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First published online November 19, 2007
Journal of Experimental Biology 210, 4233-4243 (2007)
Published by The Company of Biologists 2007
doi: 10.1242/jeb.009779
Limits to sustained energy intake. X. Effects of fur removal on reproductive performance in laboratory mice
bieta Król*
School of Biological Sciences, University of Aberdeen, Aberdeen Centre for Energy Regulation and Obesity (ACERO), Aberdeen, AB24 2TZ, UK
* Author for correspondence (e-mail: e.krol{at}abdn.ac.uk)
Accepted 24 September 2007
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Summary |
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 TOP Summary IntroductionMaterials and methodsResultsDiscussionReferences |
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Key words: heat dissipation limit, lactation, digestive efficiency, metabolizable energy intake, daily energy expenditure, milk production, pup energy content
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Introduction |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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;
Kirkwood, 1983;
Peterson et al., 1990;
Weiner, 1992;
Hammond and Diamond, 1997;
Speakman, 2000;
Speakman and Król,
2005a; Anderson and Jetz,
2005). It is therefore a key component of our understanding of
limits on reproductive output, which is essential for modelling the impact of
global climate change on animal distributions
(Thomas et al., 2001;
Humphries et al., 2002). In
addition, limits on SusEI have important ramifications for our understanding
of human endurance performance (Hammond
and Diamond, 1997) and productivity of domesticated livestock.
Recent attempts to elucidate the nature of the limits to SusEI have focused
on lactation, which is energetically the most demanding period for female
mammals (e.g. Perrigo, 1987;
Weiner, 1987;
Kenagy et al., 1989;
Hammond and Diamond, 1992;
Hammond and Diamond, 1994;
Hammond et al., 1994;
Rogowitz and McClure, 1995;
Hammond et al., 1996;
Koteja, 1996;
Speakman and McQueenie, 1996;
Rogowitz, 1998;
Hammond and Kristan, 2000;
Johnson and Speakman, 2001;
Johnson et al., 2001a;
Johnson et al., 2001b;
Johnson et al., 2001c). Early
studies of food intake at peak lactation suggested that the limits on SusEI
were imposed centrally by the capacity of the alimentary tract and associated
organs, such as the liver, to process the ingested food (e.g.
Kirkwood, 1983;
Perrigo, 1987;
Hammond and Diamond, 1992;
Hammond and Diamond, 1994;
Koteja, 1996). This idea was
supported by experiments on lactating mice and guinea pigs Cavia
porcellus that were energetically challenged by adding extra pups
(Hammond and Diamond, 1992;
Künkele, 2000;
Johnson et al., 2001a) or by
extending lactation (Hammond and Diamond,
1994), and did not breach the upper limit of food intake
established in unmanipulated mothers. This `central limitation hypothesis' was
not supported, however, when animals were challenged with different modes of
energy expenditure. Specifically, mice and hispid cotton rats Sigmodon
hispidus forced to lactate at low ambient temperatures were able to
increase their food intake well beyond a previously supposed centrally
mediated limit (Hammond et al.,
1994; Rogowitz,
1998; Hammond and Kristan,
2000; Johnson and Speakman,
2001). Consequently, it was suggested that lactating animals were
limited not centrally, but peripherally, most likely by the capacity of
mammary glands to produce milk (Hammond et
al., 1994). According to this idea, females with artificially
enlarged litters or prolonged lactation could not respond to the increased
demands, because their mammary glands were already working at maximal
capacity, and any further increase in food intake could not be transformed
into greater milk production. However, animals lactating at low temperatures
were able to elevate their food intake because the increased demands for
thermoregulation were additional to and independent from milk production.
A fundamental prediction of the `peripheral limitation hypothesis' is that
mammary glands at peak lactation should work at maximal capacity regardless of
ambient temperature. To evaluate this assumption, we performed a series of
measures of food intake and milk production in mice exposed to 30°C,
21°C and 8°C (Johnson et al.,
2001a; Johnson and Speakman,
2001; Król and
Speakman, 2003a; Król
and Speakman, 2003b). Consistent with the peripheral limitation
model, food intake was the lowest at 30°C and the highest at 8°C.
Conflicting with the peripheral limitation idea, however, milk energy output
(MEO) was not constant across the different temperatures, but mirrored the
pattern of food intake. Greater MEO at lower ambient temperatures (87.7 kJ
day–1 at 30°C, 166.7 kJ day–1 at
21°C and 288.0 kJ day–1 at 8°C) could potentially be
explained by increasing energy demands of the pups. However, pups weaned at
lower temperatures were also heavier (6.1 g at 30°C, 7.0 g at 21°C and
7.3 g at 8°C). Hence, the colder it got, the more food lactating mice ate,
the more milk they produced, and the heavier pups they raised. Our results
were inconsistent with a limitation imposed either centrally by the capacity
of the alimentary tract or peripherally by the capacity of the mammary glands.
Other attempts to test these limits were also inconclusive (for a review, see
Speakman and Król,
2005a). The lack of clear separation between the central
(alimentary tract) and peripheral (mammary gland) effects suggests that the
limits on SusEI may act elsewhere.
We have recently proposed that the limits to SusEI at peak lactation might
be imposed by the capacity of the animal to dissipate body heat generated as a
by-product of processing food and producing milk
(Król and Speakman,
2003a; Król and
Speakman, 2003b; Król
et al., 2003). The heat flow between an animal and its environment
strongly depends on the temperature gradient (the difference between body and
ambient temperatures) and the thermal conductivity of the insulating surface
(subcutaneous fat, skin and fur) (e.g.
Scholander et al., 1950;
Hammel, 1955;
Conley and Porter, 1980).
Accordingly, when lactating females were challenged with manipulations that
did not involve alterations of ambient temperature and/or thermal conductivity
to enhance heat flow, they could not increase their food intake because this
would make them dangerously hyperthermic. Examples of such manipulations are
experiments performed at room temperature (approximately 21°C) that aimed
to increase litter size (Hammond and
Diamond, 1992; Künkele,
2000; Johnson et al.,
2001a), prolong lactation
(Hammond and Diamond, 1994),
increase demands of precocial pups by withholding solid food early in
lactation (Laurien-Kehnen and Trillmich,
2003), force lactating animals to run for food
(Perrigo, 1987) or make them
simultaneously pregnant during lactation
(Biggerstaff and Mann, 1992;
Johnson et al., 2001a;
Koiter et al., 1999). However,
when the capacity of animals to dissipate heat was increased, by lowering
ambient temperature (8°C), mice were able to elevate their food intake to
produce more milk and heavier pups
(Johnson and Speakman, 2001).
By contrast, when we reduced the temperature gradient between the animal and
the environment by exposing mice to 30°C, they responded by decreasing
food intake, milk production and the size of their offspring
(Król and Speakman,
2003a; Król and
Speakman, 2003b).
Importantly, raising ambient temperature affects not only the amount of
body heat that can be dissipated by the mother, but also by her pups. If the
limits to SusEI are, for example, set by the capacity of homeothermic pups to
dissipate heat associated with their intense growth rate, then the expected
response of the pups exposed to higher ambient temperatures would be to slow
down their growth. Under this scenario, pups would have decreased demands for
milk which, via reduced stimulus from suckling, would lead to
downregulation of milk production and an associated decrease in maternal food
intake. Hence, the reduced SusEI of females lactating at higher ambient
temperatures (Król and Speakman,
2003a) does not indicate whether the limits to SusEI are intrinsic
(heat dissipation capacity of the mother) or extrinsic (heat dissipation
capacity of the pups), or perhaps unrelated to the dissipation of heat [for
examples of other potential limiting factors see
(Speakman and Król,
2005a)]. These effects can be separated by manipulation of the
heat flow between the female and the environment without affecting the thermal
balance of the pups. This can be achieved by changing the thermal conductivity
of the female by shaving off some of the fur.
The insulation provided by mammalian fur significantly reduces heat loss to
the environment (e.g. Scholander et al.,
1950; Barnett,
1959; Knight,
1987; Reynolds,
1993). Seasonal increases in the density, thickness and length of
fur have been shown to decrease thermal conductance in winter (e.g.
Hart, 1956;
Morrison and Tietz, 1957;
Conley and Porter, 1980;
Jacobsen, 1980). Furthermore,
the presence of fur substantially contributes to energy savings during torpor
or hibernation by decreasing body heat loss and reducing energy expenditure
during periodic arousals from torpor and subsequent intervals of normothermia
(Snapp and Heller, 1981;
Kauffman et al., 2001a;
Kauffman et al., 2004).
Conversely, partial or complete fur removal in non-reproductive mice, hamsters
and voles is associated with increased food intake and energy expenditure,
reflecting enhanced costs of thermoregulation
(Pearson, 1960a;
Kenagy and Pearson, 2000;
Kauffman et al., 2001b;
Kauffman et al., 2003). The
association between high thermal conductance due to lack of fur and elevated
costs of thermoregulation has also been demonstrated in genetically hairless
mice (Mount, 1971;
Heldmaier, 1974). The effects
of fur removal on the energy budget during reproduction, however, have not
previously been investigated.
To explore the nature of the limits to SusEI, we bred MF1 laboratory mice (Mus musculus L.) at 21°C and then dorsally shaved lactating females to reduce their external insulation and thereby elevate their capacity to dissipate body heat. The heat dissipation limit hypothesis predicts that under such conditions mice should have elevated food intake and milk production that would result in enhanced reproductive performance. Any increases in food intake could also be explained by higher costs of thermoregulation due to fur removal, and would not be inconsistent with the peripheral or extrinsic limitation hypotheses. However, data on milk production and reproductive performance can unambiguously separate the heat dissipation hypothesis from alternative ideas. Reduction in milk production and reproductive performance by shaved mice would be consistent with the central limitation hypothesis. The peripheral and extrinsic limit hypotheses would be consistent with unchanged milk production and reproductive performance. To test the heat dissipation hypothesis, we measured food intake, daily energy expenditure, milk production and reproductive performance (evaluated by litter size, litter mass, pup body mass, and pup energy content) of shaved mice and compared these traits with the same parameters measured in unshaved mice. The effects of shaving were also evaluated in non-reproductive mice.
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Materials and methods |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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Body mass, food intake and reproductive performance
The body mass, food intake, litter size and litter mass of females raising
10 or more pups (N=40) were recorded (±0.01 g) every other
day, from day 4 to the end of lactation (day 18). Food intake was calculated
from the difference between the amount of food provided and that left in the
hopper. The amount of food consumed over each 2-day period was averaged and
presented as the daily food intake for the second day of the period.
Simultaneous measurements of body mass and food intake were also performed on
non-reproductive females.
To evaluate metabolizable energy intake, sawdust was collected from the cages between days 12–14 of lactation. Sorting through the sawdust revealed that all mice removed some food from the hopper that was then fragmented and left uneaten in the cage as orts. The amount of fragmented food did not vary significantly between shaved and unshaved mice (data and statistical analyses not shown). Since sorting through the sawdust was performed for only one of the seven food-intake trials, the consistency of food wasting across trials repeated on the same individuals was unknown. Therefore, food-intake data reported for days 4–18 of lactation were not corrected for the amount of orts generated by mice on days 12–14 of lactation, but detailed calculations of energy balance and milk production at peak lactation were made using the food intake corrected for orts production (details below).
Shaving protocol
Once the measurements on day 6 of lactation were completed, we assigned the
mice to two groups (shaved and unshaved) that were matched for litter size
(lactating mice) or body mass (non-reproductive mice). All mice were then
anaesthetized with gaseous isoflurane for approximately 10 min. During this
time, 20 lactating and 8 non-reproductive mice were shaved dorsally (using a
Wella Contura Hair Clipper, Basingstoke, Hants, UK) to remove approximately
0.18–0.30 g of fur (Fig.
1); the remaining mice were not shaved (lactating and
non-reproductive unshaved control groups). Based on the data available for 6
mice that went through the shaving protocol and were then completely shaved
post mortem, the amount of fur shaved off during lactation
corresponded to 72.1±2.2% (range 69.4–74.4%) of total fur mass.
Hair regrowth was prevented by repeating the shaving protocol on days 10 and
14 of lactation. The mean litter sizes for shaved and unshaved mice were
11.4±1.1 and 11.3±1.0 pups, respectively (N=20 for both
groups).
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d,
1975). The apparent digestive efficiency was calculated as the
percentage of gross energy intake that was digested.
Daily energy expenditure
We measured daily energy expenditure (DEE) using the doubly labelled water
(DLW) technique (Lifson and McClintock,
1966; Speakman,
1998) on days 15–17 of lactation. This method has been
previously validated by comparison to indirect calorimetry in a range of small
mammals (e.g. Speakman and Racey,
1988a) and provides an accurate measure of DEE over periods of
several days (Speakman and Król,
2005b). Day-to-day variability in estimated energy metabolism
suggests measurements spanning multiple 24 h periods may give a superior
representation of energy metabolism
(Speakman et al., 1994;
Berteaux et al., 1996). Studies
of lactating mammals suggest that recycling of isotopes between a mother and
her offspring is negligible (Scantlebury
et al., 2000).
Animals were weighed (±0.01 g) and injected intraperitoneally with
approximately 0.25 g (lactating mice) or 0.15 g (non-reproductive mice) of
water containing enriched 18O (27.8 atom%) and 2H (15.9
atom%). Syringes were weighed before and after administration (±0.0001
g) to calculate the mass of DLW injected. Blood samples were taken after 1 h
of isotope equilibration to estimate initial isotope enrichments
(Król and Speakman,
1999; Visser et al.,
2000a) and were also collected from unlabelled animals to estimate
the background isotope enrichments
(Speakman and Racey, 1987)
(method D). Blood samples were immediately heat sealed into 2x50 µl
glass capillaries and stored at room temperature. A final blood sample was
taken approximately 48 h later (Speakman
and Racey, 1988b) to estimate isotope elimination rates.
Capillaries that contained the blood samples were then vacuum distilled
(Nagy, 1983) and water from
the resulting distillate was used to produce CO2
(Speakman et al., 1990) and
H2 (Speakman and Król,
2005b). The isotope ratios 18O:16O and
2H:1H were analysed using gas source isotope ratio mass
spectrometry (ISOCHROMµGAS system and IsoPrime IRMS, Micromass, Manchester,
UK). We ran three high-enrichment standards each day alongside the samples and
corrected all the raw data to these standards to minimise the problems of
inlet cross contamination (Meijer et al.,
2000).
Initial isotope dilution spaces (mol) were calculated by the intercept
method (Coward and Prentice,
1985), then converted to g assuming a molecular mass of body water
of 18.020 and expressed as a percentage of body mass before injection. We used
the intercept method because the actual body water pool estimated by
desiccation (using mice excluded from the main part of this experiment due to
small or unstable litter sizes), was more accurately predicted by the
intercept approach than by the plateau approach. The actual body water pool
for the mice that were desiccated averaged 69.1±5.3% (day 17 of
lactation), whereas their mean oxygen dilution space (No) was
calculated as 69.4±3.0% by the intercept method and 72.3±3.7% by
the plateau method (N=14 for all means). Similarly, the deuterium
dilution space (Nd) averaged 74.9±2.4% for the intercept
method and 77.3±3.0% for the plateau method (N=14 for both
means). Final dilution spaces were inferred from the final body mass, assuming
the same percentage of body water as measured for the initial dilution spaces.
The isotope elimination rate (k) was calculated following published
methods (Lifson et al., 1955).
We used the single-pool model equation 7.17
(Speakman, 1997) to calculate
the rate of CO2 production, as recommended for animals weighing
under 1 kg (Visser and Schekkerman,
1999; Visser et al.,
2000b; Speakman and
Król, 2005b). Energy equivalents of the rate of
CO2 production were calculated using a conversion factor of 24.026
J ml–1 CO2, derived from the Weir equation
(Weir, 1949) for a respiratory
quotient of 0.85 (Speakman,
1997).
Milk energy output (MEO)
We used the DLW data to evaluate MEO, calculated from the difference
between MEI and DEE (Król and
Speakman, 2003b). MEI and DEE were not measured simultaneously to
avoid possible changes in animal behaviour or feeding patterns as a result of
DLW injections/bleeding (Król and
Speakman, 2003a; Speakman and
Król, 2005b). Animals were in energy balance when MEO was
evaluated, as indicated by their stable body mass
(Fig. 2A).
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Statistics
Data are reported as mean ± s.d. (N=sample size).
Measurements repeated on the same individuals (body mass, food intake, litter
mass and pup body mass) were analysed using two-way repeated measures ANOVA,
with group (shaved versus unshaved mice) and day of lactation as
factors. When the effect of group or the interaction `groupxday' was
significant, the Holm–Sidak multiple comparison procedure was applied to
determine differences between the groups within each day. For single
measurements, we used either t-test or ANCOVA with an appropriate
covariate that was selected by screening the data for potential relationships
between measured variables. These relationships were described using Pearson
correlation coefficients and P-values. Arcsine-square-root
transformations were performed prior to analysis for percentage data (apparent
digestive efficiency and pup dry mass content). For non-reproductive mice, we
calculated the percentage error in DEE relative to MEI as
100x(DEE-MEI)/MEI. All statistical analyses were conducted using
SigmaStat for Windows, version 3.5 (Systat Software Inc.). Statistical
significance was determined at P<0.05. All tests were two-tailed
unless stated otherwise.
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Results |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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The body mass of non-reproductive females allocated to shaved and unshaved groups averaged 32.2±1.6 g and 31.5±1.4 g, respectively, prior to shaving and 32.0±1.5 g and 31.9±2.5 g, respectively, at the end of the experiment (N=8 for all means, Fig. 2A). Body mass was not affected by shaving or the day of experiment (two-way repeated measures ANOVA; group, F1,126=0.01, P=0.92; day, F9,126=1.5, P=0.14; interaction groupxday, F9,126=1.8, P=0.08). The mean food intake of non-reproductive shaved and control mice was 5.2±0.6 g day–1 and 5.5±0.8 g day–1, respectively, prior to shaving and 6.4±0.7 g day–1 and 5.4±1.1 g day–1, respectively, at the end of the experiment (N=8 for all means, Fig. 2C). The response of mice to the treatment strongly depended on the day of the experiment (two-way repeated measures ANOVA; group, F1,84=3.0, P=0.10; day, F6,84=4.5, P<0.001; interaction groupxday, F6,84=3.4, P=0.005). For days 5–11, the difference in food intake between shaved and control mice was not significant (P>0.05). However, on days 12–13, 14–15 and 16–17, shaved mice had a significantly higher food intake than unshaved individuals; the differences between the groups averaged 0.8 g day–1 (P=0.007), 0.9 g day–1 (P=0.015) and 1.0 g day–1 (P=0.037), respectively. Over the whole period of manipulation (days 6–18), non-reproductive shaved mice ate on average 8.1 g more food than non-reproductive unshaved individuals.
Metabolizable energy intake and digestive efficiency
The faecal production of lactating mice was highly correlated with food
consumption (r=0.84, P<0.001, N=40; data not
shown). When food consumption was included as a covariate, faecal production
did not vary significantly between shaved and unshaved mice (ANCOVA, food,
F1,36=69.6, P<0.001; group,
F1,36=0.5, P=0.47; interaction foodxgroup,
F1,36=0.5, P=0.49). The gross energy content of
faeces did not vary significantly between shaved (17.02±0.26 kJ
g–1 dry mass) and control mice (17.00±0.18 kJ
g–1 dry mass) (t-test, t34=0.3,
P=0.80, N=20 for both groups). On days 12–14 of
lactation, the mean MEI of shaved and unshaved mice was 297.2±33.2 kJ
day–1 and 266.3±35.2 kJ day–1,
respectively (N=20 for both groups). The difference between the
groups (30.9 kJ day–1) was significant (t-test,
t37=2.9, P=0.007). Apparent digestive efficiency
did not vary significantly between lactating shaved (79.9±1.8%) and
unshaved (79.8±1.2%) mice (t-test,
t33=0.2, P=0.84, N=20 for both
groups).
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Daily energy expenditure
Results of the DLW measurements are presented in
Table 1. Between days 15 and 17
of lactation, DEE of shaved and unshaved mice averaged 130.7±13.5 kJ
day–1 and 121.8±14.8 kJ day–1,
respectively (N=20 for both groups). The difference between the
groups (8.9 kJ day–1), as anticipated with higher expenditure
in the shaved mice, was significant (one-tailed t-test,
t37=2.0, P=0.027).
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In non-reproductive mice, the DEE of shaved and unshaved individuals averaged 77.0±8.6 kJ day–1 and 63.7±7.0 kJ day–1, respectively (N=8 for both groups). The difference between the groups (13.3 kJ day–1) was significant (t-test, t13=3.4, P=0.005), and DEE and MEI were highly correlated (r=0.76, P=0.001, N=16) (Fig. 3). On average, estimated DEE (70.4± 10.2 kJ day–1) differed from MEI (70.9±9.3 kJ day–1) by –0.5±9.1% (N=16 for all means).
Milk energy output and reproductive performance
On average, shaved mice exported 22.0 kJ day–1 more energy
as milk at peak lactation (166.4±31.1 kJ day–1) than
control individuals (144.5±30.8 kJ day–1)
(t-test, t37=2.2, P=0.031, N=20
for both groups). Milk production was not correlated with maternal body mass
(Fig. 4A,
r=–0.02, P=0.91, N=40) or litter size
(Fig. 4B, r=0.14,
P=0.38, N=40). There was, however, a significant positive
correlation between MEO and litter growth rate
(Fig. 4C, r=0.80,
P<0.001, N=40).
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The effect of shaving on litter mass varied during the lactation period (two-way repeated measures ANOVA; group, F1,266=5.7, P=0.022; day, F7,266=473.7, P<0.001; interaction groupxday, F7,266=6.0, P<0.001) (Fig. 5A). For days 4–10 of lactation, there was no significant difference between the litter mass of shaved and unshaved mice (P>0.05). However on days 12, 14, 16 and 18, litters raised by shaved mice were heavier than litters of unshaved females by an average of 7.7 g (P=0.016), 9.6 g (P=0.003), 9.8 g (P=0.003) and 12.2 g (P<0.001), respectively (note that no measurements were made on odd numbered days). At the end of lactation, the litter masses of shaved and control mice averaged 91.8±13.0 and 79.6±17.9 g, respectively (N=20 for both groups). The analyses performed on mean pup body mass (litter mass divided by litter size) yielded similar results (two-way repeated measures ANOVA; group, F1,266=5.9, P=0.020; day, F7,266=433.8, P<0.001; interaction groupxday, F7,266=5.7, P<0.001) (Fig. 5B). For days 4–10, the body mass of the pups of shaved and control mothers did not vary significantly (P>0.05). On days 12, 14, 16 and 18, pups raised by shaved mothers were heavier than pups of unshaved females by an average of 0.7 g (P=0.016), 0.8 g (P=0.003), 0.9 g (P=0.002) and 1.1 g (P<0.001), respectively. At the end of lactation, mean pup mass was 8.1±1.2 g for shaved mice and 7.1±1.5 g for control individuals (N=20 for both groups).
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Greater pup body mass was associated with higher dry mass content (Fig. 6A, r=0.95, P<0.001, N=17) and higher gross energy content of pup dry mass (Fig. 6B, r=0.89, P<0.001, N=17). Consequently, there was also a positive correlation between pup body mass and gross energy content of pup live mass (Fig. 6C, r=0.94, P<0.001, N=17). Pups raised by shaved and unshaved mice did not differ in dry mass content (ANCOVA, pup mass, F1,13=76.0, P<0.001; group, F1,13=0.6, P=0.42; interaction pup massxgroup, F1,13=0.7, P=0.41), gross energy content of dry mass (ANCOVA, pup mass, F1,13=41.4, P<0.001; group, F1,13=1.7, P=0.21; interaction pup massxgroup, F1,13=1.7, P=0.22) or gross energy content of live mass (ANCOVA, pup mass, F1,13=80.5, P<0.001; group, F1,13=1.0, P=0.34; interaction pup massxgroup, F1,13=1.1, P=0.33).
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Discussion |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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; Król and
Speakman, 2003b; Król
et al., 2003). If this idea is correct, then any manipulation that
increases the heat dissipation capacity should result in elevated food intake
and milk production that would translate into enhanced reproductive
performance. Indeed, when lactating mice were exposed to cold conditions,
their responses were consistent with the heat dissipation limit hypothesis
(Hammond et al., 1994;
Johnson and Speakman, 2001).
However, manipulations that involve changes in ambient temperature do not
separate maternal effects from the extrinsic effects associated with the
thermoregulatory responses of the pups.
The approach we took in the current experiment was to increase the heat
dissipation capacity of females lactating at 21°C by shaving off their
dorsal fur and comparing their reproductive performance with unshaved
controls. Importantly, shaving did not induce compensatory changes in body
mass (Fig. 2A). Furthermore,
compensatory changes in body fat content were also unlikely since shaved and
unshaved mice had similar isotope-derived body water content
(Table 1). At peak lactation,
the shaved mice ate on average 12.0% (2.5 g day–1/21.1 g
day–1) more food than unshaved controls
(Fig. 2B). This result is
incompatible with the central limitation hypothesis but consistent with the
peripheral and heat dissipation limit hypotheses. When we took into account
individual variation in food wasting and individual estimates of apparent
digestive efficiency, shaved mice at peak lactation assimilated on average
30.9 kJ day–1 more energy than unshaved individuals. As
predicted exclusively by the heat dissipation limit hypothesis, this extra
energy was used to generate more milk. Milk production was calculated from the
difference between MEI and DEE
(Król and Speakman,
2003b) and we validated our application of the DLW technique in
non-reproductive mice, in which estimates of MEI and DEE closely matched each
other (Fig. 3). With nearly
identical litter sizes (11.4 pups for shaved and 11.3 pups for unshaved mice),
shaved mothers exported at peak lactation on average 15.2% (22.0 kJ
day–1/144.5 kJ day–1) more energy as milk
than control individuals (Fig.
4). If the increase in MEO induced by shaving (22.0 kJ
day–1) was fuelled by the extra energy that was assimilated
(30.9 kJ day–1), the efficiency of conversion of MEI to MEO
would be 71.1% (22.0 kJ day–1/30.9 kJ
day–1). This value is in good agreement with the net milk
production efficiencies reported for other mammals (e.g.
Romero et al., 1976;
Baldwin et al., 1980;
Freetly et al., 2006). The
litters weaned by shaved mice were on average 15.4% (12.2 g/79.6 g) heavier
than the litters produced by control mothers
(Fig. 5A). Similarly, the
individual pups raised by shaved mice were on average 1.1 g heavier than the
control pups (Fig. 5B). The
differences in litter and pup body masses were not associated with changes in
body composition of produced offspring, since pups raised by shaved and
unshaved mice had similar dry mass and energy contents
(Fig. 6). Taken together, when
we increased the capacity of lactating mice to dissipate body heat by shaving
off their dorsal fur, these mice were able to eat more food, generate more
milk and wean heavier offspring than mice with intact fur. Our results argue
against central, peripheral or extrinsic limits to SusEI at peak lactation and
provide strong support for the heat dissipation limit hypothesis.
Experimental alterations of maternal heat load by factors other than
ambient temperature have also been performed in Wistar rats
(Leon et al., 1978). The first
manipulation involved shaving off the ventral fur of females on the day of
parturition. These rats spent more time with their pups than unshaved
controls, supporting the idea that chronic hyperthermia of lactating females
may constrain maternal behaviour if contact with the litter further increases
their body temperature, forcing the termination of nest bouts to dissipate the
heat load (e.g. Croskerry et al.,
1978; Leon et al.,
1985; Scribner and
Wynne-Edwards, 1994). The second manipulation
(Leon et al., 1978) aimed to
decrease ability of female rats to dissipate body heat by removing their tails
– a major avenue for heat flow when heat production exceeds heat loss to
the environment. Accordingly, rats without tails spent less time with their
offspring than sham-operated intact mothers, whereas females with half their
tail removed had an intermediate amount of contact time. If we assume that the
time spent suckling is positively correlated with the daily rate of milk
transfer from mother to offspring, then the behavioural observations of rats
that were shaved or had their tails removed would be consistent with the heat
dissipation limit hypothesis.
Further support for the heat dissipation hypothesis comes from the
correlated responses of mice that have been divergently selected for many
generations for high and low heat loss
(Nielsen et al., 1997a;
Nielsen et al., 1997b). As
indicated by a weigh-suckle-weigh method, mice selected for high heat loss
produced on average 20.6% (1.70 g/1.41 g) more milk over a 2 h collection
period than the low heat loss mice
(McDonald and Nielsen, 2006).
Furthermore, the litters weaned by mice selected for high heat loss were on
average 10.1 g heavier than the litters produced by the low heat loss line.
These results are in good agreement with the effects of fur removal presented
in the current study, but lack direct measures of daily MEO to be a complete
proof of the heat dissipation limit hypothesis.
When we removed dorsal fur in non-reproductive mice, they responded to the
elevated heat dissipation capacity in a similar way to lactating mice –
by increasing their food intake (Fig.
2C). On average, shaving increased food intake of non-reproductive
mice by 17.6% (0.9 g day–1/5.3 g day–1). A
similar result was found for non-reproductive Siberian hamsters (Phodopus
sungorus) that were housed under a long day photoperiod at an ambient
temperature of 23°C – these animals increased their food intake by
approximately 20% following complete fur removal
(Kauffman et al., 2001b). In
the current study, non-reproductive mice with the reduced external insulation
assimilated on average 9.5 kJ day–1 more energy than control
mice. Moreover, shaving increased energy expenditure of non-reproductive mice
by 13.3 kJ day–1, which corresponds to 20.8% increase above
the level measured in unshaved controls (13.3 kJ day–1/63.7
kJ day–1) (Table
1). Surprisingly, as a consequence of complete fur removal, DEE of
non-reproductive California voles (Microtus californicus) living in
natural winter conditions increased by only 10%
(Kenagy and Pearson, 2000).
However, the degree to which fur conserved energy in that study was probably
underestimated because of behavioural adjustments of the voles, which in
winter tend to form social aggregations by sharing nests with four or five
other individuals (Pearson,
1960b; Hayes et al.,
1992). Much higher (35%) increases in oxygen consumption following
fur removal were reported in non-reproductive harvest mice
(Reithrodontomys megalotis) held in a short-day photoperiod at
18°C and 24°C, although these conclusions should be interpreted with
caution as only one animal was tested
(Pearson, 1960a). Coupled with
the observation that fur removal does not induce changes in locomotor activity
(Kauffman et al., 2003),
simultaneous occurrence of elevated assimilation (9.5 kJ
day–1) along with the elevated energy expenditure (13.3 kJ
day–1) in non-reproductive mice that were shaved (present
study), clearly suggests that these increases may reflect a compensatory
response to increased costs of thermoregulation. This conclusion is supported
by 38% increase in the mass and activity of brown adipose tissue (BAT)
reported in genetically hairless mice
(Heldmaier, 1974).
By contrast to the non-reproductive mice, the majority of the additional
energy that was assimilated by lactating mice as a result of fur removal (30.9
kJ day–1) was not metabolised, and therefore did not reflect
possible costs of thermoregulation. Instead, the extra energy assimilated was
exported as milk and therefore was not included as carbon dioxide production
in the direct evaluation of total DEE. As indicated by the DLW data
(Table 1), the increases in
energy expenditure of lactating mice induced by shaving averaged 8.9 kJ
day–1 (7.3% increase above the level measured in unshaved
controls). This increase probably represents metabolism associated with
converting dietary energy into milk (e.g.
Romero et al., 1976;
Baldwin et al., 1980;
Freetly et al., 2006) and/or
metabolism associated with postabsorptive processing (heat increment of
feeding), because these two processes are difficult to discriminate (e.g.
Parry, 1983;
Blaxter, 1989;
Wieser, 1994).
Shaving did not appear to induce thermogenic heat production in lactating
mice as it did in non-reproductive animals, suggesting that the amount of heat
generated as a by-product of lactogenesis was great enough to substitute for
the heat that would otherwise be needed to maintain normal body temperature.
Our estimates of lactogenic heat production are 48.0 kJ day–1
(166.4 kJ day–1x28.9%) for shaved mice and 41.7 kJ
day–1 (144.5 kJ day–1x28.9%) for
unshaved controls, whereas the estimated cost of thermoregulation caused by
fur removal in non-reproductive mice was 13.3 kJ day–1. If
the performance of lactating mice is limited by the total amount of heat they
can dissipate, then the increasing heat load due to milk production is
expected not only to substitute for thermogenic heat, but also to effectively
reduce non-reproductive components of the energy budget. Otherwise, completely
additive lactogenic and thermogenic costs would exacerbate heat dissipation
problems and increase a risk of developing maternal hyperthermia. The ability
to utilise lactogenic heat to substitute for heat generated solely for
thermogenesis is supported by downregulation of the thermogenic function of
BAT during lactation, as shown by tissue hypothrophy, a decrease in
mitochondrial biogenesis, reduced expression of the gene encoding uncoupling
protein 1 (UCP1), lower protein levels of UCP1 and a reduction in the
noradrenaline-induced non-shivering thermogenesis (e.g.
Agius and Williamson, 1980;
Trayhurn et al., 1982;
Villarroya et al., 1986;
Trayhurn, 1989;
Nizielski et al., 1993;
Johnson et al., 2001b;
Xiao et al., 2004;
Zhang and Wang, 2007).
In summary, we have shown that lactating mice with their dorsal fur shaved
off ate more food, produced more milk and weaned heavier offspring than
unshaved mice. Our verification of the heat dissipation limit hypothesis is
important because it indicates that ambient temperature changes will have
direct effects on reproductive performance, as well as indirect effects
via impacts on food supply. This means that the consequences of
climate change may be felt more immediately and directly by endotherms than
has hitherto been suspected. Understanding the links between constraints on
heat dissipation, life-history traits and biogeography may improve our ability
to model the ecosystem impacts of global climate change. More generally, we
see many situations where heat dissipation limits could be a previously
unrecognised factor constraining the evolution of endothermic animals. For
example, the ability to dissipate body heat may provide valuable insights into
latitudinal and altitudinal trends in clutch and litter sizes (e.g.
Bohning-Gaese et al., 2000;
Cooper et al., 2005;
Virgos et al., 2006).
Furthermore, thermal constraints during flapping flight could explain why
birds tend to migrate at high altitude and/or during cloudless nights (e.g.
Klaassen, 1996;
Léger and Larochelle,
2006).
List of abbreviations
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Agius, L. and Williamson, D. H. (1980). Lipogenesis in interscapular brown adipose tissue of virgin, pregnant and lactating rats. The effects of intragastric feeding. Biochem. J. 190,477 -480.[Medline]
Anderson, K. J. and Jetz, W. (2005). The broad-scale ecology of energy expenditure of endotherms. Ecol. Lett. 8,310 -318.[Medline]
Baldwin, R. L., Smith, N. E., Taylor, J. and Sharp, M.
(1980). Manipulating metabolic parameters to improve growth rate
and milk secretion. J. Anim. Sci.
51,1416
-1428.
Barnett, S. A. (1959). The skin and hair of mice living at low environmental temperature. J. Exp. Physiol. 44,35 -42.
Berteaux, D., Thomas, D. W., Bergeron, J.-M. and Lapierre, H. (1996). Repeatability of daily field metabolic rate in female Meadow Voles (Microtus pennsylvanicus). Funct. Ecol. 10,751 -759.[CrossRef]
Biggerstaff, S. and Mann, M. (1992). Consummatory behaviors and weight regulation in pregnant, lactating, and pregnant-lactating mice. Physiol. Behav. 52,485 -491.[CrossRef][Medline]
Blaxter, K. (1989). Energy Metabolism in Animals and Man. Cambridge: Cambridge University Press.
Bohning-Gaese, K., Halbe, B., Lemoine, N. and Oberrath, R. (2000). Factors influencing the clutch size, number of broods and annual fecundity of North American and European land birds. Evol. Ecol. Res. 2,823 -839.
Conley, K. E. and Porter, W. P. (1980). Heat loss from deer mice (Peromyscus): evaluation of seasonal limits to thermoregulation. J. Exp. Biol. 126,249 -269.
Cooper, C. B., Hochachka, W. M., Butcher, G. and Dhondt, A. A. (2005). Seasonal and latitudinal trends in clutch size: thermal constraints during laying and incubation. Ecology 86,2018 -2031.[CrossRef]
Coward, W. A. and Prentice, A. M. (1985).
Isotope method for the measurement of carbon dioxide production rate in man.
Am. J. Clin. Nutr. 41,659
-661.
Croskerry, P. G., Smith, G. K. and Leon, M. (1978). Thermoregulation and the maternal behaviour of the rat. Nature 273,299 -300.[CrossRef][Medline]
Drent, R. H. and Daan, S. (1980). The prudent parent: energetic adjustments in avian breeding. Ardea 68,225 -252.
Drod, A. (1975). Metabolic cages
for small mammals. In Methods for Ecological Bioenergetics,
International Biological Programme Handbook No. 24 (ed. W.
Grodzinski, R. Z. Klekowski and A. Duncan), pp.346
-351. Oxford: Blackwell Scientific
Publications.
Freetly, H. C., Nienaber, J. A. and Brown-Brandl, T.
(2006). Partitioning of energy during lactation of primiparous
beef cows. J. Anim. Sci.
84,2157
-2162.
Hammel, H. T. (1955). Thermal properties of
fur. Am. J. Physiol.
182,369
-376.
Hammond, K. A. and Diamond, J. (1992). An experimental test for a ceiling on sustained metabolic rate in lactating mice. Physiol. Zool. 65,952 -977.
Hammond, K. A. and Diamond, J. (1994). Limits to dietary nutrient intake and intestinal nutrient uptake in lactating mice. Physiol. Zool. 67,282 -303.
Hammond, K. A. and Diamond, J. (1997). Maximum sustained energy budgets in humans and animals. Nature 386,457 -462.[CrossRef][Medline]
Hammond, K. A. and Kristan, D. M. (2000). Responses to lactation and cold exposure by deer mice (Peromyscus maniculatus). Physiol. Biochem. Zool. 73,547 -556.[CrossRef][Medline]
Hammond, K. A., Konarzewski, M., Torres, R. M. and Diamond, J. (1994). Metabolic ceilings under a combination of peak energy demands. Physiol. Zool. 67,1479 -1506.
Hammond, K. A., Lloyd, K. C. K. and Diamond, J. (1996). Is mammary output capacity limiting to lactational performance in mice? J. Exp. Biol. 199,337 -349.[Abstract]
Hart, J. S. (1956). Seasonal changes in insulation of the fur. Can. J. Zool. 34, 53-57.
Hayes, J. P., Speakman, J. R. and Racey, P. A. (1992). The contributions of local heating and reducing exposed surface area to the energetic benefits of huddling by short-tailed field voles (Microtus agrestis). Physiol. Zool. 65,742 -762.
Heldmaier, G. (1974). Temperature adaptation and brown adipose tissue in hairless and albino mice. J. Comp. Physiol. 92,281 -292.[CrossRef]
Humphries, M. M., Thomas, D. W. and Speakman, J. R. (2002). Climate-mediated energetic constraints on the distribution of hibernating mammals. Nature 418,313 -316.[CrossRef][Medline]
Jacobsen, N. K. (1980). Differences in thermal properties of white-tailed deer pelage between seasons and body regions. J. Therm. Biol. 5,151 -158.[CrossRef]
Johnson, M. S. and Speakman, J. R. (2001).
Limits to sustained energy intake. V. Effect of cold-exposure during lactation
in Mus musculus. J. Exp. Biol.
204,1967
-1977.
Johnson, M. S., Thomson, S. C. and Speakman, J. R.
(2001a). Limits to sustained energy intake. I. Lactation in the
laboratory mouse Mus musculus. J. Exp. Biol.
204,1925
-1935.
Johnson, M. S., Thomson, S. C. and Speakman, J. R.
(2001b). Limits to sustained energy intake. II.
Inter-relationships between resting metabolic rate, life-history traits and
morphology in Mus musculus. J. Exp. Biol.
204,1937
-1946.
Johnson, M. S., Thomson, S. C. and Speakman, J. R.
(2001c). Limits to sustained energy intake. III. Effects of
concurrent pregnancy and lactation in Mus musculus. J. Exp.
Biol. 204,1947
-1956.
Kauffman, A. S., Cabrera, A. and Zucker, I. (2001a). Torpor characteristics and energy requirements of furless Siberian hamsters. Physiol. Biochem. Zool. 74,876 -884.[CrossRef][Medline]
Kauffman, A. S., Cabrera, A. and Zucker, I. (2001b). Energy intake and fur in summer- and winter-acclimated Siberian hamsters (Phodopus sungorus). Am. J. Physiol. 281,R519 -R527.
Kauffman, A. S., Paul, M. J., Butler, M. P. and Zucker, I. (2003). Huddling, locomotor, and nest-building behaviors of furred and furless Siberian hamsters. Physiol. Behav. 79,247 -256.[CrossRef][Medline]
Kauffman, A. S., Paul, M. J. and Zucker, I. (2004). Increased heat loss affects hibernation in golden-mantled ground squirrels. Am. J. Physiol. 287,R167 -R173.
Kenagy, G. J. and Pearson, O. P. (2000). Life with fur and without: experimental field energetics and survival of naked meadow voles. Oecologia 122,220 -224.[CrossRef]
Kenagy, G. J., Stevenson, R. D. and Masman, D. (1989). Energy requirements for lactation and postnatal growth in captive golden-mantled ground squirrels. Physiol. Zool. 62,470 -487.
Kirkwood, J. K. (1983). A limit to metabolisable energy intake in mammals and birds. Comp. Biochem. Physiol. 75,1 -3.[CrossRef][Medline]
Klaassen, M. (1996). Metabolic constraints on long-distance migration in birds. J. Exp. Biol. 199, 57-64.[Abstract]
Knight, F. M. (1987). The development of pelage insulation and its relationship to homeothermic ability in an altricial rodent, Peromyscus leucopus. Physiol. Zool. 60,181 -190.
Koiter, T. R., Moes, H., Valkhof, N. and Wijkstra, S.
(1999). Interaction of late pregnancy and lactation in rats.
J. Reprod. Fertil. 115,341
-347.
Koteja, P. (1996). Limits to the energy budget in a rodent, Peromyscus maniculatus: the central limitation hypothesis. Physiol. Zool. 69,981 -993.
Król, E. and Speakman, J. R. (1999). Isotope dilution spaces of mice injected simultaneously with deuterium, tritium and oxygen-18. J. Exp. Biol. 202,2839 -2849.[Abstract]
Król, E. and Speakman, J. R. (2003a).
Limits to sustained energy intake. VI. Energetics of lactation in laboratory
mice at thermoneutrality. J. Exp. Biol.
206,4255
-4266.
Król, E. and Speakman, J. R. (2003b).
Limits to sustained energy intake. VII. Milk energy output in laboratory mice
at thermoneutrality. J. Exp. Biol.
206,4267
-4281.
Król, E., Johnson, M. S. and Speakman, J. R.
(2003). Limits to sustained energy intake. VIII. Resting
metabolic rate and organ morphology of laboratory mice lactating at
thermoneutrality. J. Exp. Biol.
206,4283
-4291.
Künkele, J. (2000). Effects of litter size on the energetics of reproduction in a highly precocial rodent, the guinea pig. J. Mammal. 81,691 -700.[CrossRef]
Laurien-Kehnen, C. and Trillmich, F. (2003). Lactation performance of guinea pigs (Cavia porcellus) does not respond to experimental manipulation of pup demands. Behav. Ecol. Sociobiol. 53,145 -152.
Leon, M., Croskerry, P. G. and Smith, G. K. (1978). Thermal control of mother-young contact in rats. Physiol. Behav. 21,793 -811.[CrossRef]
Leon, M., Adels, L. and Coopersmith, R. (1985). Thermal limitation of mother-young contact in Norway rats. Dev. Psychobiol. 18,85 -105.[CrossRef][Medline]
Léger, J. and Larochelle, J. (2006). On
the importance of radiative heat exchange during nocturnal flight in birds.
J. Exp. Biol. 209,103
-114.
Lifson, N. and McClintock, R. (1966). Theory of use of the turnover rates of body water for measuring energy and material balance. J. Theor. Biol. 12, 46-74.[CrossRef][Medline]
Lifson, N., Gordon, G. B. and McClintock, R.
(1955). Measurements of total carbon dioxide production by means
of D2O18. J. Appl. Physiol.
7, 704-710.
McDonald, J. M. and Nielsen, M. K. (2006).
Correlated responses in maternal performance following divergent selection for
heat loss in mice. J. Anim. Sci.
84,300
-304.
Meijer, H. A. J., Neubert, R. E. M. and Visser, G. H. (2000). Cross contamination in dual inlet isotope ratio mass spectrometers. Int. J. Mass Spectrom. 198, 45-61.[CrossRef]
Morrison, P. R. and Tietz, W. J. (1957). Cooling and conductivity in three small Alaskan mammals. J. Mammal. 38,78 -86.[CrossRef]
Mount, L. E. (1971). Metabolic rate and thermal
insulation in albino and hairless mice. J. Physiol.
Lond. 217,315
-326.
Nagy, K. A. (1983). The Doubly Labeled Water (3HH18O) Method: A Guide to its Use (UCLA Publication no. 12-1417). Los Angeles, CA: University of California.
Nielsen, M. K., Jones, L. D., Freking, B. A. and DeShazer, J.
A. (1997a). Divergent selection for heat loss in mice: I.
Selection applied and direct response through fifteen generations.
J. Anim. Sci. 75,1461
-1468.
Nielsen, M. K., Freking, B. A., Jones, L. D., Nelson, S. M.,
Vorderstrasse, T. L. and Hussey, B. A. (1997b). Divergent
selection for heat loss in mice: II. Correlated responses in feed intake, body
mass, body composition, and number born through fifteen generations.
J. Anim. Sci. 75,1469
-1476.
Nizielski, S. E., Billington, C. J. and Levine, A. S. (1993). BAT thermogenic activity and capacity are reduced during lactation in ground squirrels. Am. J. Physiol. 264,R16 -R21.[Medline]
Parry, G. D. (1983). The influence of the cost of growth on ectotherm metabolism. J. Theor. Biol. 101,453 -477.[CrossRef][Medline]
Pearson, O. P. (1960a). The oxygen consumption and bioenergetics of harvest mice. Physiol. Zool. 33,152 -160.
Pearson, O. P. (1960b). Habits of Microtus californicus revealed by automatic photographic recorders. Ecol. Monogr. 30,231 -249.[CrossRef]
Perrigo, G. (1987). Breeding and feeding strategies in deer mice and house mice when females are challenged to work for their food. Anim. Behav. 35,1298 -1316.[CrossRef]
Peterson, C. C., Nagy, K. A. and Diamond, J.
(1990). Sustained metabolic scope. Proc. Natl. Acad.
Sci. USA 87,2324
-2328.
Reynolds, P. S. (1993). Effects of body size and fur on heat loss of collared lemmings, Dicrostonyx groenlandicus.J. Mammal. 74,291 -303.[CrossRef]
Rogowitz, G. L. (1998). Limits to milk flow and energy allocation during lactation of the hispid cotton rat (Sigmodon hispidus). Physiol. Zool. 71,312 -320.[Medline]
Rogowitz, G. L. and McClure, P. A. (1995). Energy export and offspring growth during lactation in cotton rats (Sigmodon hispidus). Funct. Ecol. 9, 143-150.[CrossRef]
Romero, J. J., Canas, R., Baldwin, R. L. and Koong, L. J.
(1976). Lactational efficiency complex of rats: provisional model
for interpretation of energy balance data. J. Dairy
Sci. 59,57
-67.
Scantlebury, M., Hynds, W., Booles, D. and Speakman, J. R. (2000). Isotope recycling in lactating dogs (Canis familiaris). Am. J. Physiol. 278,R669 -R676.
Scholander, P. F., Walters, A., Hock, R. and Irving, L.
(1950). Body insulation of some arctic and tropical mammals and
birds. Biol. Bull. 99,225
-236.
Scribner, S. J. and Wynne-Edwards, K. E. (1994). Thermal constraints on maternal behavior during reproduction in dwarf hamsters (Phodopus). Physiol. Behav. 55,897 -903.[CrossRef][Medline]
Snapp, B. D. and Heller, H. C. (1981). Suppression of metabolism during hibernation in ground squirrels. Physiol. Zool. 54,297 -307.
Speakman, J. R. (1997). Doubly Labelled Water: Theory and Practice. London: Chapman & Hall.
Speakman, J. R. (1998). The history and theory of the doubly labeled water technique. Am. J. Clin. Nutr. 68,932S -938S.[Abstract]
Speakman, J. R. (2000). The cost of living: field metabolic rates of small mammals. Adv. Ecol. Res. 30,177 -297.
Speakman, J. R. and Król, E. (2005a). Limits to sustained energy intake IX: a review of hypotheses. J. Comp. Physiol. B 175,375 -394.[CrossRef][Medline]
Speakman, J. R. and Król, E. (2005b). Comparison of different approaches for the calculation of energy expenditure using doubly labeled water in a small mammal. Physiol. Biochem. Zool. 78,650 -667.[CrossRef][Medline]
Speakman, J. R. and McQueenie, J. (1996). Limits to sustained metabolic rate: the link between food intake, basal metabolic rate, and morphology in reproducing mice, Mus musculus.Physiol. Zool. 69,746 -769.
Speakman, J. R. and Racey, P. A. (1987). The equilibrium concentration of O-18 in body-water – implications for the accuracy of the doubly-labeled water technique and a potential new method of measuring RQ in free-living animals. J. Theor. Biol. 127, 79-95.[CrossRef]
Speakman, J. R. and Racey, P. A. (1988a). Validation of the doubly labeled water technique in small insectivorous bats by comparison with indirect calorimetry. Physiol. Zool. 61,514 -526.
Speakman, J. R. and Racey, P. A. (1988b). Consequences of non steady-state CO2 production for accuracy of the doubly labeled water technique – the importance of recapture interval. Comp. Biochem. Physiol. 90A,337 -340.
Speakman, J. R., Nagy, K. A., Masman, D., Mook, W. G., Poppitt, S. D., Strathearn, G. E. and Racey, P. A. (1990). Interlaboratory comparison of different analytical techniques for the determination of O-18 abundance. Anal. Chem. 62,703 -708.
Speakman, J. R., Racey, P. A., Haim, A., Webb, P. I., Ellison, G. T. H. and Skinner, J. D. (1994). Interindividual and intraindividual variation in daily energy expenditure of the pouched mouse (Saccostomus-campestris). Funct. Ecol. 8, 336-342.[CrossRef]
Thomas, D. W., Blondel, J., Perret, P., Lambrechts, M. M. and
Speakman, J. R. (2001). Energetic and fitness costs of
mismatching resource supply and demand in seasonally breeding birds.
Science 291,2598
-2600.
Trayhurn, P. (1989). Thermogenesis and the energetics of pregnancy and lactation. Can. J. Physiol. Pharmacol. 67,370 -375.[Medline]
Trayhurn, P., Douglas, J. B. and McGuckin, M. M. (1982). Brown adipose tissue thermogenesis is `suppressed' during lactation in mice. Nature 298, 59-60.[CrossRef][Medline]
Villarroya, F., Felipe, A. and Mampel, T. (1986). Sequential changes in brown adipose tissue composition, cytochrome oxidase activity and GDP binding throughout pregnancy and lactation in the rat. Biochim. Biophys. Acta 882,187 -191.[Medline]
Virgos, E., Cabezas-Diaz, S. and Blanco-Aguiar, J. A. (2006). Evolution of life history traits in Leporidae: a test of nest predation and seasonality hypotheses. Biol. J. Linn. Soc. Lond. 88,603 -610.[CrossRef]
Visser, G. H. and Schekkerman, H. (1999). Validation of the doubly labeled water method in growing precocial birds: the importance of assumption concerning evaporative water loss. Physiol. Biochem. Zool. 72,740 -749.[CrossRef][Medline]
Visser, G. H., Dekinga, A., Achterkamp, B. and Piersma, T. (2000a). Ingested water equilibrates isotopically with the body water pool of a shorebird with unrivaled water fluxes. Am. J. Physiol. 279,R1795 -R1804.
Visser, G. H., Boon, P. E. and Meijer, H. A. J. (2000b). Validation of the doubly labeled water in Japanese Quail Coturnix c. japonica chicks: is there an effect of growth rate? J. Comp. Physiol. B 170,365 -372.[CrossRef][Medline]
Weiner, J. (1987). Limits to energy budget and tactics in energy investments during reproduction in the Djungarian hamster (Phodopus sungorus sungorus Pallas 1770). Symp. Zool. Soc. Lond. 57,167 -187.
Weiner, J. (1992). Physiological limits to sustainable energy budgets in birds and mammals: ecological implications. Trends Ecol. Evol. 7,384 -388.[CrossRef]
Weir, J. B. de V. (1949). New methods for
calculating metabolic rate with special reference to protein metabolism.
J. Physiol. 109,1
-9.
Wieser, W. (1994). Cost of growth in cells and organisms: general rules and comparative aspects. Biol. Rev. Camb. Philos. Soc. 69,1 -33.[Medline]
Xiao, X. Q., Grove, K. L., Grayson, B. E. and Smith, M. S.
(2004). Inhibition of uncoupling protein expression during
lactation: role of leptin. Endocrinology
145,830
-838.
Zhang, X. Y. and Wang, D. H. (2007).
Thermogenesis, food intake and serum leptin in cold-exposed lactating Brandt's
voles Lasiopodomys brandtii. J. Exp. Biol.
210,512
-521.
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  C. U Guillemette, Q. E Fletcher, S. Boutin, R. M Hodges, A. G McAdam, and M. M Humphries Lactating red squirrels experiencing high heat load occupy less insulated nests Biol Lett, April 23, 2009; 5(2): 166 - 168. [Abstract] [Full Text] [PDF]
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