Modulation of ingested water absorption by Palestine sunbirds: evidence for adaptive regulation
1 Department of Ecology and Evolutionary Biology, University of Arizona,
Tucson, AZ 85721, USA
2 Department of Zoology and Physiology, University of Wyoming, Laramie, WY
82071, USA
3 Mitrani Department of Desert Ecology, Jacob Blaustein Institute for Desert
Research, Ben-Gurion University of the Negev, Sede Boqer Campus, 84990,
Israel
4 Department of Life Sciences, Ben-Gurion University of the Negev, Sede
Boqer Campus, 84990, Israel
* Author for correspondence at present address: Department of Wildlife Ecology, University of Wisconsin, Madison, WI 53706, USA (e-mail: mcwhorte{at}email.arizona.edu)
Accepted 18 November 2002
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Summary |
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Key words: Palestine sunbird, Nectarinia osea, hummingbird, adaptive regulation, water absorption, water intake, water turnover, nectar
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Introduction |
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Beuchat et al. (1990)
hypothesized that when hummingbirds are ingesting large volumes of dilute
nectar, perhaps only a small fraction is absorbed in the small intestine,
leaving the rest to pass quickly through the intestinal tract. This hypothesis
would explain the ability of these birds to process such large volumes of
water rapidly but requires the rapid absorption of sugars and electrolytes and
strict regulation of transepithelial water flux
(Skadhauge, 1981
;
Beuchat et al., 1990
). If
ingested water is largely absorbed across the intestine, as appears to be the
case in most vertebrates (Powell,
1987
), nectar-feeding birds would be faced with significant renal
challenges for water elimination and glucose and electrolyte recovery when
feeding on dilute nectar (Beuchat et al.,
1990
). McWhorter and Martínez del Rio
(1999
) developed a model based
on pharmacokinetic techniques to estimate the fractional absorption of
ingested water across the gastrointestinal tract of birds. Their model
estimates fractional water absorption as the proportion of ingested water that
contributes to body water turnover
(McWhorter and Martínez del Rio,
1999
). McWhorter and Martínez del Rio
(1999
) tested and rejected the
hypothesis of Beuchat et al.
(1990
) in broad-tailed
hummingbirds; they found that approximately 80% of ingested water contributed
to the turnover of the body water pool and that fractional water absorption
was not correlated with food or water intake rate or diet energy density.
Although nectar-feeding birds are convergent in diet, and indeed often in
appearance and behavior, it is unclear if the physiological mechanisms by
which they cope with a nectar diet are also convergent. Nectar poses peculiar
problems to the animals that feed on it because it is a relatively dilute
solution of sugars containing trace amounts of amino acids and electrolytes
(Baker, 1975,
1977
;
Baker and Baker, 1983
). Here,
we revisit the hypothesis of Beuchat et al.
(1990
) in another lineage of
nectar-feeding birds. We report the results of experiments designed to examine
the relationship between nectar intake, water absorption and water turnover in
the Palestine sunbird [Nectarinia osea (Bonaparte 1856)], an Old
World nectarivore in the family Nectariniidae. Based on previous measurements
in hummingbirds, we hypothesized that water absorption by sunbirds would be
essentially complete at all sucrose concentrations naturally encountered in
floral nectars. Alternately, we hypothesized that if water absorption were
modulated, fractional absorption would decrease to some obligatory minimum
with increasing water intake. This hypothesis was based on the observation
that nutrient absorption does not take place without concomitant transport of
water, whether via hydration spheres of molecules in nutrient
transporters (e.g. Loo et al.,
1996
,
1998
) or paracellular solvent
drag (e.g. Pappenheimer and Reiss,
1987
; Pappenheimer,
1990
). As a corollary to our alternate hypothesis, we predicted
that absorbed water loads would be greater when sugar assimilation rates are
higher. Because researchers generally assume that water turnover in
nectar-feeding animals can be used to approximate nectar intake, given that
ingested water comes only from food (von
Helversen and Reyer, 1984
;
Kunz and Nagy, 1988
;
Powers and Nagy, 1988
;
Weathers and Stiles, 1989
;
Tiebout and Nagy, 1991
), our
results also test the primary assumption of a significant body of work on the
field energetics and water fluxes of nectarivorous animals.
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Materials and methods |
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Experimental design
Experiment 1: fractional absorption of ingested water as a function
of water intake rate
We relied on the behavioral responses of nectar-feeding birds to food of
varying energy density in the design of this experiment. Typically,
nectar-feeding birds decrease their food intake rate with increasing sugar
concentration (Martínez del Rio et
al., 2001). Manipulation of sugar concentration therefore leads to
a wide range of variation in the quantity of food ingested. We used a
repeated-measures design in which we measured water absorption in four
sunbirds fed on four dietary sugar concentrations (292 mmol l-1,
584 mmol l-1, 876 mmol l-1 and 1168 mmol l-1
sucrose) at one ambient temperature (30±2°C), randomizing the order
in which diets were presented to subjects.
Experiment 2: fractional absorption of ingested water as a function
of sucrose assimilation
When ambient temperatures decrease, birds must consume and assimilate more
sugar to meet increased energy demands for thermoregulation. We measured water
absorption in six sunbirds feeding on 584 mmol l-1 sucrose
solutions at both 15±1°C and 30±2°C in a
repeated-measures design to determine the effect of sucrose assimilation rate.
We randomized the order in which subjects were exposed to the two
temperatures.
Experimental measurements
Water turnover rates were estimated by injecting 1.85x105
Bq of 3H2O in 15 µl of distilled water into the
pectoralis of each bird approximately 1.5 h after the lights came on.
Injection volumes were verified gravimetrically by weighing syringes (25
µl; Hamilton Company, Reno, NV, USA) to the nearest 0.0001 g before and
after injection. Excreted fluid samples were collected, using glass
microcapillary tubes, immediately after excretion and placed in separate
scintillation vials. Samples were collected at irregular intervals for
approximately 30 h, excluding the dark portion of the photoperiod during which
sunbirds do not excrete. Sample collection was not initiated until
approximately 40 min after injection, allowing sufficient time for complete
equilibration of 3H with body water (estimates of equilibration
time vary from 15 min to 30 min in small birds;
Williams and Nagy, 1984;
Speakman, 1997
). Liquid
scintillation cocktail (ACS II; Amersham, Piscataway, NJ, USA) was added to
all excreted fluid and injection samples, which were counted, correcting for
quench and lumex, in a Packard Tri-Carb 1600TR Liquid Scintillation Analyzer.
Fractional water turnover rate (K3H; measured
in h-1) was estimated by fitting negative exponential functions to
the relationship between the specific activity of 3H in excreted
fluid and time. In most cases, 3H specific activity was high enough
on the second day to estimate water turnover and absorption. Because birds
were not injected on the second day, these measurements provided a test for
the effects of handling and injection on water turnover and absorption during
the first day. Food intake rate (µl h-1) was recorded over the
course of each experimental trial by measuring the change in food level to the
nearest 0.5 mm in a tube of constant internal diameter, correcting for
evaporation and food spillage.
Total body water volume (TBW; measured in µl) was estimated using
isotope dilution (Nagy, 1983;
Speakman, 1997
). Briefly, a
small blood sample (approximately 50 µl) was taken approximately 4 h after
injection by puncturing the brachial vein. The water microdistilled from this
sample (Nagy, 1983
) was
analyzed for specific activity of 3H as described above. The slope
of the relationship between specific activity of 3H in excreted
fluid and time was extrapolated to the zero time concentration of marker in
body water. We used this modification of the isotope dilution technique
described by Speakman (1997
)
because of the sensitivity of small birds to repeated blood sampling. We
assumed that the rate of disappearance of marker from the blood was equal to
the rate of appearance in the excreted fluid. The specific activity of marker
in each fluid would, of course, not be equal because of renal and post-renal
modification of urine and the mixing of urine with gut contents. After the
final experimental run, one bird was killed with CO2 and dried to
constant mass at 80°C to confirm TBW estimated by isotope dilution. The
TBW of this bird measured by dehydration (3591 µl or 63.8% of body mass)
was 1.6% higher than the average volume for this individual estimated by
isotope dilution.
Estimating water absorption in sunbirds
We used the mass balance approach developed by McWhorter and
Martínez del Rio (1999)
to estimate the fraction of ingested water that was absorbed by sunbirds
(fW). Simply stated, this method determines the proportion
of ingested water that contributes to the turnover of the TBW pool. Assuming
that birds were in neutral water balance, fW was estimated
as:
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Statistical analysis
Experiment 1
To describe the relationship between fractional water absorption
(fW) and water intake rate
(I) and to assess
differences among subjects and treatment days, we constructed a linear model
with fW as a dependent variable, and the reciprocal of
water intake
(
I-1),
individual bird and treatment day as independent variables. We used the
reciprocal transformation of
I to obtain a linear
relationship (i.e.
fW=a+b
I-1,
where a and b are constants) because visual inspection of the relationship
between fW and
I resembled a hyperbola
that tended asymptotically to a constant value for large values of
I. Because relationships
between volumetric food intake and sugar concentration in nectar-feeding birds
are power functions (Martínez del
Rio et al., 2001
), we determined the effects of subject and
treatment day on food, water and sucrose intake rates using linear models of
loge-transformed intake and sucrose concentration data. We
similarly used loge-transformed data to determine the significance
of the relationship between water absorbed per mass sucrose assimilated and
sucrose concentration. We used linear models on untransformed data to assess
significance and subject and treatment day effects in all other cases.
Analysis of covariance (ANCOVA) was used to check for differences in the slope
of the relationship between water flux and water intake between sunbirds and
hummingbirds. We used the Spearman rank correlation test to check for a
correlation between diet sucrose concentration and sucrose intake rate.
Experiment 2
Repeated-measures analysis of variance (RM-ANOVA) was used to test for
differences in food and sucrose intake rates, fractional water absorption,
water absorbed per mass of sucrose assimilated, and the total absorbed water
load between temperatures.
All values are presented as means ± S.E.M.
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Results |
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|
The relationships between the specific activity of 3H in
excreted fluid (d.p.m. µl-1) and time were well described by
exponential functions (r2 ranged from 0.57 to 0.96,
N=35). The decline in the specific activity of 3H in
excreted fluid with time therefore seemed to follow one-compartment,
first-order kinetics. Fractional water turnover rate ranged from 0.037
h-1 to 0.117 h-1 and was linearly correlated with water
intake rate (F1,29=169.50, P<0.0001). Because
there was no significant effect of subject (F3,29=1.4,
P>0.2) or treatment day (F1,29=2.0,
P>0.1) on K3H as a function of
I, we removed these
variables from the model and estimated a common relationship
(K3H=1.15x10-4
I+0.03;
r2=0.84). When birds were feeding on the most dilute
nectar (292 mmol l-1 sucrose), approximately 10% of their TBW pool
was turning over each hour. Average TBW estimated by isotope dilution was
3470±86 µl (or 63.6±0.7% of body mass, N=4).
Fractional water absorption (fw) ranged from 0.33 to 1.02
(averaging 0.59±0.04, N=35). Because we found no significant
effect of subject (F3,29=0.29, P>0.8) or
treatment day (F1,29=3.1, P>0.08) on
fw, but a highly significant effect of
I-1
(F1,29=40.03, P<0.0001), we estimated a common
relationship between fw and
I-1. The
reciprocal transformation adequately described the relationship between
fw and
I
(Fig. 2). These results suggest
that Palestine sunbirds may avoid absorbing up to 64% (1-0.36=0.64) of
ingested water when feeding on dilute nectars. Fractional water absorption was
also positively correlated with sugar concentration in food
(y=0.32x+0.37, r2=0.34,
F1,29=17.13, P<0.0003), which is not
surprising given the negative relationship between water intake rate and
sucrose concentration. Because we found no significant effects of subject
(F3,29=0.43, P>0.7) or treatment day
(F1,29=0.86, P>0.3) on fw as a
function of sucrose concentration, we removed these variables from the model.
Water flux estimated from fractional water turnover rate
(K3H) and total body water (TBW) measurements
ranged from 112.97 µl h-1 to 463.83 µl h-1 and
increased linearly with water intake rate (F1,29=237.29,
P<0.0001). Because we found no effects of subject
(F3,29=0.2, P>0.8) or treatment day
(F1,29=1.95, P>0.1), we estimated a common
relationship between water flux and water intake rate
(K3HxTBW=0.42
I+81.64;
r2=0.89; Fig.
3). The slope of this relationship was significantly less than 1.0
(0.42±0.03, t=22.8, d.f.=33, P<0.001) and
significantly lower than that of the same relationship in broadtailed
hummingbirds (ANCOVAslopes F1,35=27.8,
P<0.0001).
|
|
The volume of water absorbed per mass of sucrose assimilated (µl
mg-1) declined significantly with the sucrose concentration of the
diet (y=1.47x-0.9, r2=0.78,
F1,29=106.66, P<0.0001;
Fig. 4A). There was no
significant effect of subject (F3,29=0.5,
P>0.6) or treatment day (F1,29=0.44,
P>0.5), so we removed these variables from the model. Absorbed
water load
(fwxI;
measured in µl h-1) was positively correlated with food intake
rate (F1,29=152.53, P<0.0001;
Fig. 4B). There was no
significant effect of subject (F3,29=0.64,
P>0.5) or treatment day (F1,29=0.83,
P>0.3), so we removed these variables from the model and estimated
a common relationship (y=0.40x+25.09,
r2=0.84).
|
Experiment 2: fractional absorption of ingested water as a function
of sucrose assimilation
Sunbirds feeding on 584 mmol l-1 sucrose solutions consumed
approximately 1.3 times more food and sucrose at 15°C than at 30°C
(624.52±29.83 µl h-1 vs 487.23±25.47
±l h-1 and 124.84±5.96 mg h-1 vs
97.4±5.09 mg h-1, respectively;
F1,5=6.6, P=0.05 for both variables). These
values translate into energy intake rates of 29.01±1.39 kJ
day-1 and 22.64±1.18 kJ day-1, respectively.
Fractional water absorption was not significantly different between
temperatures (0.44±0.02 vs 0.43±0.02 at 15°C and
30°C, respectively; F1,5=0.22, P=0.66;
Fig. 2). The volume of water
absorbed per mass sucrose assimilated did not differ between temperatures
(1.94±0.09 µl mg-1 vs 1.88±0.08 µl
mg-1 at 15°C and 30°C, respectively;
F1,5=0.21, P=0.67). Although the absorbed water
load (fwxI)
was approximately 1.3 times greater at 15°C than at 30°C, it did not
differ significantly between treatments (237.34±11.49 µl
h-1 vs 184.72±14.46 µl h-1,
respectively; F1,5=5.06, P=0.074). We suspect
that lack of statistical significance in this case was the result of low power
due to small sample sizes.
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Discussion |
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Although fractional absorption decreased with increasing sucrose concentration, the absorbed water load increased with food intake rate (Fig. 4B). The volume of water absorbed per mass sucrose assimilated decreased with sucrose concentration in food (Fig. 4A), despite constant sucrose intake. This suggests that sunbirds can regulate transepithelial water flux independently of sugar absorption. These intriguing results open the door to many questions about how water transport is regulated in the vertebrate gastrointestinal tract. In this discussion, we explore the differences in water turnover and fractional absorption between sunbirds and hummingbirds, members of separate evolutionary radiations of nectar-feeding birds. We posit that differences in mechanisms of sugar absorption and mass-specific food intake rates between these groups may explain the apparent ability of sunbirds to modulate water absorption. We discuss the implications of our findings for estimating food intake in nectarivorous animals based on water flux rates and suggest that intestinal water and body water form two separate but interacting pools in nectar-feeding birds.
Our results provide empirical support for the hypothesis posed by Beuchat
at al. (1990) for
nectar-feeding birds. Sunbirds did not absorb all ingested water, and the
fraction of water absorbed in the intestine decreased with the ingested water
load. Our results and those of McWhorter and Martínez del Rio
(1999
) highlight important
differences between sunbirds and hummingbirds. Fractional water turnover rates
in Palestine sunbirds ranged from 0.037 h-1 to 0.117
h-1, while those in broad-tailed hummingbirds ranged from 0.12
h-1 to 0.61 h-1
(McWhorter and Martínez del Rio,
1999
). In other words, sunbirds feeding on the most dilute nectar
(292 mmol l-1 sucrose) turned over approximately 10% of their TBW
pool each hour, compared with over 50% in hummingbirds. Because daily food
intake by broad-tailed hummingbirds may reach 5.4 times their body mass while
that of Palestine sunbirds only reaches approximately 2.2 times body mass in
birds feeding on 292 mmol l-1 sucrose solutions, this difference
may not be surprising. However, when similar rates of water intake are
considered, hummingbirds and sunbirds show large differences in fractional and
total water turnover rates. The slope of the linear relationship between water
flux and water intake rate (Fig.
3) provides a relative estimate of the fraction of ingested water
that contributes to body water turnover. If 100% of ingested water were
contributing to the turnover of the TBW pool at all water intake rates, the
slope of this relationship would be equal to 1. The slope of this relationship
in sunbirds was significantly less than one and shallower than that of the
same relationship in hummingbirds. Sunbirds appear to regulate water flux from
the gastrointestinal tract to the body, whereas hummingbirds do not.
Convergence in diet has led to the evolution of many similar traits in
hummingbirds and sunbirds (e.g. elongated bill, small body size and
pugnacity). The physiological traits of these two groups that allow the
processing of a water and sugar diet, however, may be very different.
The mechanisms of intestinal water absorption in nectar-feeding birds are
unknown but are probably facilitated by sugar uptake. Active transport appears
to account for essentially all intestinal glucose absorption in hummingbirds
(Karasov et al., 1986). Loo et
al. (1996
) have shown that the
translocation of each glucose molecule by the mammalian intestinal
Na+/glucose cotransporter (SGLT1) is coupled with the transport of
up to 260 water molecules (potentially transporting 4.8 liters of water per
mol of glucose). Hummingbirds, which appear to absorb all ingested water, also
exhibit the highest rate of carrier-mediated glucose uptake measured in a
vertebrate (Karasov et al.,
1986
). McWhorter and Martínez del Rio
(1999
) estimated that the
amount of water potentially accompanying mediated glucose absorption in
broad-tailed hummingbirds exceeded the water content in food by 1.7-5.5-fold,
depending on sucrose concentration. Sunbirds in this study assimilated on
average 3.1x10-3±1.6x10-4 mol of
glucose in 14h. The mediated uptake of this quantity of glucose could be
responsible for the transport of 15 ml of water. Average daily water intake by
sunbirds in this study ranged from 2 ml to 8.7 ml. As in hummingbirds, this
amount exceeds the water ingested in food by a large margin (approximately
1.7-7.5-fold, depending on sucrose concentration). This comparison is
perplexing because sunbirds appear to be able to modulate water absorption
whereas hummingbirds do not.
Modulation of intestinal water absorption requires the rapid absorption of
dissolved sugars and efficient extraction of electrolytes and amino acids
present at low levels in ingested nectar
(Beuchat et al., 1990). It also
requires that the permeability of the intestine to transepithelial water flux
is regulated. How may sunbirds regulate water flux while rapidly absorbing
osmotically active sugars and electrolytes? One possibility is that the
permeability of the intestine to transepithelial water flux by solvent drag
increases with sugar concentration. This would require that sunbirds have a
low capacity for mediated glucose uptake relative to hummingbirds and
significant passive absorption of nutrients at high sugar concentrations.
Although passive absorption appears to be insignificant in hummingbirds
(Karasov et al., 1986
), it is
an important route for nutrient absorption in some passerine and psittacine
birds (Karasov and Cork, 1994
;
Caviedes-Vidal and Karasov,
1996
; Afik et al.,
1997
; Chediack et al.,
2001
). It would be instructive to measure the capacity for
mediated glucose uptake and determine whether the magnitude of passive
absorption of carbohydrate probes varies with water intake (given constant
energy intake) in sunbirds. Another possibility is that water cotransported
into enterocytes during mediated nutrient absorption does not contribute to
turnover of the TBW pool but rather is secreted rapidly back into the
intestinal lumen (Chang and Rao,
1994
). Our estimates of the capacity for water absorption
via mediated Na+/glucose cotransport in birds are based on
measurements made by Loo et al.
(1996
) on the mammalian SGLT1
expressed in the Xenopus oocyte. Their measurements, however, sought
to isolate water transport by that cotransporter and represent one element in
a complex membrane system. The links between nutrient absorption, electrolyte
balance and the regulation of transepithelial water flux in birds remain
unknown.
Beuchat et al. (1990) raised
their hypothesis to explain the ability of hummingbirds to cope with
extraordinary water fluxes. Daily food intake by broad-tailed hummingbirds may
reach 5.4 times their body mass while that of Palestine sunbirds only reaches
approximately 2.2 times body mass in birds feeding on 292 mmol l-1
sucrose solutions. Metabolic mass-specific sucrose intake rate (mg
h-1 kg-0.75) is approximately three times higher in
hummingbirds than in sunbirds. Why may sunbirds modulate water absorption
while hummingbirds do not? Perhaps there are significant physiological
differences in nutrient absorption and the regulation of transepithelial water
flux between these groups. It is also possible that the extraordinarily high
mass-specific energy demands of hummingbirds lead to water intake rates that
simply overwhelm their physiological capacities to regulate water absorption.
We speculate that water ingestion and subsequent absorption are unlikely to
constrain energy intake by sunbirds. The apparent ability of sunbirds to
modulate water absorption may allow them to feed profitably on dilute floral
nectars by minimizing the metabolic cost of recovering glucose and
electrolytes filtered in the kidney. Indeed, we have preliminary data
suggesting that glomerular filtration rates in sunbirds are lower than
expected based on allometric estimates.
Implications for doubly labeled water studies
Water turnover in nectar-feeding animals has often been used to approximate
nectar intake, assuming that ingested water comes only from food
(von Helversen and Reyer,
1984; Kunz and Nagy,
1988
; Powers and Nagy,
1988
; Weathers and Stiles,
1989
; Tiebout and Nagy,
1991
). These approximations are based on the assumption that
isotope concentrations in water leaving the body are the same as those in the
body water at the same time (Lifson and
McClintock, 1966
). Differences in isotope concentrations between
these pools can arise from both physical and biological fractionation
(Lifson and McClintock, 1966
;
Speakman, 1997
;
Visser et al., 2000
).
Biological fractionation is due to incomplete mixing of the isotope label
between the body and ingested water. Although physical fractionation can be
accounted for mathematically, the issue of incomplete mixing has received very
little attention (Visser et al.,
2000
). Nagy and Costa
(1980
) argued that biological
fractionation might occur in birds eating bulky, energy-dilute foods with
consequent high gastrointestinal passage rates, but there are no data to
support this argument. Visser et al.
(2000
) recently determined
that ingested water reaches isotopic equilibrium with the body water pool
regardless of water intake rate in red knots (Calidris canutus),
which may have water fluxes up to 17 times greater than predicted for
free-living birds. By contrast, our results and those of McWhorter and
Martínez del Rio (1999
)
suggest that biological fractionation is occurring in nectar-feeding birds,
i.e. that intestinal water and body water form two separate but interacting
pools. Our model estimates the proportion of ingested water that contributes
to the turnover of the TBW pool. We assumed that the rates of appearance of
marker in excreted fluid and disappearance from TBW were equal, rather than
assuming that the concentrations of markers were equal. If complete
equilibration of intestinal water and body water were occurring, our model
would estimate fw as 1.0 regardless of water flux rate, which was not
the case for either sunbirds or hummingbirds. Thus, our results tell a
cautionary tale for the estimation of food intake based on water flux rates in
nectar-feeding animals: nectar intake will be underestimated if water
absorption is not complete. Our data also suggest that additional attention
needs to be paid to the issue of biological fractionation when using stable
and radioactive hydrogen isotopes to measure whole body rates of water
turnover in animals.
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Acknowledgments |
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References |
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---|
Afik, D., McWilliams, S. R. and Karasov, W. H. (1997). A test for passive absorption of glucose in yellow-rumped warblers and its ecological implications. Physiol. Zool. 70,370 -377.[Medline]
Baker, H. G. (1975). Sugar concentrations in nectars from hummingbird flowers. Biotropica 7, 37-41.
Baker, H. G. (1977). Non-sugar chemical constituents of nectar. Apidologie 8, 349-356.
Baker, H. G. and Baker, I. (1983). Floral nectar constituents in relation to pollinator type. In Handbook of Pollination Biology (ed. C. E. Jones and R. J. Little), pp.117 -141. New York: Scientific & Academic.
Beuchat, C. A., Calder, W. A., III and Braun, E. J. (1990). The integration of osmoregulation and energy balance in hummingbirds. Physiol. Zool. 63,1059 -1081.
Caviedes-Vidal, E. and Karasov, W. H. (1996). Glucose and amino acid absorption in house sparrow intestine and its dietary modulation. Am. J. Physiol. Reg. I 40,R561 -R568.
Chang, E. B. and Rao, M. C. (1994). Intestinal water and electrolyte transport. In Physiology of the Gastrointestinal Tract (ed. L. R. Johnson), pp.2027 -2081. New York: Raven Press.
Chediack, J. G., Caviedes-Vidal, E., Karasov, W. H. and
Pestchanker, M. (2001). Passive absorption of hydrophilic
carbohydrate probes by the house sparrow Passer domesticus. J. Exp.
Biol. 204,723
-731.
Goldstein, D. L. and Bradshaw, S. D. (1998). Regulation of water and sodium balance in the field by Australian Honeyeaters (Aves: Meliphagidae). Physiol. Zool. 71,214 -225.[Medline]
Karasov, W. H. and Cork, S. J. (1994). Glucose absorption by a nectarivorous bird: the passive pathway is paramount. Am. J. Physiol. 267,G16 -G26.
Karasov, W. H., Phan, D., Diamond, J. M. and Carpenter, F. L. (1986). Food passage and intestinal nutrient absorption in hummingbirds. Auk 103,453 -464.
Kunz, T. H. and Nagy, K. A. (1988). Methods of energy budget analysis. In Ecological and Behavioral Methods for the Study of Bats (ed. T. A. Kunz), pp.277 -302. Washington, DC: Smithsonian Institution Press.
Lifson, N. and McClintock, R. (1966). Theory of use of the turnover rates of body water of measuring energy and material balance. J. Theor. Biol. 12, 46-74.[Medline]
Loo, D. D. F., Hirayama, B. A., Gallardo, E. M., Lam, J. T.,
Turk, E. and Wright, E. M. (1998). Conformational changes
couple Na+ and glucose transport. Proc. Natl. Acad. Sci.
USA 95,7789
-7794.
Loo, D. D. F., Zeuthen, T., Chandy, G. and Wright, E. M.
(1996). Cotransport of water by the Na+/glucose
cotransporter. Proc. Natl. Acad. Sci. USA
93,13367
-13370.
Lotz, C. N. and Nicolson, S. W. (1999). Energy and water balance in the lesser double-collared sunbird (Nectarinia chalybea) feeding on different nectar concentrations. J. Comp. Physiol. B 169,200 -206.[CrossRef]
Martínez del Rio, C., Schondube, J. E., McWhorter, T. J. and Herrera, L. G. (2001). Intake responses in nectar feeding birds: digestive and metabolic causes, osmoregulatory consequences, and coevolutionary effects. Am. Zool. 41,902 -915.
McWhorter, T. J. and Martínez del Rio, C.
(1999). Food ingestion and water turnover in hummingbirds: how
much dietary water is absorbed? J. Exp. Biol.
202,2851
-2858.
Nagy, K. A. (1983). The doubly labeled water (3HH18O) method: a guide to its use. UCLA Publication No. 12-1417. Los Angeles: University of California, Los Angeles.
Nagy, K. A. and Costa, D. P. (1980). Water flux
in animals: analysis of potential errors in the tritiated water method.
Am. J. Physiol. 238,R454
-R465.
Nicolson, S. W. and Fleming, P. A. (2003). Energy balance in the whitebellied sunbird, Nectarinia talatala: constraints on compensatory feeding, and consumption of supplementary water. Funct. Ecol. 17, in press.
Pappenheimer, J. R. (1990). Paracellular
intestinal absorption of glucose, creatinine, and mannitol in normal animals:
relation to body size. Am. J. Physiol.
259,G290
-G299.
Pappenheimer, J. R. and Reiss, K. Z. (1987). Contribution of solvent drag through intercellular junctions to absorption of nutrients by the small intestine of the rat. J. Membr. Biol. 100,123 -126.[Medline]
Powell, D. W. (1987). Intestinal water and electrolyte transport. In Physiology of the Gastrointestinal Tract (ed. L. R. Johnson), pp.1267 -1305. New York: Raven Press.
Powers, D. R. (1991). Diurnal variation in mass, metabolic rate, and respiratory quotient in Anna's and Costa's hummingbirds. Physiol. Zool. 64,850 -870.
Powers, D. R. and Conley, T. M. (1994). Field metabolic rate and food consumption of two sympatric hummingbird species in Southeastern Arizona. Condor 96,141 -150.
Powers, D. R. and Nagy, K. A. (1988). Field metabolic rate and food consumption by free-living Anna's hummingbirds (Calypte anna). Physiol. Zool. 61,500 -506.
Rooke, I. J., Bradshaw, S. D. and Langworthy, R. A. (1983). Aspects of water, electrolyte and carbohydrate physiology of the silvereye, Zosterops lateralis (Aves). Aust. J. Zool. 31,695 -704.
Skadhauge, E. (1981). Osmoregulation in Birds. New York: Springer Verlag.
Speakman, J. R. (1997). Doubly Labelled Water: Theory and Practice. London: Chapman and Hall.
Suarez, R. K., Lighton, J. R. B., Moyes, C. D., Brown, G. S., Gass, C. L. and Hochachka, P. W. (1990). Fuel selection in rufous hummingbirds: ecological implications of metabolic biochemistry. Proc. Natl. Acad. Sci. USA 87,9207 -9210.[Abstract]
Tiebout, H. M., III and Nagy, K. A. (1991). Validation of the doubly labeled water method (3HH18O) for measuring water flux and CO2 production in the tropical hummingbird Amazilia saucerottei. Physiol. Zool. 64,362 -374.
Visser, G. H., Dekinga, A., Achterkamp, B. and Piersma, T.
(2000). Ingested water equilibrates isotopically with the body
water pool of a shorebird with unrivaled water fluxes. Am. J.
Physiol. 279,R1795
-R1804.
von Helversen, O. and Reyer, H. U. (1984). Nectar intake and energy expenditure in a flower visiting bat. Oecologia 63,178 -184.
Weathers, W. W. and Stiles, F. G. (1989). Energetics and water balance in free-living tropical hummingbirds. Condor 91,324 -331.
Williams, J. B. (1993). Energetics of incubation in free-living orange-breasted sunbirds in South Africa. Condor 95,115 -126.
Williams, J. B. and Nagy, K. A. (1984). Daily energy expenditure of savannah sparrows: comparison of time-energy budget and doubly-labeled water estimates. Auk 101,221 -229.