Oecologia (2006) 150:190–201
DOI 10.1007/s00442-006-0522-0
ECOPHYSIOLOGY
Correlation of metabolism with tissue carbon and nitrogen
turnover rate in small mammals
Stephen E. MacAvoy Æ Lynne S. Arneson Æ
Ethan Bassett
Received: 20 February 2006 / Accepted: 18 July 2006 / Published online: 12 September 2006
! Springer-Verlag 2006
Abstract Stable isotopes have proven to be a useful
tool for deciphering food webs, examining migration
patterns and determining nutrient resource allocation.
In order to increase the descriptive power of isotopes,
an increasing number of studies are using them to
model tissue turnover. However, these studies have,
mostly by necessity, been largely limited to laboratory
experiments and the demand for an easier method of
estimating tissue turnover in the field for a large
variety of organisms remains. In this study, we have
determined the turnover rate of blood in mice and
rats using stable isotope analysis, and compared these
rates to the metabolic rates of the animals. Rats
(Rattus norvegicus) (n=4) and mice (Mus musculus)
(n=4) were switched between isotopically distinct
diets, and the rate of change of d13C and d15N in
whole blood was determined. Basal metabolic rates
(as CO2 output and O2 consumption per unit time,
normalized for mass) were determined for the rats
and mice. Rats, which were an order of magnitude
larger and had a slower metabolic rate per unit mass
than mice (0.02 vs. 0.14 O2/min/g), had a slower blood
turnover than mice for 13C (t1/2 =24.8 and 17.3 days,
respectively) and 15N (t1/2 =27.7 and 15.4 days, respectively). A positive correlation between metabolic
rate and blood isotopic turnover rate was found.
These are the only such data for mammals available,
Communicated by Jim Ehleringer.
S. E. MacAvoy Æ L. S. Arneson (&) Æ E. Bassett
Department of Biology, American University,
Hurst Hall 101, 4400 Massachusetts Ave NW,
Washington, DC 202-885-2186, USA
e-mail: [email protected]
123
but the literature for birds shows that mass and
whole-body metabolic rates in birds scale logarithmically with tissue turnover. Interestingly, the mammalian data graph separately from the bird data on a
turnover versus metabolic rate plot. Both mice and
rat tissue in this study exhibited a slower turnover
rate compared to metabolic rate than for birds. These
data suggest that metabolic rate may be used to
estimate tissue turnover rate when working with
organisms in the field, but that a different relationship
between tissue turnover and metabolism may exist for
different classes of organisms.
Keywords Stable isotope Æ Turnover Æ Metabolism Æ
Blood Æ Mammal
Introduction
Stable isotope analysis has become an important tool
used by ecologists when determining food webs, resource use, migration patterns, and species interactions. The basic premise behind using stable isotopes in
dietary studies is that a consumer’s tissues will isotopically resemble what is consumed (Fry and Sherr
1984; Minagawa and Wada 1984; Peterson and
Howarth 1987). Tissues can only be built from available nutrients, such as carbohydrates, proteins, and
lipids, although the extent to which a tissue resembles
the different dietary components depends on the
percent composition of the food as well as the type of
tissue examined (Hobson et al. 1995; MacAvoy et al.
2005; Tieszen et al. 1983; Tieszen and Farge 1993).
Stable isotope analysis can be a valuable tool in
differentiating the relative importance of protein and
Oecologia (2006) 150:190–201
carbohydrate in diet as long as the isotope ratios of the
various nutrient components are known (Arneson and
MacAvoy 2005; Hobson and Bairlein 2003; Koch
and Phillips 2002; Phillips and Gregg 2003; Phillips and
Koch 2002). By utilizing stable isotopes, studies have
shown that protein is the primary source material used
for building new tissues whereas carbohydrates are
used mainly as an energy source (Hobson et al. 2000;
MacAvoy et al. 2005). Because organisms resemble the
isotopic signatures of their diets, many ecological
studies use stable isotopes to gain insights into the food
web of a particular community or ecosystem (see
Hobson 1999; Hobson and Wassenaar 1999 for reviews). These studies often assume that consumers are
in isotopic equilibrium with their diet. However, this
may not always be the case in the event of a shifting
diet or migration of consumer or prey (Gannes et al.
1997; Hobson 1999).
The isotopic turnover rates of various tissues have
been published for several animals, including birds,
mammals, and marine organisms. For example, it has
been shown that garden warblers (Sylvia borin) and
Japanese quails (Coturnix japonica) have similar
blood d13C half-lives at 5.4 and 11.4 days, respectively (Hobson and Bairlein 2003; Hobson and Clark
1992), while larger birds such as the American crow
(Corvus brachyrhynchos) and canvasback duck
(Aythya valisineria) have longer blood half-lives:
approximately 30 and 21 days, respectively (Haramis
et al. 2001; Hobson and Clark 1993). MacAvoy et al.
(2005) found that blood d13C turnover in mice (Mus
musculus) has a half-life of approximately 17 days,
while Voigt et al. (2003) found blood d13C half-lives
in bats were much longer, 113 or 120 days in two
different species. Turnover in poikilotherms is much
slower than that of homeotherms, which is likely due
to low metabolic rates (MacAvoy et al. 2001). In
cases where turnover in poikilotherms is relatively
fast, it has been shown that the rapid turnover is due
mostly (and in some cases exclusively) to growth
(Tieszen and Farge 1993; Frazer et al. 1997; Herzka
and Holt 2000; Maruyama et al. 2001; Vander Zanden
et al. 1997).
Studies continue to show that large and significant
differences exist among organisms in how quickly they
incorporate the isotopic signature of their diet. Although an increasing number of studies are appearing
in the literature, measurements of isotope turnover are
still limited to a few species. However, if a relationship
between isotope turnover and a more common, welldocumented parameter, such as metabolic rate, could
be established, then estimates of isotope turnover rates
could be more widely applied. Researchers using
191
isotopes to characterize food webs could evaluate
when the organisms could be expected to be in isotope
equilibrium.
Metabolism refers to the sum of all catabolic
(degrading) and anabolic (synthesizing) processes that
occur in a living system. Metabolic processes produce
energy by consuming O2 in order to break down molecules that store energy, producing CO2 as a waste
product. Anabolic metabolic processes use this energy
to produce macromolecules, carbohydrates, proteins or
lipids, in order to either increase tissue mass (resulting
in growth) or to replace macromolecules that have
been degraded (resulting in tissue replacement). Thus,
metabolic processes contribute to both components of
tissue turnover as measured by stable isotope analysis,
growth and tissue replacement (MacAvoy et al. 2005).
Metabolic rate, which can be measured by oxygen
consumption per unit time, correlates to the body mass
of the organism according to the ‘‘3/4 rule’’ (Kleiber
1932, 1947), in which metabolic rate is proportional to
M0.75, where M is body mass.
In this study we examine the relationship between
tissue turnover rate and resting metabolic rate for two
species of small mammal, rats (Rattus norvegicus) and
mice (Mus musculus). We also compile existing
information on metabolic rate and tissue turnover for
a variety of avian species in order to broadly describe
the overall relationship between the two variables for
birds. We found that rats, which were an order of
magnitude larger and had a slower metabolic rate per
unit mass than mice, had a slower blood carbon and
nitrogen turnover than mice (carbon t1/2 = 24.8 vs.
17.3 days; nitrogen t1/2 = 27.8 and 15.4 days). Metabolic rates and blood turnover rates of different bird
species show that mass and whole-body metabolic
rates scale logarithmically with tissue turnover.
However, mice and rats both had a slower blood
isotopic turnover rate versus MR/gram than birds of
similar body mass, suggesting that the relationship
between these two variables may differ between classes
of organism.
Methods
Experimental design and tissue sampling
Adult female Sprague-Dawley rats (R. norvegicus)
(n=6) and adult female BALB/c mice (M. musculus)
(n=6) were allowed to equilibrate to a control diet
for approximately 120 days (2018, Harlan Teklad,
Madison, WI, USA). Four of each species began an
isotopically distinct experimental diet, while two
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192
Oecologia (2006) 150:190–201
remained on the control diet. The control diet contains both corn and wheat, and contains 18.9% protein. The experimental diet is 21% protein from
casein, 59% carbohydrate from beet sugar and 7%
soybean oil, with the remainder of the diet composed
of fiber, vitamins and minerals (Arneson and MacAvoy 2005). Carbon and nitrogen isotope values for
each diet are given in Table 1. Four blood samples
(lateral tail vein bleeding, ~100 ll) were taken from
each animal during the equilibration period, and
every seven days (R. norvegicus) and every ten days
(M. musculus) during the experimental period, which
lasted for 80 days. Blood samples were placed in a
drying oven at 60 "C for three days and then refluxed
for 35 min in dichloromethane for lipid extraction
(Knoff et al. 2002). After air drying, ~1 mg of tissue
was measured and placed in tin capsules in preparation for stable isotope analysis. Blood tissue samples were analyzed for d13C and d15N using a Europa
Hydra 20/20 (University of California, Davis, CA,
USA) stable isotope ratio mass spectrometer (IRMS)
to obtain d13C and d15N. Standards were run in
duplicate every twelve measurements (within a run
of 100 samples, which included 15 standards, the
standard deviation of the standard material was 0.2&
for nitrogen and 0.1& for carbon).
When determining isotopic signatures, the heavy to
light isotopic ratio of a particular element in the
sample is measured relative to that of a standard. The equation for an isotopic signature is as
follows:
d X¼
" "H !L #
X X sample
ðH X=L XÞstandard
CO2 output was measured using a Qubit Systems
(Kingston, ON, USA) S153 CO2 Analyzer attached to
an open air respiration chamber. Two readings were
taken per animal during the equilibration phase, and
two more were taken prior to blood sampling
throughout the course of the experiment. In this study,
MR is measured as O2/min.
Growth rates
Masses of individual animals were taken prior to each
blood collection and metabolic measurement. The
growth rate constant, k, for each group was determined
using
k ¼ ln ðMS =M0 Þ=t;
ð2Þ
where M0 is the initial mass in grams and MS is the
mass in grams on day t of the experiment. This was
done so that the contribution of growth to tissue
turnover could be ascertained (see below, Modeling
turnover).
Modeling turnover
The rate of isotope turnover in blood can best be
described by the following equation
Stable isotope analysis
H
Metabolic rates (MR)
#
$ 1 % 1000:
ð1Þ
Here X is any element, and H and L are the heavy and
light mass numbers, respectively; units are in per mil
(&).
dC
¼ $ðk þ mÞ % ðC $ CE Þ;
dt
ð3Þ
which describes the change in isotope value over time.
C is the signature at day t, CE is the signature in
equilibrium with the new diet, k is the specific growth
rate, m is the metabolic tissue replacement rate, and k
can be measured directly as a function of time (see Eq.
2 above). Integrating equation 3 results in
C $ CE ¼ ae$ðkþmÞt ;
ð4Þ
Table 1 Average tissue signatures at t0 and tend and diet-to-tissue discrimination averages for each diet. ±SD is the standard deviation
and Frac stands for ‘‘fractionation,’’ the name given to the diet-to-tissue discrimination value
d13C
Control diet (N=10)
M. musculus (N=15)
R. norvegicus (N=6)
Experimental diet (N=8)
M. musculus (N=8)
R. norvegicus (N=12)
123
–21.4&
–19.2
–19.0
–26.8
–25.6
–25.2
±SD
0.7
0.1
0.1
0.1
0.1
0.1
Frac
+2.3&
+2.4
+1.2
+1.6
d15N
±SD
2.8&
6.1
6.1
5.5
8.5
8.4
0.6
0.1
0.1
0.4
0.1
0.1
Frac
+3.3&
+3.3
+3.0
+2.9
Oecologia (2006) 150:190–201
193
where a is the difference between the initial and final
isotope signatures. Therefore, Eq. 4 becomes
CðtÞ ¼ CE þ ðC0 $ CE Þe$ðkþmÞt ;
ð5Þ
where C0 is the initial signature. This is an equation
describing isotope change over time as used by Hesslein et al. (1993); m can be calculated by rearranging
Eq. 5 so that
%
2 $
3
E
ln CC$C
0 $CE
m ¼ $4
þ k5 :
t
ð6Þ
The relative importance of k to m changes during different life stages. For young, fast-growing organisms,
the m component is negligible relative to k (Frazer
et al. 1997; Fry and Arnold 1982; Herzka and Holt
2000). The opposite is true for very slow-growing or
adult organisms. In these cases it has been shown that
metabolic tissue replacement is an important driver of
isotopic change. MacAvoy et al. (2005) found that, in
adult mice, the rate of change is almost entirely
(>90%) due to m, as little or no growth occurs. It is also
clear that m (metabolic tissue replacement rate) is
apparently higher in animals with high metabolic rates
(measured as oxygen consumption or carbon dioxide
consumption per gram). Using the growth and metabolic tissue replacement constants, the half-lives of
various elements within a tissue can be calculated,
yielding a measure of how quickly an organism
resembles the isotopic signature of its diet.
When solving Eq. 5 for the contribution of growth to
tissue turnover, m was set to zero. The growth
constant, k, used in Eq. 5 was the value obtained from
the day equilibrium was reached, tE. Any isotopic
turnover in excess of what was attributable to growth
was considered metabolic tissue replacement. The
metabolic constant, m, was determined using observed
data in Eq. 6 from day 0 to day tE. The best estimate
of m resulted in the least absolute sum of the differences between calculated and observed isotope values
for each measurement up until the time of equilibrium
(which was day 65 in all cases). We interpreted an
approximate 0.1& fluctuation between measurements
made at successive time intervals as being indicative of
the animals reaching isotopic equilibrium. The reproducibility of tissue isotope (d13C and d15N) measurements usually has a standard deviation of ±0.2&, so
we accepted a 0.1& fluctuation as a reasonable
approximation of isotope equilibrium. Earlier feeding
experiments with this particular strain of mouse and
diets similar to those in this study observed the time
to isotopic equilibrium to be approximately 70 days.
Therefore, the experimental mice were fed for
85 days, and we expected equilibrium to be reached
during this time, although it was some months before
all isotope data could be gathered and analyzed.
Although we did not have isotope equilibrium time
estimates for rats, weekly sampling was halted at
72 days. However, the experimental group continued
to be fed the new diet, and a final blood sample was
taken at 176 days to ensure there was no difference
between the last weekly measurement and the one
100 days later. No significant difference (d13C –25.1 vs.
–25.1& and d15N 8.3 vs. 8.4&, respectively) was
observed between the 72- and 176-day samples.
Half-life
In this study, half-life refers to the time it takes for half
of the existing tissue to resemble the isotopic signature
of the new diet. Half-life is calculated by the following
equation:
t1=2 ¼
ln 2
:
mþk
ð7Þ
Metabolic rate and m
Equations were developed relating the rate of blood
turnover (m and t1/2) to average mass, metabolic rate
and metabolic rate per unit mass. To complete these
models, a literature review was also performed with
any published turnover data available. In addition to
using data obtained in this study with R. norvegicus and
M. musculus, data found in published papers on other
species was also used to support/strengthen correlations (birds: Bearhop et al. 2002; Chamberlain et al.
1997; Evans-Ogden et al. 2004; Haramis et al. 2001;
Hobson and Clark 1992a, 1993; Pearson et al. 2003;
mammals: Ayliffe et al. 2004; Tieszen et al. 1983).
Results
Growth rates
The animals used in this study were mature at the
beginning of the experiment. Both the rats and the
mice were a minimum of eight months old on day zero
of the experiment, and were sexually mature. The mass
of each animal was determined every seven (rats) or
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194
Oecologia (2006) 150:190–201
Turnover rates
The animals used in this study were weaned on the
control diet, and were also fed this diet until they were
switched to the experimental treatment. The animals
were maintained on the control diet for an equilibration period of four months to ensure that the different
lots of the commercially available diet did not affect
the stable isotope ratio. Blood samples taken during
the equilibrium phase prior to the beginning of the
experiment from both species of animals indicated that
the carbon and nitrogen stable isotope ratios did not
change during this time (data not shown).
On day zero of the experiment, two mice and two
rats were maintained on the control diet, while the
diets of four mice and four rats were changed to the
experimental diet. Blood samples were collected from
all animals in order to determine the starting carbon
and nitrogen stable isotope ratios. The average blood
d13C at the start of the experiment (t0) was –19.2&
(±0.1) in M. musculus (n=15), and –19.0& (±0.1) in R.
norvegicus (n=6; Table 1). The average blood d15N at
the beginning of the experiment was 6.1& (±0.1) in M.
musculus (n=15), and 6.1& (±0.1) in R. norvegicus
(n=6; Table 1). The diet–tissue discrimination values at
the beginning of the experiment were 2.3 and 2.4& for
carbon and 3.3 and 3.3& for nitrogen in M. musculus
and R. norvegicus, respectively.
Blood samples were collected every ten (mice,
Fig. 2) or seven (rats, Fig. 3) days and carbon and
nitrogen stable isotope ratios were analyzed. Blood
carbon and nitrogen turned over at similar rates
within the same species. The half-life of tissue carbon
and nitrogen was 17.3 and 15.4 days, respectively, for
mice (Table 2). Blood tissue carbon and nitrogen
turned over more slowly in rats, with a half-life of 24.8
and 27.7 days, respectively (Table 2). Diet–tissue discrimination values at the end of the experiment were
1.2 and 1.6& for carbon and 3.0 and 2.9& for nitrogen in mice and rats, respectively (Table 1). For both
a
-16
-18
-20
-22
δ13C
ten (mice) days to determine rate of growth (Fig. 1).
Although some change in mass was seen in both the
control and experimental groups of rats and mice, it
was minimal. R. norvegicus in the experimental group
(n=4) had an average growth rate of 8.62·10–5 g/day
(±3.13·10–4), while the control group (n=2) grew at an
average rate of 4.50·10–4 g/day. M. musculus on the
experimental diet (n=4) had an average growth rate of
–3.95·10–4 g/day (±6.89·10–4), while the control group
grew at an average rate of 3.64·10–4 g/day. Any change
in mass was most likely due to change in fat deposition
from excess caloric uptake. This is suggested by the
relatively sharp decrease in mass after 15–24 h of
fasting prior to metabolic testing (Fig. 1).
-24
-26
-28
-30
0
10
20
30
40
50
65
72
85
Days
b
R. norvegicus
10
360
320
9
280
240
8
200
0
10
20
30
40
50
60
70
M. musculus
δ15N
Mass (g)
400
7
6
35
5
30
4
25
20
0
0
10
20
30
40
50
60
70
Days
Fig. 1 Mass of individual Rattus norvegicus and Mus musculus
versus time
123
10
20
30
40
50
65
72
85
Days
Fig. 2a–b a Carbon and b nitrogen isotope change in Mus
musculus blood over time. Controls are squares and experimentals are circles. Bars represent ±1 standard deviation
Oecologia (2006) 150:190–201
a
195
Metabolic rates
-16
-18
Metabolic rate averages were determined using data
from the experimental period, including day zero.
Average whole-body metabolic rate for mice was
3.2 ml O2/min compared to 8.9 ml O2/min for rats.
However, as expected, metabolic rate per gram for
mice was considerably higher than that for rats
(6.84 ml O2/h/g for mice compared to 1.84 ml O2/h/g
for rats) (Fig. 4).
-20
δ13C
-22
-24
-26
-28
-30
0
7
14
21
28
35
42
49
65
72
Days
b
Discussion
9
This study examined the blood turnover rate in mice
and rats using stable carbon and nitrogen isotopes. We
have found that blood carbon and nitrogen turnover
was roughly equivalent in mice and in rats, and that
mouse blood turned over slightly faster than rat blood.
We have also determined the resting metabolic rate
(MR) of the animals used in this study. The measured
MR and the relationship between mass and metabolic
rate are consistent with what other researchers have
observed (Fig. 5). Our findings are also in accordance
8
δ15N
7
6
5
4
0
7
14
21
28
35
42
49
65
72
Days
1.2
Table 2 Average half lives (t1/2) and metabolic constants (m) for
each isotope in each species
M. musculus
R. norvegicus
M. musculus
R. norvegicus
13
C
15
N
m
t1/2 (days)
0.04
0.028
0.0449
0.025
17.3
24.8
15.4
27.7
Log (MR (mL O2/h))
mice and rats, equilibrium of blood tissue with the
experimental diet occurred by day 65. Values at the
end of the experiment were used as the equilibrium
values in Eq. 5 to determine turnover constants. The
data do not allow the extent to which the animals
were actually at equilibrium to be determined, and we
cannot rule out the existence of a very long turnover
pool (see Ayliffe et al. 2004). However, the values at
the end of the experiment are likely to be close
(within ±0.1&) to the true equilibrium values, and
calculated turnover rates are relatively insensitive to
error in this parameter.
1.0
Rats
0.8
0.6
Mice
0.4
0.2
0.0
0.0
0.5
1.0
1.5
2.0
2.5
3.0
2.5
3.0
14.0
MR/Mass (O2/h/g)
Fig. 3a–b a Carbon and b nitrogen isotope change in Rattus
norvegicus blood over time. Controls are squares and experimentals are circles. Bars represent ±1 standard deviation
12.0
10.0
8.0
Mice
6.0
4.0
Rats
2.0
0.0
0.0
0.5
1.0
1.5
2.0
Log (Mass(g))
Fig. 4a–b a Log metabolic rate (O2/h) versus log mass (g) and b
metabolic rate per gram (O2/h/g) versus log mass (g) for Rattus
norvegicus and Mus musculus
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Oecologia (2006) 150:190–201
0.02
0.01
0
0
1
2
3
4
5
6
Log (mass(g))
Fig. 5 Metabolic rates (ml O2/min)/g for mammals (filled diamonds) and all birds (open squares) in Tables 3 and 4 were plotted
versus average mass of the animal. Trend lines are shown for both
categories of animals. For mammals, y = –0.0265ln(x) + 0.0462,
r2=0.9246, whereas for all birds, y = –0.0313ln(x) + 0.0529,
r2=0.8854
with the long-known relationship between size and
metabolism (Kleiber 1932, 1947). The mice exhibited a
faster MR per gram tissue but slower whole-body MR
relative to rats (Fig. 4), and our data indicate that
faster isotope turnover is associated with higher MR/g
(Fig. 7).
The carbon isotope turnover rate of the mice was
slightly slower in this study than that reported for the
same strain by MacAvoy et al. (2005) (half-lives for
carbon: 17.3 vs. 16.9). However, the mice used in the
previous study were younger than the adults used here
and the shorter half-life likely reflects an increase in
mass superimposed on turnover from metabolic tissue
replacement. MacAvoy et al. (2005) estimated that
growth accounted for approximately 10% of the observed isotope turnover, whereas in the current study
mass gain was negligible.
In this study, as in our previous studies (MacAvoy
et al. 2005; Arneson et al. 2006), the carbon and
nitrogen tissue turnover rates are roughly similar
within a tissue. This is in contrast to results from
Carleton and del Rio (2005) in which they find that
d13C incorporation is approximately 1.5 times faster
than d15N in birds exposed to low temperatures.
Although we do frequently see differences between
nitrogen and carbon incorporation rates, we cannot
apply significance to these differences.
In the final section of this study, we compared the
tissue turnover rate to MR in rats and mice, and found
that there is a correlation between these two measurements. Specifically, we found that a faster MR per
gram tissue correlates with a faster rate of tissue
turnover (Fig. 7). This relationship is not unexpected,
as the equation describing stable isotope turnover rate
123
30
Rat
C
t1/2 (days)
0.03
20
Mouse
13
0.04
10
0
0
5
10
15
20
25
30
30
N
t1/2 (days)
0.05
contains both a growth and a metabolic component
(Hesslein et al. 1993; MacAvoy et al. 2005). For adult
animals with relatively high metabolic rates and very
little mass gain over time, isotope turnover should be
particularly highly correlated with MR. Indeed, Sponheimer et al. (2006) suggest the existence of just such a
relationship between alpacas and gerbils. However, our
results are in opposition to the findings of Carleton and
del Rio (2005), which conclude that metabolism and
isotope incorporation into tissues are not directly related in birds.
In this study we could not determine the type of
correlation between tissue turnover and metabolism
because two species were utilized and two data points
cannot yield a predictive equation. Only blood was
sampled in this study, so we could not compare our
isotopic turnover values with those reported for other
mammals (horse, Ayliffe et al. 2004; gerbil, Tieszen
et al. 1983), as these studies did not measure blood
Rat
20
Mouse
15
MR (mL O2/min)/g
0.06
10
0
0
5
10
15
20
25
30
MR (mLO2/min)
Fig. 6a–b Carbon (a) and nitrogen (b) stable isotope half-lives in
blood for the birds given in Table 3 are plotted versus metabolic
rate (closed diamonds). The trend lines for these relationships
are shown, and the equations are a y = 3.802ln(x) + 6.978,
r2=0.90 and b y = 3.2919ln(x) + 8.2191, r2=0.72. The metabolic
rates for mouse and rat are overlaid for blood stable carbon (a)
and nitrogen (b) isotope turnover. Note that values for gerbil and
horse are not included, as blood isotopic turnovers were not
determined for these species
Oecologia (2006) 150:190–201
197
isotope turnover. Therefore, we examined the literature for other studies determining whole blood turnover rate in species for which the metabolic rate has
been determined (Table 4). These data, exclusively
from studies of birds, describe a logarithmic relationship between isotope turnover and metabolic rate
(Fig. 6) or metabolic rate per gram (Fig. 7) when
plotted. The various studies pooled to examine this
relationship examined birds ranging in mass from 11.5
to 1,250 g and metabolic rates ranging from 0.56 to
27.5 O2/min (warbler and canvasback, respectively)
(McKechnie and Wolf 2004). Yet even with the order
of magnitude differences in these variables, the correlation of metabolic rate per unit mass with 13C and 15N
turnover is very high (0.87 and 0.9, respectively, Fig. 7).
Unfortunately, almost all of the tissue turnover
studies published have only examined avian species.
While these data show that within a class of organism
there is a high degree of predictability regarding how
quickly an organism will resemble the isotope signature of its diet that comes from knowledge of its
metabolic rate, it is clear from Figs. 6 and 7 that
mammalian MR versus isotope half-life are not welldescribed by the relationship in birds. Both mice and
30
25
Rat
13
C
t1/2 (days)
20
Mouse
15
10
5
0
0
0.01
0.02
30
N
t1/2 (days)
0.04
0.05
0.06
Rat
25
15
0.03
20
Mouse
15
10
5
0
0
0.01
0.02
0.03
0.04
0.05
0.06
MR/g (ml O 2 /min/g)
Fig. 7a–b Carbon (a) and nitrogen (b) stable isotope half-lives in
blood for the birds given in Table 3 are plotted versus metabolic
rate per gram (closed diamonds). The trend lines for these
relationships are shown, and the equations are a y = –7.5508ln(x)
– 16.536, r2=0.87 and b y = –3.2376ln(x) – 1.6114, r2=0.90. The
metabolic rates for mouse and rat are overlaid for blood stable
carbon (a) and nitrogen (b) isotope turnover. Note that values
for gerbil and horse are not included, as blood isotopic turnover
was not determined for these species
rats have longer blood tissue turnover rates per gram
body mass than birds (Figs. 6, 7). While we realize that
the relationship between passerines and nonpasserine
birds and metabolic rate are not equivalent (McNabb
1988), when the birds are grouped as a whole, they
tend to have faster metabolic rates per gram tissue than
mammals, especially in smaller animals (Fig. 5). Because mammals have a slower metabolic rate per gram
than birds, and metabolic rate per gram is positively
correlated to tissue turnover rate, it is not surprising
that mice and rats have a slower turnover rate than
similarly sized birds. Although the metabolic rate and
blood turnover rate have only been correlated for two
mammals in this study, the relationship seen in various
avian species suggests that the correlation in mammals
would also be logarithmic. However, it is important to
stress that additional data points are needed to verify
this relationship and construct a model.
It should also be noted that the relationship between
isotope turnover and MR found in the study likely only
holds for adults (where growth rate is effectively zero)
and homeotherms. We postulate that in growing animals or poikilotherms there would be a markedly different relationship and likely a weaker correlation
between the two variables than what we show here.
Determining the relationship between metabolic
rate and tissue turnover rate could allow researchers to
predict the turnover rate of the tissue in question based
on knowledge of the metabolic rate of the animal being
studied. Currently, tissue turnover rates are determined in laboratory studies by changing the diet of the
organism and measuring the stable isotope ratios of the
tissues over time as they come into equilibrium with
the new diet. The relationship proposed in this study
will be valuable for researchers seeking a way to predict time to isotope equilibrium for species for which
metabolic rate is known but the isotope turnover is not.
If a relationship between metabolic rate and turnover
rate can be modeled, the researcher could determine
the metabolic rate for the organism and use this measurement to predict the isotope turnover rate.
The main caveat to this work is that metabolic rate
can change, and therefore likely cause the tissue turnover rate to be altered. Most published metabolic rates
are reported as basal metabolic rates (BMR), which
require the measurement of metabolic rate under
standard conditions (McNabb 1988). These standard
conditions require the animal be (1) resting during its
normal time of rest (as in circadian rhythm); (2) in
thermoneutrality; (3) postabsorptive; and (4) an adult
(McNabb 1988). In contrast, metabolic rates measured
in the field may be affected by photoperiod, temperature (ambient and body), animal activity, age and diet.
123
198
Oecologia (2006) 150:190–201
Table 3 Mass, metabolic rate (MR in ml O2/min), and isotope (13C and
15
N) turnover rates of various tissues in four mammals
MR
(ml O2/min)
Tissue
13
Ct1/2
(days)
15
Nt1/2
(days)
(1) Turnover reference
(2) MR andmass reference
Mammalian: Rodents 101 g
Gerbil
64.8
Meriones unguiculates
1.61
(1) Tieszen et al. (1983)
(2) Lovegrove (2003),
White and Seymour (2003)
18.55
18.55
0.84
0.84
Mouse
Mus musculus
27.7
3.16
6.4
15.8
27.7
27.7
46.2
16.9
ND
16.5
18.6
ND
Mouse
Mus musculus
Liver
Fat
Muscle
Brain
Hair
Blood
Liver
Muscle
Blood
19.3
7.3
24.8
19.6
(1) Macavoy et al. (2005)
(2) Lovegrove (2003),
Heusner (1991)
(1) This Study
(2) Lovegrove (2003);
Heusner (1991)
288
5.90
Blood
24.7
21.3
(1) This Study
(2) Hart (1971)
488,636
409,778
1125.22
487.45
Hair
136.0
ND
(1) Ayliffe et al. (2004)
(2) Bromham et al. (1996),
Heusner (1991)
Species
Size (g)
Mammalian: Rodents 102g
Rat
Rattus norvegicus
Mammalian: 105g
Horse
Equus caballus
Equus asinus
Table 4 Mass, metabolic rate (in mL O2/min), and isotope (13C and
15
N) turnover data for seven species of birds
Tissue
Size (g) MR
(ml O2/min)
13
Ct1/2
(days)
15
Nt1/2
(days)
(1) Turnover reference
(2) MR and mass reference
1,250
27.50
1.23
Yellow-rumped warbler 11.5
Dendrorica coronta
0.56
26.5
25.4
17.6
23.2
11.2
5.0
8.1
7.5
American crow
Corvus brachyrhynchos
Avian: Nonpasserine
Japanese quail
Coturni japonica
384.8
9.70
26.2
19.8
16.0
20.7
5.0
5.8
5.4
3.9
5.0
6.1
5.0
2.9
30.1
(1) Haramis et al. (2001)
(2) Woodin and Stephenson (1998)
24.8
Blood (clam/corn Diet)
Blood (tuber diet)
Blood (clam diet)
Average
Blood (meal worm diet)
Blood (blk elderberry diet)
Average
Blood (49% insect diet)
Blood (73% insect diet)
Blood (97% insect diet)
Average
Plasma
Blood cells
115
2.89
970
14.03
Blood
Liver
Muscle
Bone
Blood
11.2
2.5
12.4
173.3
15.1
12.0
44
1.57
Blood
11.2
10.0
Species
Avian: Passerine
Canvasback
Aythya valisineria
Garden warbler
Sylvia borin
Great skua
Catharacta skua
Dunlin
Calidris alpina pacifica
It has been shown that the metabolic rate that can
be measured in the field (FMR) scales differently with
body size than BMR (Kojeta 1991). Kojeta (1991) and
Ricklefs et al. (1996) showed that a correlation does
exist between BMR and FMR with mammals (rodents), but a weak correlation exists with birds, and
123
(1) Hobson and Bairlein (2003)
(2) Mckechnie and Wolf (2004)
(1) Pearson et al. (2003)
(2) Mckechnie and Wolf (2004)
(1) Hobson and Clark (1993)
(2) Mckechnie and Wolf (2004)
(1) Hobson and Clark (1992)
(2) Roberts and Baudinette (1986)
(1)
(2)
(1)
(2)
Bearhop et al. (2002)
Mckechnie and Wolf (2004)
Evans-Ogden et al. (2004)
Lindstrom (1997)
none with marsupials. The lack of evidence supporting
a relationship between FMR and BMR is probably
due to the fact that MR is highly variable in the field.
BMR, therefore, is important as a standardized tool
that justifies comparisons between individuals and
species.
Oecologia (2006) 150:190–201
The potential difference between FMR and BMR
indicates that using BMR to predict the tissue turnover
rate could result in inaccurate estimates. However,
most current studies determining tissue turnover rate
using stable isotope analysis do not conduct their
experiments in a field-type setting. Animals are usually
housed in a laboratory, with limited living space and
food supplied ad libitum, likely leading to decreased
activity levels. Laboratory temperatures and photoperiod are generally stably and optimally maintained.
Thus, the tissue turnover rates experimentally measured in laboratories are also likely obtained using
animals that do not exhibit metabolic rates similar to
those observed in the field.
However, recent data from Carleton and del Rio
(2005) suggest that a cold-induced change in metabolic rate in an avian species has a negligible effect on
carbon and nitrogen incorporation into red blood
cells. The metabolic rates measured in this study are
not basal MR, but are affected by environmental
components that could be expected to affect MR in
the field. These results suggest that FMR, although
different then BMR, may have only an indirect, and
perhaps negligible, effect on the isotopic tissue turnover rate, allowing laboratory studies relating BMR
and tissue turnover to apply to studies of animals in
the field.
An additional interesting component of this work is
that low nitrogen dietary intake may negate the relationship between MR and tissue turnover, as seen in
bats (Voigt et al. 2003). Nectar-feeding bats have
higher mass-specific metabolic rates than similar sized
terrestrial mammals, and yet blood carbon turnover
rates are considerably slower than those seen in terrestrial mammals of the same mass, seemingly negating
the relationship found earlier in this study. However,
the bats were fed sugar-water, with little nitrogen
content (Voigt et al. 2003), and nectar-feeding bats in
the wild could also be expected to take in low levels of
dietary nitrogen. As protein turnover and replacement
is a major driver in metabolism, and the bats are
deficient in dietary nitrogen and thus would need to
recycle tissue components, it seems likely that both
tissue nitrogen and carbon are being recycled, resulting
in longer tissue carbon half-lives, despite the faster
MR. As seen in other species (Hobson and Stirling
1997; Arneson et al. 2006), the carbohydrate in the
nectar or sugar-water is likely primarily oxidized to
provide energy and expelled as breath CO2 (Sponheimer et al. 2006), rather then being incorporated into
tissues. A similar disconnect between MR and tissue
isotope turnover rate would also be expected in other
199
mammals that consume low dietary nitrogen, as well as
in animals experiencing starvation.
A second caveat to this work is that studies have
shown that different tissues turn over at different rates
(Arneson and MacAvoy 2005; Evans-Ogden et al.
2004; MacAvoy et al. 2005; Tieszen et al. 1983; Voigt
et al. 2003). Therefore, the relationship between
whole-body metabolic rate and blood carbon or
nitrogen tissue turnover could not be used to predict
the turnover rates of other tissues. The relationship will
likely need to be modeled for each type of tissue
studied before any predictions can be made.
In a 2004 paper, Ogden, Hobson and Lank observed that studies are needed to determine the
relationship between metabolic rate and isotope
turnover. They point out that variable metabolic rate,
even among individuals of the same species, could
have a significant impact on isotope turnover rates.
The study reported here presents the first examination we are aware of relating tissue isotope turnover
to metabolic rate in an effort to determine a predictive relationship between the two. While we understand that directly applying these results to field
situations where different types of animals experience
variable physiological stresses would not be advisable
(Carleton and del Rio 2005), this examination has
been a useful step towards a better understanding of
the effects of metabolic rate on isotope turnover.
Given the strong and predictive relationship between
metabolism and isotope turnover in adult homeotherms apparent from this study, we believe that
knowledge of the metabolic rates of organisms within
an ecosystem will allow researchers to make wellgrounded assumptions about the isotope equilibrium
status of each system studied.
Acknowledgments The authors would like to thank the COSMOS Foundation and the Mellon Fund (American University)
for partial funding of this study, and two anonymous reviewers
for their constructive comments. The experiments described in
this paper comply with the current laws of the United States.
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