Nitrogen stress causes unpredictable enrichments of 15N in two nectar-feeding bat species
1 Institute for Zoo and Wildlife Research, Evolutionary Ecology Research
Group, Postfach 601103, 10252 Berlin, Germany
2 Institute of Zoology II, University Erlangen-Nürnberg, Staudtstr. 5,
91058 Erlangen, Germany
* Author for correspondence (e-mail: voigt{at}izw-berlin.de)
Accepted 6 February 2004
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Summary |
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Key words: nitrogen isotope, nitrogen stress, fractionation, mixing, Glossophaga soricina, Leptonycteris curasoae
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Introduction |
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An important assumption in stable isotope studies is the premise that
tissue isotope ratios equal dietary isotope ratios plus an offset called
fractionation factor or the consumer-diet enrichment (DeNiro and Epstein,
1978,
1981
;
Mizutani et al., 1992
).
Fractionation describes the phenomenon that light and heavy isotopes pass
through the body at different rates due to enzymatic and physical processes;
heavy isotopes are preferentially held back in the body, whereas light
isotopes are preferentially released. The isotopic analysis of whole animals
shows, for example, an enrichment of 15N by 3.0±2.6
15N relative to the diet
(DeNiro and Epstein, 1981
).
Within animals, tissues differ in their fractionation of isotopes according to
their molecular composition and the specific enzymes involved. In muscles of
birds, diet-tissue enrichments of 15N vary between 0.2
and
1.4
15N (Hobson
and Clark, 1992
) and in feathers between 2
and 6
15N (Hobson and Clark,
1992
; Mizutani et al.,
1992
). Fractionation effects also occur in other animal groups,
such as mammals or insects, and within all groups fractionation factors vary
between species and tissues (e.g. DeNiro
and Epstein, 1981
; Hilderbrand
et al., 1996
; Webb et al.,
1998
).
Catabolic animals are more enriched in heavy nitrogen isotopes than animals
with a balanced energy budget (Hobson and
Clark, 1992; Hobson et al.,
1993
). In the remainder of this text, we call this phenomenon
`additional fractionation', because an additional offset leads to a larger
difference in isotope ratios between diet and tissue than in animals with a
balanced energy budget. Recently, Vanderklift and Ponsard
(2003
) discussed the effect of
poor-quality diets (i.e. with a high C:N ratio) on the 15N
enrichments of animals and the overall lack of experiments. In the present
study, we investigated the effect of nitrogen stress on 15N
enrichments in two nectar-feeding bat species, i.e. we studied whether
estimates of tissue turnover based on nitrogen isotopes are reliable in
animals under nitrogen stress. In an earlier paper, we described the
fractionation and half-life of carbon isotopes in an experiment with two
nectar-feeding bat species: Leptonycteris curasoae (20 g) and
Glossophaga soricina (10 g)
(Voigt et al., 2003b
). We
switched the carbon isotope ratio of the diet by approximately 14
15N and traced the change in carbon isotope enrichments in
different tissues (wing membrane, blood and hair) over a period of 60 days.
Weobserved that the half-life of stable carbon isotopes was consistently low
for both species and all tissues. In contrast to our expectation, we could not
find a negative correlation between half-life of carbon isotopes and losses of
body mass. We concluded that the mobilization of body reserves and associated
mixing effects did not bias the estimates of tissue turnover of carbon
isotopes because the amount of carbon from internal sources was negligible in
relation to the amount of carbon from external sources.
During the same experiment, the nitrogen concentration of the diet was
changed from 1.3% to 0.1% nitrogen, and the nitrogen isotopic ratio from
4.2
to 13.1
. Here, we present and discuss the results of this
aspect of the experiment. For wing membrane and blood, we predicted that
15N would increase due to the replacement of light with
heavy nitrogen isotopes from external sources. Secondly, we predicted that the
loss of body mass would lead to an overestimate of half-life because light
nitrogen isotopes from internal sources (body substance) mix with heavy
nitrogen isotopes from the diet. Therefore, the half-life of nitrogen isotopes
should be negatively correlated with losses of body mass. Thirdly, we
predicted that carbon and nitrogen isotope turnover rates would be correlated
within the same tissue if additional fractionation is absent.
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Materials and methods |
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After seven days of habituation, we switched the diet of the bats to plants
representative of the carbon isotope composition of C and CAM photosynthetic
pathways and a high 15N value
(Table 1). We refer to this day
as day one of the experiment. As before, all different food sources were mixed
to a sugar concentration of 18% (mass/mass). To complement the diet of the
bats, we added several mg of vitamins and minerals to the diet each day.
Before and after each night, the sugar water was weighed to an accuracy of
1 g and we refer to the ingested food as the difference between the two
measurements. The overall nitrogen isotope enrichment of the ingested food was
estimated each day by measuring the portion of each of the three food sources
ingested by the animals by the percentage concentration of nitrogen in the dry
substance of the food (Tables
1,
2) and by multiplying these
values by the corresponding 15N (Tables
1,
2). To estimate the mean
isotopic enrichment of the ingested food before and after switching the diet,
we calculated the average value of isotopic enrichment of the food for seven
days before day one and for 60 days following day one. Then,
we calculated mean values for the period before day one and following day one.
The change in isotopic composition of diets equalled 8.9
15N for both species (L. curasoae: diet
1=4.1
15N, diet 2=13.0
15N; G. soricina: diet 1=4.3
15N, diet 2=13.2
15N).
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At the end of the first week and during each subsequent sampling event (in
two-week intervals), bats were weighed to an accuracy of 0.01 g (electronic
balance, Mettler PM-100) and two types of samples were taken from each bat. We
took two tissue samples from the wing membrane using a sterile 3 mm-diameter
biopsy punch. Next, we drew approximately 30 µl of blood from the animals
by puncturing the propatagial vein with a small sterile needle. All samples
were placed into Eppendorf tubes, labelled and transferred immediately into a
drying oven, where they were dried to constant mass at 60°C. Subsequently,
samples were stored in a refrigerator below 0°C. After the change in
diets, we captured the bats and took 30 µl blood and a small piece of
wing membrane at the end of the second, fourth, sixth and eighth week. To
remove external contaminants from the skin, we washed the samples with a
chloroform:methanol solvent (1:1; Voigt et
al., 2003b
).
Sample analysis and conversion to the notation
Samples were combusted and the resultant gases (N2 and
CO2) were sequentially measured in a CE 1110 elemental analyzer
connected via a continuous flow system to a Thermo Finnigan Delta
Plus isotope ratio mass spectrometer (Bremen, Germany). The sample isotope
ratios were compared with international gas standards (USGS-24 and IAEA-N1).
Accuracy was greater than ±0.1 (1
). Isotope ratios are
expressed in the
notation in parts per thousands (
) using the
following equation:
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Statistical analysis and curve estimation
To test for differences in mean isotope enrichments between tissues, we
performed a one-way analysis of variance (ANOVA). We ran post-hoc
Tukey HSD tests to make pair-wise comparisons to evaluate differences in mean
values. We calculated mean values of isotopic composition for all sample
periods. In theory, changes in isotopic composition should follow an
exponential curve (e.g. Tieszen et al.,
1983). Hence, equations of the type
y=a+bxecxt were fitted to
the
15N data from each tissue and each bat species. In this
equation, a represents the asymptotic
15N value for
the tissue equilibrated on a C4/CAM-diet, b equals the
overall change in isotope ratio, c is the turnover rate of nitrogen
isotopes in the tissues, and y is the mean nitrogen isotope ratio in
the tissue at time t. For reasons of simplicity, we refer to
c as the regression coefficient in the exponential model. We assumed
that the different tissues equilibrate to an isotope ratio that is equal to
the baseline value at day one plus the difference in isotope enrichment
between the two diets. Estimation of c was performed on an iterative
basis starting with a value of 0.05. To estimate the half-life of nitrogen
isotopes, we calculated t50 using the following equation:
t50=loge(0.5)/c, with
t50 representing the time in days in which half of the
nitrogen isotopes were exchanged in the corresponding tissue, loge
representing the natural logarithm and 0.5 representing the exchange of 50%
isotopes. Some tissue and blood samples contained amounts of nitrogen that
were insufficient for mass spectrometer analysis. Therefore, we excluded the
estimated regression coefficients of these individuals from further analysis
(one blood and wing membrane sample of L. curasoae and two blood and
wing membrane samples of G. soricina).
Possible effect of body mass changes on regression coefficients
To evaluate a possible influence of the loss of body mass on the estimates
of t50, we calculated exponential exchange curves for
nitrogen isotopes on an individual basis. We then tested whether the half-life
of nitrogen in individual tissues was correlated with the change in body mass.
We predicted that the half-life of nitrogen isotopes decreases with loss of
body mass. The level of significance was Bonferroni-corrected to 2.5%, because
two data sets were tested in each individual. Mean values ± 1
S.D. are expressed. Unless stated otherwise, two-tailed
tests were performed. For all statistical analysis and regression models we
used SPSS (version 9.0).
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Results |
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Data on nutritional intake are reported in Voigt et al.
(2003b) and repeated briefly
here for reasons of completeness. The nutritional intake of the bats remained
constant throughout the experiment. On average, individuals of L.
curasoae ingested 20.3±2.9 ml day1 and those of
G. soricina ingested 19.2±1.8 ml day1. The
mean initial body mass equalled 23.6±2.1 g in L. curasoae and
10.2±0.7 g in G. soricina
(Fig. 2). Both species lost on
average 8% of their initial body mass during the course of the experiment.
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After switching the diet to plant products with higher
15N values, the enrichment of 15N increased
significantly in both tissues (Fig.
3). At the end of the experiment at day 60, none of the tissues
was equilibrated to the expected point of nitrogen isotopic enrichment (first
numerical in the exponential regression equation plotted in the graphs). The
nitrogen isotope turnover was estimated as 821 days in wing membrane and 514
days in blood for L. curasoae, or 329 days in wing tissue and 274
days in blood for G. soricina (see
Fig. 3). Estimates of
t50 that were based on nitrogen isotopes were
significantly higher than estimates that were based on carbon isotopes
(Wilcoxon signed-rank test L. curasoae blood, T=0,
N=10, P<0.05; wing membrane, T=7, N=10,
P<0.05; G. soricina blood, T=3, N=10,
P<0.05; wing membrane, T=0, N=10,
P<0.05; carbon isotope data from
Voigt et al., 2003b
).
According to Levene's test, the variances of estimated half-lifes were
significantly higher for nitrogen isotopes than for carbon isotopes (L.
curasoae blood F1,18=15.2, P=0.001; wing
membrane F1,18=6.5, P=0.02; G. soricina
blood F1,18=14.0, P<0.001; wing membrane
F1,18=9.1, P<0.007; carbon isotope data from
Voigt et al., 2003b
). The
regression coefficients of nitrogen exchange curves of individual bats and
consequently also the t50 values were not affected by body
mass losses (Fig. 4;
Table 2). Additionally, within
the same tissue of the same individual, the corresponding pairs of turnover
rates and t50 values, respectively, were not significantly
correlated with each other (Table
3).
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Discussion |
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During the experiment, individuals of both species ingested approximately
19 ml sugar water each day, which is similar to their daily food intake in
similar experimental setups (e.g. Voigt,
2003; Voigt et al.,
2003a
). The amount of ingested sugar water did not change over
time (Voigt et al., 2003b
).
Both facts argue against a situation of energetic malnutrition. Nonetheless,
almost all bats lost body mass during the experiment, and this loss of body
mass could be caused by an insufficient supply of nitrogen (Tables
1,
2). Our previous study showed
that the estimates of half-life based on carbon isotopes remained almost
constant irrespective of the amount of mobilized body substance
(Fig. 4; Voigt et al., 2003b
). This
lack of correlation is probably due to the fact that (1) the amount of carbon
from internal sources, i.e. body substance, is negligible compared with that
from dietary sources and (2) fractionation effects are less pronounced in
carbon isotopes. The larger portion of nitrogen from internal sources in
relation to external sources from a nitrogen-poor diet
(Table 1) may, however,
influence the estimates of nitrogen half-life. In the following paragraph, we
will simulate the extent of this effect.
Howell (1974) estimated a
protein requirement of 84106 mg protein day1 for
L. curasoae. Dividing this value by 6.25 (a standard factor for the
conversion of protein to nitrogen;
Kleiber, 1961
) yields a
nitrogen requirement of 1317 mg nitrogen day1 for
L. curasoae. The percentage concentration of nitrogen in the second
diet was low (mean 0.07%), but even lower values were found in the nectar of
bat- or bird-pollinated plants; for example, Baker and Baker
(1982
) measured a mean protein
content of 0.04% in dry matter, which is equivalent to 0.0064% nitrogen in dry
nectar. L. curasoae ingested on average 20 ml day1
of 18% (mass/mass) sugar water or 3.6 g day1 pure sugar.
This is equivalent to an average daily nitrogen uptake of 3.6
gx0.07%=2.52 mg nitrogen from sugar water. In addition to the dietary
nitrogen uptake, L. curasoae also acquired nitrogen from body
substance. We consider two scenarios: (1) all loss of body mass is due to fat
mobilization and (2) all loss of body mass is due to protein catabolism.
Body fat contains 20% water and therefore mobilization of 1.9 g body
mass in 60 days equals 1.5 g fat. According to Kleiber
(1961
), dry fat consists of 7%
protein or 7/6.25=1.1% nitrogen. Mobilization of 1.5 g fat released 1.5
gx0.011%=0.0165 g nitrogen in 60 days, or 0.28 mg nitrogen
day1. Thus, individual L. curasoae had a total
amount of 2.82 mg nitrogen available per day: 0.3 mg nitrogen from fat
mobilization and 2.52 mg nitrogen from sugar water.
Protein consists of 16% nitrogen
(Kleiber, 1961). As non-fat
body substance consists of 70% water in nectar-feeding bats
(Voigt et al., 2003a
), L.
curasoae catabolized
0.38 g of protein dry matter in 60 days.
According to this, bats mobilized 0.0608 g nitrogen in 60 days, which is
equivalent to 1.0 mg nitrogen day1. Under the second
scenario, L. curasoae had a total amount of 3.52 mg nitrogen
available per day: 1.0 mg nitrogen from protein catabolism and 2.52 mg from
sugar water. Both simulations do not take into account that bats supplemented
their diet with small amounts of nitrogen-rich opuntia fruits. Therefore, the
estimates of available nitrogen are probably underestimated. Nonetheless, does
this simulation emphasize that (1) the nitrogen requirements of the bats
(
1317 mg day1;
Howell, 1974
) were not met
during the experiment although mobilized body reserves provided additional
nitrogen and that (2) more nitrogen was available from external sources than
from internal sources.
In the present study, the mobilization of body reserves may have influenced
the isotopic enrichments within tissues, because nitrogen isotopes of body
reserves (internal substrate) with a low 15N ratio mixed
with those of the diet (external substrate). If both isotope pools are
combined to intermediate levels of
15N, the half-life
(t50) is overestimated. In agreement with this,
t50 estimates were higher than the values based on carbon
isotopes (Figs 3,
4). The estimates of blood
half-life derived from carbon isotopes equalled 120 days in L.
curasoae and 113 days in G. soricina, and the estimates for wing
membrane equalled 134 days in L. curasoae and 102 days in G.
soricina. Comparing these values with those of
Fig. 3 reveals that the
estimates of turnover rates based on nitrogen isotopes were, on average, two
to six times higher. Interestingly, the estimates of half-life based on
nitrogen isotopes (Fig. 3) were
approximately twice as high in L. curasoae than in G.
soricina: 1.8 times in blood (514/274) and 2.5 times in wing tissue
(821/329). This factor of two could be explained by the observation that
L. curasoae mobilized approximately twice as much body reserves as
did G. soricina (1.9 g in L. curasoae and 0.8 g G.
soricina), while ingesting approximately the same amount of sugar water
as G. soricina (
19 ml day1 in both species).
Following this, the ratio between external and internal nitrogen pools
differed by a factor of two between the study species; in other words, L.
curasoae derived twice as much nitrogen isotopes from internal sources
than did G. soricina. We conclude that mixing effects of nitrogen
isotopes occurred during the experiment. In summary, estimates of half-life
based on nitrogen isotopes were most likely influenced by mixing effects,
whereas those based on carbon isotopes were not influenced by loss of body
mass because the amount of mobilized carbon from internal sources was small
compared with the amount of carbon from external sources
(Voigt et al., 2003b
).
In contrast to our expectation, the estimates of t50
that were based on nitrogen isotopes did not correlate with losses of body
mass (Table 2). A possible
explanation for this finding is that simultaneous fractionation effects, as
described for example in birds (Hatch et
al., 1995), obscured the true turnover rates. Several authors have
suggested that the fractionation of nitrogen isotopes most likely occurs
during the deamination and transamination of amino acids (e.g.
Gaebler et al., 1966
;
Minagawa and Wada, 1984
).
Enzymatic processes necessary for homeostasis may then lead to even higher
enrichments of heavy nitrogen within tissues. Hobson and Clark
(1992
) proposed that
"nutritional stress cause[d] substantial increases in diet-tissue
fractionation values due either to: (1) mobilization and redeposition
of proteins elsewhere in the body; or (2) amino acid composition changes in
tissues". Following this argument, one could expect
15N in mobilized organic molecules to be higher in
comparison with the initial state of the organic molecules in the body
reserves. Hatch et al. (1995
),
for example, found that chicks kept under a restricted food regime grew less
and had higher
13C values in tissues than chicks fed ad
libitum. This effect of additional fractionation should be even more
pronounced in nitrogen isotopes because nitrogen isotopes fractionate to a
larger extent than carbon isotopes (present study;
Mizutani et al., 1992
). Hobson
et al. (1993
) observed high
enrichments of 15N in tissues of Ross's geese (Chen
rossii) that had been fasting for four weeks. Similar results were
obtained in Japanese quails (Coturnix japonica) when raised on a
restricted diet (Hobson et al.,
1993
). A second possible explanation is that the different extent
of metabolic processing of two nitrogen sources, i.e. internal body reserves
(fat) and external food sources (carbohydrates), may lead to different
nitrogen enrichments because the enzymes involved in the catabolism of fat and
carbohydrates are active to different degrees and because these enzymes may
fractionate nitrogen isotopes at different rates. This hypothesis could also
explain the well-known phenomenon in feeding trials that the same organism
shows different enrichments in 15N when fed on diets of different
quality (e.g. Webb et al.,
1998
; Adams and Sterner,
2000
; Oelberman and Scheu,
2002
); i.e. the assimilation of diets of different composition
requires corresponding enzymes with specific fractionation characteristics,
which in turn results in diet-specific enrichments of 15N in the
tissue of the organism. We argue in the present study that bats complemented
their nitrogen requirements by recycling nitrogen from catabolized body
substance. Thus, complex fractionation effects, in addition to the mixing of
nitrogen isotopes from internal and external pools, led to the observed
overestimate of t50 and the large variance of
t50 values (see Fig.
4). In addition, individual differences in metabolic rates could
have attributed to the variance in t50 estimates, as
enzymatic action correlates with the metabolism of an animal.
In summary, half-lifes of blood and wing membrane estimated using nitrogen isotopes were higher than those estimated using stable carbon isotopes when bats were sustained under a nitrogen-poor diet. In addition, isotope turnover rates estimated with carbon and nitrogen isotopes were not correlated. These findings are most likely explained by mixing effects, additional fractionation of nitrogen isotopes and individual metabolic rates. The present study shows that nitrogen stress and the associated mobilization of body substance or compensatory metabolic responses may alter the isotopic composition of tissues. This could violate two basic assumptions of stable isotope studies: (1) isotopic balance within tissues and (2) constant and predictable fractionation factors between diet and tissues. Therefore, seasonal changes in the diet, for example, may be difficult to track with nitrogen isotopes if diet quality and body masses change seasonally as well, e.g. in reproducing, migrating or hibernating animals.
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Acknowledgments |
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References |
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![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Adams, S. A. and Sterner, R. W. (2000). The effect of dietary nitrogen content on trophic level 15N enrichment. Limn. Oceanogr. 45,601 -607.
Baker, H. G. and Baker, I. (1982). Chemical constituents of nectar in relation to pollination mechanisms and phylogeny. In Biochemical Aspects of Evolutionary Biology (ed. H. M. Nitecki), pp. 131-171. Chicago: Chicago University Press.
Ben-David, M., Flynn, R. W. and Schell, D. M. (1997). Annual and seasonal changes in diets of martens: evidence from stable isotope analysis. Oecologia 111,280 -291.[CrossRef]
Cabana, G. and Rasmussen, J. B. (1994). Modelling food chain structure and contaminant bioaccumulation using stable nitrogen isotopes. Nature 372,255 -257.[CrossRef]
DeNiro, M. J. and Epstein, S. (1978). Influence of diet on the distribution of carbon isotopes in animals. Geochim. Cosmochim. Acta 42,495 -506.[CrossRef]
DeNiro, M. J. and Epstein, S. (1981). Influence of diet on the distribution of nitrogen isotopes in animals. Geochim. Cosmochim. Acta 45,341 -351.[CrossRef]
Eggers, T. and Jones, T. H. (2000). You are what you eat... or are you? Trends Ecol. Evol. 15,265 -266.[CrossRef][Medline]
Fleming, T. H., Nunez, R. A. and Sternberg, L. (1993). Seasonal changes in the diets of migrant and non-migrant nectarivorous bats as revealed by carbon stable isotope analysis. Oecologia 94,72 -75.
Gaebler, O. H., Vitti, T. G. and Vukmirovich, R. (1966). Isotope effects in metabolism of 14N and 15N from unlabelled dietary proteins. Can. J. Biochem. 44,1249 -1257.[Medline]
Hatch, K. A., Sacksteder, K. A., Treichel, I. W., Cook, M. E. and Porter, W. P. (1995). Early detection of catabolic state via change in 13C/12C ratios of blood proteins. Biochem. Biophys. Res. Commun. 212,719 -726.[CrossRef][Medline]
Hilderbrand, G. V., Farley, S. D., Robbins, C. T., Hanley, T. A., Titus, K. and Servehen, C. (1996). Use of stable isotopes to determine diets of living and extinct bears. Can. J. Zool. 74,2080 -2088.
Hobson, K. A. (1999). Tracing origins and migration of wildlife using stable isotopes: a review. Oecologia 120,314 -326.[CrossRef]
Hobson, K. A. and Clark, R. G. (1992). Assessing avian diets using stable isotopes II: factors influencing diet-tissue fractionation. Condor 94,189 -197.
Hobson, K. A. and Welch, H. E. (1992).
Determination of trophic relationships within a high Arctic marine food web
using 13C and
15N analysis.
Mar. Ecol. Prog. Ser.
84, 9-18.
Hobson, K. A., Alisauskas, R. T. and Clark, R. G. (1993). Stable-nitrogen isotope enrichment in avian tissues due to fasting and nutritional stress: implications for isotopic analysis of diet. Condor 95,388 -394.
Hobson, K. A., McLellan, B. N. and Woods, J. G.
(2000). Using stable carbon (13C) and nitrogen
(
15N) isotope to infer trophic relationships among black and
grizzly bears in the upper Columbia River basin British Columbia.
Can. J. Zool. 78,1332
-1339.[CrossRef]
Howell, D. J. (1974). Bats and pollen: physiological aspects of the syndrome of chiropterophily. Comp. Biochem. Physiol. A 48,263 -276.[CrossRef]
Kleiber, M. (1961). The Fire of Life. New York: Wiley.
Minagawa, M. and Wada, W. (1984). Stepwise
enrichment of 15N along food chains: further evidence and
the relation between
15N and animal age. Geochim.
Cosmochim. Acta 48,1135
-1140.[CrossRef]
Mizutani, H., Fukuda, M. and Kabaya, Y. (1992). 13C and 15N enrichment factors of feathers of 11 species of adult birds. Ecology 73,1391 -1395.
Oelbermann, K. and Scheu, S. (2002). Stable
isotope enrichment (15N and
13C) in a
generalist predator (Pardosa lugubris, Araneae: Lycosidae): effects
of prey quality. Oecologia
130,337
-344.[CrossRef]
Owens, N. J. P. (1987). Natural variations in 15N in the natural environment. Adv. Mar. Biol. 24,389 -451.
Peterson, B. J. and Fry, B. (1987). Stable isotopes in ecosystem studies. Annu. Rev. Ecol. Syst. 18,293 -320.[CrossRef]
Tieszen, L. L., Boutton, T. W., Tesdahl, K. G. and Slade, N.
A. (1983). Fractionation and turnover of stable carbon
isotopes in animal tissues: implications for 13C analysis of
diet. Oecologia 57,32
-37.
Vanderklift, M. A. and Ponsard, S. (2003).
Sources of variationin consumerdiet 15N enrichment: a
meta-analysis. Oecologia
136,169
-182.[CrossRef][Medline]
Voigt, C. C. (2003). The energetics of reproduction in the nectar-feeding bat Glossophaga soricina. J. Comp. Physiol. B 173,79 -85.[Medline]
Voigt, C. C., von Helversen, O., Michener, R. and Kunz, T. H. (2003a). Validation of a non-invasive blood-sampling technique for doubly-labelled water experiments. J. Exp. Zool. 296A,87 -97.[CrossRef]
Voigt, C. C., Matt, F., Michener, R. and Kunz, T. H.
(2003b). Low rates of carbon isotope turnover in tissues of two
nectar-feeding bats. J. Exp. Biol.
206,1419
-1427.
Webb, S. C., Hedges, R. E. M. and Simpson, S. J.
(1998). Diet quality influences the 13C and
15N of locusts and their biochemical components.
J. Exp. Biol. 201,2903
-2911.[Abstract]