
,b
;
Hochachka and Somero, 2002
).
We focus here on how modulation of fatty acid composition of fat stores
affects exercise performance in birds.
Although few studies document fatty acid composition of fat stores in birds
during migration, evidence to date shows that the majority of lipids in
migrating birds comprise 16- or 18-carbon fatty acids, and unsaturated fatty
acids (mostly 16:1, 18:1, 18:2) usually predominate over saturated fatty acids
(mostly 16:0 and 18:0; Blem,
1976; Conway et al.,
1994
; Egeler and Williams,
2000
). Whether a certain fatty acid composition enhances exercise
performance of birds is unknown, although studies with exercising mammals
provide a basis for developing some hypotheses. Mammalian adipocytes release
fatty acids preferentially based on chain length and degree of unsaturation:
fatty acids with the same number of carbon atoms but with more double bonds
are preferentially released, and those with the same number of double bonds
but with shorter chain lengths are preferred
(Raclot and Groscolas, 1995
;
Raclot, 2003
). In rats and
humans, high levels of essential n-6 polyunsaturated fatty acids (e.g. 18:2n6)
in muscle membrane phospholipids have been associated with improved endurance
capacity, and n-6 fatty acids appear to be depleted from membranes by
repetitive exercise (Ayre and Hulbert,
1996
,
1997
;
Andersson et al., 1998
). Such
preferential mobilization and oxidation of unsaturated fatty acids has not
been demonstrated in birds. If similar mechanisms exist in birds, then we
predict that birds with more unsaturated fatty acids, in general, or more n-6
polyunsaturated fatty acids, in particular, would have enhanced exercise
performance.
We tested this hypothesis by measuring metabolic rate at rest and during
intense exercise in two groups of red-eyed vireos Vireo olivaceous
fed one of two semi-synthetic diets for 4 months. Red-eyed vireos are
abundant, medium-sized (1325 g) neotropical migrants that spend the
summer in northern United States and southern Canada and spend the winter in
Central and South America (Cimprich et
al., 2000). They store relatively large fat reserves during
migration and thus are an excellent species in which to study composition of
fat reserves and its energetic consequences.
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Materials and methods |
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Influence of diet on fatty acid composition of vireo body fat
To determine how dietary fatty acids influenced fatty acid composition of
vireos, all vireos fed each diet were killed immediately following exercise
and stored at 20°C for later analysis. Carcasses were thawed and
whole liver and both pectoral muscles were removed, rinsed in distilled water,
blotted dry and weighed (±0.1 mg). Intestines were removed, perfused
with distilled water, blotted dry and weighed (±0.1 mg). We also
collected and weighed ca. 1 g (±0.1 mg) fat from the furcular region of
each bird. Each tissue sample was placed into a glass scintillation vial and
stored at 20°C for later analysis.
All organic solvents used were of HPLC grade (Fisher Scientific,
Pittsburgh, PA, USA). All bird tissues and diet samples were freeze-dried,
weighed (±0.1 mg), and cut into fine particles using surgical scissors.
Lipids were extracted from ca. 100 mg of sample using a modified version of
Folch et al. (1956) as
described in Jackson and Place
(1990
). Briefly, samples were
homogenized with 3.0 mldichloromethane:methanol (2:1
CH2Cl2:methanol), centrifuged for 15 min at 537
g and the supernatant transferred to a large test tube. This
procedure was repeated with 1:1 CH2Cl2:methanol and 2:1
CH2Cl2:methanol. Lipid extract was first washed with
0.88% potassium chloride water solution, and then with
dichloromethane:methanol:water (3/48/47). Samples were dried under nitrogen,
weighed (±0.1 mg), resuspended in 500 µl of 1:1
dichloromethane:methanol and capped under nitrogen.
Quantification of fatty acid methyl esters was achieved by hydrolyzing ca. 500 µg of extracted lipid with methanolic HCl, adding 25 µg of internal standard mixture of equal amounts of nonadecanoic acid (C19:0) and heinecosanoic acid (C21:0) (Nu-chek PreP Inc., Elysian, MN, USA) to each sample, and extracting the methyl esters into dichloromethane. An aliquot sample of the dichloromethane extract was subjected to gas chromatography directly on a Hewlett-Packard (Paolo Alto, CA, USA) model 6890 instrument equipped with a flame ionization detector at 300°C and a J&W DBWAX fused silica capillary column (30 mx0.25 mm i.d. with 0.25 mm film thickness; J. & W. Scientific Inc., Folsom, CA, USA). Helium was used as the carrier gas with a constant flow rate of 1.5 ml min1. Oven temperature was programmed from an initial temperature of 50°C for 0.5 min to 195°C for 15 min after ramping at 40°C min1, to 220°C for 7 min after ramping at 2°C min1, with a total run time of 38.13 min. Peaks were identified by comparison with retention times of quantitative standards from Nu-Check Prep, Inc. (stds 3B, GLC 17AA') and expressed as percentages of fatty acid methyl esters. Fatty acids that comprised on average less than 1% of the fatty acids in all tissue and diet samples are reported but were excluded from statistical analysis.
Analysis of blood metabolites
We measured concentrations of blood metabolites in resting and
post-exercise birds to demonstrate that birds used fats to fuel their exercise
in the enclosed running wheel. We sampled ca. 200 µl of blood from the
brachial vein of vireos 1 week prior to and then immediately following their
exercise trial. Blood was sampled between 12:00 and 15:00 h and after
90120 min without food. Whole blood was centrifuged at 2817
g for 10 min. Plasma was stored in cryogenic vials at
80°C until further analysis.
Metabolites were measured using commercial kits modified for small volumes. Triacylglycerols (TAG) and free glycerol (GLYC) were measured by endpoint assay (Sigma, St Louis, MO, USA; Trinder reagent A, 300 µl to 5 µl plasma). Uric acid, phospholipids (PL), and non-esterified fatty acids (NEFA) were measured by endpoint assay (Wako Diagnostics, Richmond, VA, USA; 3 µl plasma, 120 µl reagent A, 240 µl reagent B). ß-hydroxybutyrate was measured by kinetic assay (Roche Biopharma, Indianapolis, IN, USA). All samples (2.55 µl) were analyzed in duplicate on a microplate spectrophotometer (Biotek PowerwaveX 340, Winooksi, VT, USA). If coefficients of variation (CV) were greater than 10% for duplicate samples, additional samples were analyzed until CV<10%.
Resting and active metabolic rates of vireos
Oxygen consumption was measured using open-circuit respirometry. The same
flow system was used for measuring both resting metabolic rates (RMR) and peak
metabolic rates (MRpeak). We measured both RMR and
MRpeak so that we could calculate the metabolic scope
(MRpeak/RMR) of birds, which provides an indication of the
intensity of exercise. Flow rates of dry, CO2-free air (800 ml
min1 for RMR and 5000 ml min1 for
MRpeak) were regulated by thermal mass flowmeters (RMR; Brooks,
Veenendaal, The Netherlands; MRpeak; Sable, Las Vegas, NV, USA).
Approximately 100 ml min1 of the excurrent air from the
chambers was diverted, dried (using Drierite), passed through a CO2
analyzer (Ametek CD-3A, Pittsburgh, PA, USA), redried, scrubbed of
CO2 (Ascarite), redried and passed through an oxygen analyzer
(Sable FC-1). Outputs from these instruments were sampled by a MacIntosh
computer equipped with an analog-to-digital converter and Warthog©
software (Mark A. Chappell and the Regents of the University of
California).
Each day one vireo from each of the two diet groups was randomly selected
and the resting metabolic rates (RMR) of both birds were measured. Starting at
20:00 h, 60 min prior to lights off, vireos were weighed (±0.1 g) and
placed into individual stainless-steel metabolic chambers (volume 1.9 l).
These chambers were then placed in a temperature-controlled cabinet at
32±1.0°C, which is within the thermoneutral zone of a songbird
species similar in size to vireos (Root et
al., 1991). Each vireo fasted and rested in the chamber for
34 h, then oxygen consumption was measured during the remaining
67 h of the overnight period. Excurrent air from the two chambers was
routed to the carbon dioxide and oxygen analyzers through a
computer-controlled airstream selector (Sable Systems Respiratory
Multiplexer). Excurrent streams from each chamber were sampled for 30
min,followed by a 5 min reference sample, before switching to sample the
alternate chamber for 30 min. This sequential sampling continued throughout
the measurement period. Thus, the RMR values reported are for postabsorptive
individuals, resting in the inactive phase of their daily cycle. The RMR of an
individual was taken as the minimum 10 min mean of O2 consumption
during the overnight test period. After RMR determination, vireos were placed
back into their regular cages at 06:45 h and given ad libitum food
and water for 4 h. Since digesta retention time of lipids in red-eyed vireos
fed similar diets is ca. 68 min (Pierce et
al., 2004
), we removed food 1 h before energetic trials to promote
emptying of the gut and the use of endogenous energy reserves by vireos during
exercise.
Oxygen consumption of vireos during exercise was measured using an enclosed
running wheel modified for flying birds. The wheel was constructed of acrylic
plastic, carpet-lined, manually driven, and contained four ping-pong balls to
discourage birds from walking (Chappell et al.,
1996,
1999
). Birds were weighed
(±0.1 g), placed in the wheel (which was covered with a cloth at this
time in order to reduce stress in the bird), and allowed to acclimate for 10
min within the wheel. The cloth was then removed and the wheel was spun slowly
for 1015 s to initiate exercise. The wheel was then kept in constant
motion so that vireos were forced to hop and hover for ca. 3040 min.
This type of exercise provides a significant aerobic challenge to the birds
and allowed us to determine a peak metabolic rate during short-term intense
exercise, as done in other studies of peak metabolic rates in running and
flying birds (Chappell et al.,
1996
,
1999
). The MRpeak
of an individual was the maximum mean of `instantaneous' O2
consumption achieved during this exercise over a 1 min period, calculated
using the equation of Bartholomew et al.
(1981
).
Statistical analysis
Fatty acid composition (%) of the diets and tissues were arcsine
transformed and Hotelling's T multivariate analysis of variance (MANOVA) was
used to compare the proportion of each fatty acid in body tissues of birds fed
each diet. Analysis of variance (ANOVA) was used to compare the proportion of
unsaturated fat in the diets and tissues of vireos fed those diets.
Repeated-measures analysis of variance (RMANOVA) was used to compare blood
metabolite levels of vireos before and after exercise. ANOVA was used to
compare metabolic rates of vireos fed each diet and Student t-test
was used to compare metabolic scope of vireos fed each diet. All ANOVA tests
were performed using the general linear model in SPSS 11.0 (SPSS, Inc.) and
Tukey's HSD (honestly significant difference) was used for all post
hoc comparisons. Results are reported as means ± standard error
(S.E.M.).
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Results |
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Influence of diet on fatty acid composition of vireo body fat
Vireos fed the 82%U diet had significantly more unsaturated fat and less
18:2n6 in their furcular fat than birds fed the 58%U diet (Diet:
F(1,32)=182.6, P<0.001;
Fig. 1). Fatty acids 16:0,
18:0, 18:1n9 and 18:2n6 comprised >94% of dietary fatty acids and were also
the primary fatty acids (>85%) in tissues of birds fed either diet
(Table 3). However, tissues
from vireos fed the 58%U diet had significantly more 16:0
(F(1,32)=169.9, P<0.001), 18:0
(F(1,32)=7.04, P=0.01) and 18:2n6
(F(1,32)=42.6, P<0.001) and significantly less
18:1n9 (F(1,32)=266.2, P<0.001) than vireos
fed the 82%U diet (Table 3).
For vireos fed either diet, the proportions of 16:0
(F(3,32)=11.8, P<0.001), 18:0
(F(3,32)=65.1, P<0.001) and 18:1n9
(F(3,32)=38.0, P<0.001) were significantly
different among tissues (Table
3). In general, liver contained more 16:0 and 18:0 than other
tissues, and furcular fat and intestine contained more 18:1n9 than pectoral
muscle and liver (Table 3). Fatty acids 10:0, 18:1n7, 22:5 and 22:6n3 comprised <6% of either diet, and
none of these fatty acids comprised >5% of the lipids in the furcular fat,
intestine and liver of birds fed these diets
(Table 3). However, these four
fatty acids combined comprised ca. 10% and 7% of the fatty acids in the
pectoral muscle of birds fed the 58%U and the 82%U diets, respectively
(Table 3). Of the remaining
less-common fatty acids in the tissues of vireos, furcular fat contained the
largest proportion of 20:1n9, and pectoral muscle and liver contained the
largest proportions of 20:4n6 (Table
3).
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Blood metabolites
Diet did not influence plasma metabolite concentrations prior to or
immediately following exercise (P>0.21 for each metabolite; the
KolmogorovSmirnov test revealed 9 of 12 metabolites were normally
distributed and transformation of data did not effect level of significance).
TAG and phospholipid levels decreased by 42% and 11%, respectively, after
exercise (F(1,8)=53.64, P<0.001;
F(1,8)=6.47, P=0.035, respectively) and NEFA and
ß-hydroxybutyrate increased by 68% and 67%, respectively, after exercise
(F(1,8)=134.78, P<0.001;
F(1,8)=23.17, P=0.001, respectively;
Fig. 2). In addition, glycerol
levels decreased by 82% and uric acid increased by 30% after exercise
(F(1,8)=12.15, P=0.008;
F(1,8)=12.12, P=0.008, respectively;
Fig. 2).
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Resting and active metabolic rates of vireos
We report metabolic rates on a whole-animal basis (per bird) and in
mass-specific units (g1) because both sets of data
demonstrate the effect of fatty acid composition of birds on metabolic rate
during exercise, although variation in body mass between treatment groups
influenced the level of statistical significance for estimates of whole-animal
metabolic rate. Whole-animal RMR of vireos fed the 58%U diet (0.83±0.09
ml O2 min1) was similar to that of vireos fed the
82%U diet (0.81±0.07 ml O2 min1;
F(1,8)=0.04, P=0.8). Whole-animal
MRpeak of vireos fed the 58%U diet (8.67±0.22 ml
O2 min1 MRpeak) was higher than
whole-animal of vireos fed the 82%U diet (7.68±0.56 ml O2
min1). However, the variance in whole-animal
MRpeak between individuals on the same diet was noticeably higher
for birds fed the 82%U diet compared to birds fed the 58%U diet (Levene's
test: F(1,8)=4.7, P=0.06) so that the difference
in MRpeak was not statistically significant
(F(1,8)=2.7, P=0.14).
We calculated mass-specific MR using the average body mass of a vireo before and after the RMR trial (i.e. 18.4±1.17 g for vireos fed the 58%U diet and 21.7±1.18 g for vireos fed the 82%U diet). Mass-specific RMR of vireos fed the 58%U diet (2.75±0.32 ml O2 g1 h1) was similar to that of vireos fed the 82%U diet (2.30±0.30 ml O2 g1 h1; F(1,8)=1.09, P=0.33; Fig. 3). However, the mass-specific MRpeak of vireos fed the 58%U diet (28.55±1.47 ml O2 g1 h1) was significantly higher than the MRpeak of vireos fed the 82%U diet (21.50±1.76 ml O2 g1 h1) F(1,8)=9.45, P=0.015; Fig. 3). Thus, metabolic rate of vireos during exercise was different between diet groups, indicating that fatty acid composition of birds affected peak metabolic rate of vireos. This increase in mass-specific MR was 10.89±1.19 times the RMR for birds fed the 58%U diet and 9.95±1.41 times RMR for birds fed the 82%U diet and was not significantly different between the two diet groups (t=0.51, P=0.31).
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Discussion |
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Influence of diet on fatty acid composition of vireo body fat
As expected, diet significantly influenced the fatty acid composition of
body fat in vireos. The four predominant fatty acids in the diets (16:0, 18:0,
18:1n9 and 18:2n6) also predominated in the body fat of birds fed these diets.
When vireos were fed a diet with mostly 18:1n9, their tissues (fat, breast,
intestines, liver) also contained large proportions of this fatty acid.
Likewise, when vireos were fed a diet with relatively equal proportions of
16:0, 18:1n9 and 18:2n6, their tissues also contained relatively similar
proportions of these fatty acids. Lipids are the only dietary component that
is deposited intact into tissues (Klasing,
1998). Consequently, the fatty acid composition of the diet can
primarily determine the fatty acid composition of fat stores in birds
(West and Meng, 1968
;
Thomas and George, 1975
;
West and Peyton, 1980
),
although some conversion of dietary fatty acids occurs and this selective
metabolism can create some differences between fatty acid composition of diet
and body fat, as is evident in our results and those of others
(Blem, 1990
;
Klasing, 1998
). For example,
the livers of vireos fed each diet contained more 16:0 and 18:0, and less
18:1n9, than the other three tissues. This may be due, in part, to differences
in oxidation rates of fatty acids in the liver
(Leyton et al., 1987
) as
compared to other tissues. The majority of the long-chain unsaturated fatty
acids in the breast, intestines and liver are likely to be found in the
phospholipids of the cell membranes (Mead
et al., 1986
), although we cannot confirm this because we did not
distinguish the different lipid classes (i.e. triacylglycerols, phospholipids)
in our analysis.
Fuel use during exercise
Few studies have documented how concentrations of plasma metabolites change
in a migratory songbird following intense exercise (Jenni-Eiermann and Jenni,
1991,
1992
,
1996
,
2001
). Plasma metabolite
changes in a migratory songbird are most commonly studied by comparisons of
free-living birds captured at night during their annual migration
(post-exercise) and free-living birds captured and maintained overnight (at
rest). The increased levels of NEFA and ß-hydroxybutyrate, and the
decreased levels of PL, in the plasma of vireos after exercise demonstrate
that vireos were metabolizing lipids while exercising in the running wheel.
NEFA levels of vireos before (ca. 0.85 mmol l1) and after
exercise (ca. 2.5 mmol l1) were similar to those found for
garden warblers Sylvia borin at rest (1.0 mmol l1)
and during migratory flight at night (2.0 mmol l1;
Jenni-Eiermann and Jenni,
1992
). The levels of ß-hydroxybutyrate (ca. 2.0 mmol
l1) and uric acid (0.7 mmol l1) in vireos
after exercise were also similar to those found in migrating garden warblers
(2.2 and 0.6 mmol l1, respectively;
Jenni-Eiermann and Jenni,
1996
). Plasma GLYC levels in vireos after exercise (ca. 0.7 mmol
l1) were similar to those found in garden warblers during
migratory flight at night (0.9 mmol l1; Jenni-Eiermann and
Jenni, 1992
,
2001
). However, vireos had
significantly higher GLYC plasma levels prior to exercise (ca. 510 mmol
l1) than did garden warblers at rest (0.5 mmol
l1; Jenni-Eiermann and
Jenni, 1992
). High GLYC levels in vireos prior to exercise might
be the result of substantial recent catabolism of exogenous fats in the
high-lipid diet, since vireos were not in a fasted state when blood samples
were taken.
Our results are relevant when considering an interesting supplementary mode
of circulatory delivery of exogenous fatty acids to muscles that was proposed
by Jenni-Eiermann and Jenni
(1992) based on their studies
of migratory passerines. They found that plasma TAG and
very-low-density-lipoproteins (VLDL) were elevated in migrants captured in
mid-flight, compared to birds fasted for 60 min or overnight. They suggested
that the high lipid uptake and processing capacity of the liver allows it to
act as an alternative sink for exogenous fatty acids originating from adipose
tissue, thus freeing plasma albumin to transport more fatty acids per unit
time. Fatty acids taken up by the liver would be re-esterified and released
back to the plasma in VLDL. This pathway could provide large amounts of fatty
acids to muscle without the osmotic effects of increased plasma albumin.
However, elevated plasma TAG levels during flight have not been further
confirmed in controlled field or laboratory studies with other species. Plasma
TAG concentration declined in flying pigeons
(Bordel and Haase, 1993
;
Schwilch et al., 1996
), and
declined initially and stabilized in red knots Calidris canutus
flying for up to 10 h in a wind tunnel
(Jenni-Eiermann et al., 2002
).
We found that plasma TAG levels also declined during flight wheel exercise in
the red-eyed vireo. These results show that plasma TAG concentration is not
necessarily elevated in birds during exercise, and so do not support the
supplementary mode of fatty acid transport proposed by Jenni-Eiermann and
Jenni (1992
).
Resting metabolic rate and active metabolic rate in red-eyed vireos
Average peak metabolic rate of flying birds is 16 times resting metabolic
rate (Hinds et al., 1993).
Average peak metabolic rates in our vireos were ca. 10 times higher than
resting metabolic rates and similar to those of adult house sparrows
Passer domesticus (10.8±2.11 ml O2 g
l1 min1) exercised in a comparable
enclosed running wheel (Chappell et al.,
1999
). In addition, mass-specific RMR of vireos fed either diet
(2.32.75 ml O2 g l1
min1), was similar to those found for house sparrows
(2.52.8 ml O2 g l1 min1;
Chappell et al., 1999
).
However, whole-animal RMR of vireos fed the 58%U diet (0.83 ml O2
min1) and 82%U diet (0.81 ml O2
min1) were ca. 20% lower than those predicted by an
allometric equation for passerine birds (1.021.15 ml O2
min1 depending on diet treatment;
Lasiewski and Dawson, 1967
).
Mass-specific MRpeak of our vireos fed the 58%U diet
(28.5±1.5 ml O2 g1 h1)
was higher than those of adult (26.9 ml O2 g1
min1) and juvenile house sparrows (22.9 ml O2
g1 min1;
Chappell et al., 1999
),
whereas mass-specific MRpeak of our vireos fed the 82%U diet
(21.5±1.8 ml O2 g1 h1)
were lower than those of adult and juvenile house sparrows
(Chappell et al., 1999
).
Influence of fatty acid composition on energetic performance
Fatty acid composition of fat reserves affects exercise performance in rats
and fish in part because specific unsaturated fatty acids are preferentially
used during metabolism over saturated fatty acids
(Leyton et al., 1987;
Raclot and Groscolas, 1995
;
McKenzie et al., 1997
,
1998
). Ours is the first study
to examine the influence of fatty acid composition of depot fat on the
energetic performance in birds. Our results contradict the simple hypothesis
that migratory birds with more unsaturated fatty acids comprising their fat
depot have enhanced aerobic performance during intense exercise compared to
birds with less unsaturated fatty acids. Determining which specific fatty
acids were responsible for the differences in peak metabolic rate between
birds fed each diet was not the goal of this study, although our results can
be used to suggest and cautiously evaluate a few possible hypotheses. Below we
discuss two alternative hypotheses that seem most likely, given the observed
differences in fatty acid composition of our birds and the results from
studies of other vertebrates.
Our results suggest that migratory birds with more essential n-6
polyunsaturated fatty acids (i.e. 18:2n6) comprising their fat depot have
improved exercise performance. The hypothesis that the amount of essential n-6
polyunsaturated fatty acids such as 18:2n6 improves exercise performance in
birds is also supported by a recent field study of Western sandpipers
Calidris mauri. Muscle phospholipids in sandpipers were more
monounsaturated during migration and n-6 fatty acids decreased between
premigration and migration periods, suggesting that n-6 fatty acids may be
depleted by migratory flight (Guglielmo et
al., 2002b).
Alternatively, animal performance may be enhanced when fat stores comprise
some intermediate amount of unsaturated fatty acids. For example, McKenzie et
al. (1998) found that sturgeon
fed a diet containing menhaden oil (rich in polyunsaturated fatty acids) had
reduced maximum swimming speed compared to fish fed a diet containing canola
oil (rich in monounsaturated fatty acids). In addition, it is known that the
polyunsaturated fatty acids in mammalian cells undergo lipid peroxidation
(producing toxic lipid peroxides) more readily than saturated and
monounsaturated fatty acids (Mead et al.,
1986
). Frank et al.
(1998
) found that
golden-mantled ground squirrels Spermophilus lateralis, when given
choices of diets with various proportions of unsaturated and saturated fatty
acids, restricted their intake of polyunsaturated fatty acids to the minimal
level required for proper hibernation. Similar processes may be occurring in
migratory songbirds, although little is known about lipid peroxidation in
avian tissues, and no study has tested whether birds combine diets with
certain ratios of unsaturated and saturated fatty acids to achieve a specific
fatty acid composition in their body fat prior to a certain energetically
demanding event (e.g. migration, egg-laying, cold-tolerance).
Fatty acid storage and use: implications for the ecology of migratory songbirds
We have demonstrated that the fatty acid composition of the diet largely
determines the composition of a migratory bird and this, in turn, affects the
bird's energetic performance during intense exercise. These results suggest
that birds during migration could benefit from selecting foods with certain
fatty acids. Unfortunately, we know little about the fatty acid composition of
foods eaten by free-living birds and the extent to which diet selection in
birds is influenced by composition of dietary fats. Migratory birds prefer
diets with more long-chain monounsaturated fatty acids (e.g. 18:1;
Bairlein, 1991;
McWilliams et al., 2002
;
Pierce et al., 2004
). What
remains to be demonstrated is how these diet preferences interact with food
availability and composition to determine the fatty acid composition of birds
during migration. We find particularly intriguing the untested hypothesis that
there is some `optimum' fatty acid composition of migratory birds that
enhances performance of birds during migratory flight and which the birds
attempt to achieve by carefully choosing their diet, thus influencing the pace
of their migration.
List of abbreviations
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Acknowledgments |
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References |
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