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First published online August 22, 2008
Journal of Experimental Biology 211, 2759-2766 (2008)
Published by The Company of Biologists 2008
doi: 10.1242/jeb.019463
Allometry of evaporative water loss in marsupials: implications of the effect of ambient relative humidity on the physiology of brushtail possums (Trichosurus vulpecula)
1 Centre for Ecosystem Diversity and Dynamics in the Department of Environmental
Biology, Curtin University of Technology, PO Box U1987, Perth, Western
Australia, 6845
2 Zoology, School of Animal Biology, University of Western Australia, Crawley,
Western Australia, 6009
* Author for correspondence (e-mail: c.cooper{at}curtin.edu.au)
Accepted 21 May 2008
 |
Summary |
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 TOP Summary INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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WVP) in the chamber
during measurement significantly increased, rather than decreased, the
variability of the allometric relationship for EWL. For the brushtail possum,
both ambient temperature (Ta) and RH significantly
affected EWL. At Ta=25°C, EWL was independent of RH at
63% RH, but decreased linearly at higher RH values. At
Ta=30°C, EWL was significantly related to RH from 26%
to 92% RH. There was a significant effect of Ta on
Tb and dry thermal conductance (Cdry;
higher at 30°C), but no effect of RH. For MR and wet thermal conductance
(Cwet) there was a significant effect of
Ta (MR higher and Cwet lower at
25°C), and RH at Ta=30°C (MR higher and
Cwet lower at the lowest RH) but not at 25°C. Our
results indicate that brushtail possums do not necessarily show the linear
relationship between ambient RH and EWL expected for an endotherm, possibly
because of behavioural modification of their immediate microclimate. This may
account for the failure of WVP deficit correction to improve the allometric
EWL relationship for marsupials. Chamber RH is an important environmental
factor to be considered when measuring standard physiological variables such
as MR and Cwet.
Key words: evaporative water loss, respirometry, relative humidity, marsupial, brushtail possum, methodology, water vapour pressure deficit
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INTRODUCTION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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; Hulbert and
Dawson, 1974; Dawson,
1969; Dawson et al.,
1969; Dawson,
1973; MacMillen and Hinds,
1983; Bell et al.,
1983; Hinds and MacMillen,
1986; Schmidt-Nielsen et al.,
1970).
Total EWL (EWL hereafter refers to total EWL unless otherwise stated) is in
turn affected by factors that influence cutaneous and/or respiratory water
loss, including activity, and a number of environmental parameters including
water vapour pressure (WVP) and Ta. The principal
environmental factor determining EWL is the water vapour pressure deficit
(WVP) between an animal and ambient air
(Christian, 1978). High ambient
relative humidity, RH (small WVP), retards EWL, whereas low RH (large
WVP) enhances EWL (Lasiewski et
al., 1966; Proctor and
Studier, 1970). Thus animals should have higher EWL under drier
ambient conditions. This relationship between EWL and ambient WVP is generally
inverse and linear for small animals including birds
(Lasiewski et al., 1966),
rodents (Baudinette, 1972;
Christian, 1978;
Edwards and Haines, 1978) and
bats (Proctor and Studier,
1970; Webb et al.,
1995). There is also in theory an influence of ambient RH on
metabolic rate, as reduced evaporative heat loss due to high RH and low WVP
will decrease wet thermal conductance (Cwet) and thus
might reduce metabolic heat production and/or increase body temperature
(Tb) and dry thermal conductance
(Cdry). This is of particular importance at
Ta values above the upper critical temperature
(Tuc) of the thermoneutral zone where an endotherm must
dissipate much or even all of its metabolic heat production by evaporative
heat loss. A high ambient RH may therefore reduce or prevent heat loss, having
dire thermoregulatory consequences. There is, however, little evidence of a
relationship between RH and metabolic rate (MR) and/or Tb
below Tuc (Proctor and
Studier, 1970; Baudinette,
1972; Ewing and Studier,
1973; Kay,
1975).
The standard technique for studying metabolism and EWL of endotherms in the
laboratory is flow-through respirometry, where dry air flows through a
metabolic chamber containing an animal, and the gas composition (water vapour,
oxygen and/or carbon dioxide) of the excurrent air is measured (e.g.
Withers, 2001). However, the
flow rate together with the Ta and EWL of the animal will
determine the RH within the metabolic chamber, as the water evaporated from
the animal will mix with incoming dry air and humidify the chamber air
(Lasiewski et al., 1966). This
humidification will potentially reduce EWL and evaporative heat loss (and thus
affect Tb and/or MR). Low flow rates are often used in
flow-through respirometry to maximise the O2/CO2
differential between incurrent and excurrent air to 
1% (e.g.
Willis and Cooper, in press)
but may not be optimal in terms of chamber washout and the maintenance of a
low ambient RH within the chamber, especially at high Ta
(Lasiewski et al., 1966).
Chamber RH is often not considered in flow-through respirometry experiments
(despite its potential to influence MR and EWL, particularly at high
Ta). The effect of ambient RH on EWL makes it difficult to
(1) compare EWL and possibly MR between different Ta
values within a single study, since ambient RH changes with
Ta, and (2) compare `standard' values, e.g. basal
metabolic rate (BMR) and standard EWL, between studies (e.g.
Cooper and Withers, 2002;
Cooper et al., 2005) because
ambient RH differs with varying flow rate and Ta. Some
authors have attempted to correct the EWL of birds and mammals for the effect
of ambient RH (e.g. Salt,
1964; Lasiewski et al.,
1966; Coulombe,
1970; Larcombe et al.,
2003; Larcombe,
2004; Larcombe and Withers,
2006; Larcombe et al.,
2006) but to do this effectively the relationship between EWL and
RH must be well understood.
For marsupials, there is a strong allometric effect on EWL
(Withers et al., 2006);
log10EWL=0.96+0.68log10M with
R2=0.95, i.e. body mass (M, in g) explains 95% of
the variability in EWL (mg h–1). Presumably some of the
remaining variability is explained by differences in measurement technique
(gravimetric vs hygrometric) and respirometry conditions (flow rate
and chamber ambient RH). To better understand the allometry of EWL in
marsupials and sources of variability in the data, we examined here the
effects of correcting EWL for ambient WVP. To interpret the
implications of RH correction on the EWL data for marsupials, we measured the
effect of ambient RH on the EWL (and Tb, MR,
Cwet and Cdry) of a medium-sized
marsupial, the brushtail possum (Trichosurus vulpecula Kerr
1792).
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MATERIALS AND METHODS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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;
Cooper and Withers, 2002;
Cooper et al., 2005;
Cooper et al., in press;
Dawson, 1969;
Dawson, 1973;
Dawson and Bennett, 1978;
Dawson and Degrabrielle, 1973;
Dawson et al., 1969;
Hinds and MacMillen, 1986;
Hudson and Dawson, 1975;
Larcombe, 2004;
Withers, 1992a;
Withers et al., 1990). These
data were analysed to determine the allometry of EWL, and to examine the
effect of correction for chamber RH on the allometric relationship for
marsupials. The chamber WVP (in Torr; 1 Torr
133 Pa) for each species was
obtained from the data source or was calculated from flow rate,
Ta and EWL. EWL (mg h–1) was corrected
for the WVP [calculated as
WVP=WVPsat–WVPchamber, where
WVPsat is WVP saturation at Ta
(Coulombe, 1970;
Edwards and Haines, 1978)] to
units of mg H2O h–1 Torr–1.
Allometric relationships for log-transformed uncorrected EWL data (mg
H2O h–1), and EWL corrected for WVP (mg
H2O h–1 Torr–1), were determined
by least squares linear regression, and the variance ratio test
(Zar, 1999) was used to test
for differences in remaining variance for uncorrected and corrected
allometries, using the regression residual mean square errors.
Brushtail possums
Six adult male brushtail possums were captured at Mount Caroline Nature
Reserve (31°47'S, 117°38'E), near Quairading, Western
Australia. The possums were housed at Curtin University in large outdoor
enclosures, where they experienced natural weather and photoperiod for Perth,
Western Australia. Possums were fed rabbit and guinea pig pellets, fruit,
vegetables, cheese and Eucalyptus leaves, with ad libitum
water. Experiments were conducted from November to January 2007/2008.
Metabolic rate (oxygen consumption,
O2; ml
O2 g–1 h–1), carbon dioxide
production (CO2;
ml CO2 g–1 h–1) and EWL (mg
H2O g–1 h–1) were determined by
flow-through respirometry. The respirometry system consisted of a mass flow
controller, either an Aalborg GFC171 (Orangeburg, NY, USA) or an Omega
FMA-A2412 (Stamford, CT, USA) that regulated the flow of compressed air
through a Perspex metabolic chamber (8000 cm3) at a rate of
2300–3000 ml min–1. The metabolic chamber was located
inside a controlled temperature cabinet or room. Excurrent air passed over a
thin film capacitance RH/Ta sensor (Vaisala HMP 45A,
Helsinki, Finland), then through a column of Drierite (W. A. Hammond Drierite
Co. Ltd, Xenia, OH, USA), an oxygen analyser (Servomex OA184 or 572,
Crowborough, East Sussex, UK), and finally a carbon dioxide analyser (Qubit
S153, Kingston, Ontario, Canada; or Sable Systems CA-2A, Las Vegas, NV, USA).
The gas analysers were interfaced to a PC via a RS232 serial port
using Brymen (Taipei, Taiwan) multimeters (BM202 for RH, CO2 and
Ta; TBM859CF for O2) or a Pico Technology ADC11
analog-to-digital converter (St Neots, Cambs, UK). Custom-written data
acquisition software (using Visual Basic v6, Microsoft, Redmond, WA, USA) was
used to record O2, CO2, RH and Ta
every 10–20 s throughout the experimental period. The O2
analysers were 2-point calibrated using compressed N2 (0%
O2) and dry ambient air (20.95% O2); the CO2
analysers were calibrated with compressed N2 (0% CO2)
and a certified gas mix (0.53% CO2; BOC, Perth, Western
Australia).
The humidity of the incurrent air was controlled by bubbling it through an
aerator in water at a specified temperature
(Ewing and Studier, 1973;
Christian, 1978). The
temperature of the water was regulated by a Techne Tempette TE-8A heater
(Duxford, Cambs, UK) and a FST LC-20 liquid cooler (Stone Ridge, NY, USA) and
was varied to provide inlet RH values of 5–11%, 25%, 50%, 75% and 85%.
Factory calibration of the RH probes was confirmed by comparing actual and
theoretical RH during initial and final baseline measurements. The actual RH
within the chamber with a possum was higher than the incurrent RH due to
mixing of incurrent air and chamber air containing water evaporated from the
animal (Lasiewski et al.,
1966). Excurrent RH values were considered to be indicative of
chamber RH. All six possums were measured at five RH values for thermoneutral
temperatures of 25°C and 30°C
(Dawson, 1969) (C.E.C. and
P.C.W., unpublished data).
Baseline values for background O2, CO2 and RH were
measured for at least 30 min before and after experiments. All measurements
were conducted for post-absorptive (fasted for at least 24 h) possums during
their inactive phase (daytime) for a period of 6–9 h, during which
O2,
CO2 and EWL
invariably reached a constant and minimal value for at least 20 min. A possum
was removed from its enclosure in the morning, weighed to the nearest
±1g, and placed in the metabolic chamber. The possum was observed in
the chamber during experiments without disturbance with a Swann Max-IP-cam
camera under infrared light (Richmond, Victoria, Australia). At the conclusion
of the experiment, the possum was removed from the chamber and its
Tb measured with a plastic-tipped thermocouple inserted 3
cm into the cloaca and/or an Omron MC-510 (Singapore) infrared thermometer
via the ear canal (the two methods recorded identical
Tb values when used simultaneously). The possum was then
re-weighed and returned to its enclosure.
O2,
CO2 and EWL were
calculated (Withers, 2001)
using a custom-written data analysis program (Visual Basic v6). For each
individual possum, the minimum 20 min mean
O2,
CO2 and EWL were
determined for the experiment.
CO2 data are not
presented here, as they mirror
O2, but
CO2 was measured to enable accurate calculation of
O2 without use
of a CO2 scrubber (Withers,
2001), and the conversion of
O2 to J using
the measured respiratory exchange ratio (RER) for calculation of thermal
conductance. Cwet
(Jg–1h–1°C–1) was
calculated separately for each experiment as
MHP/(Tb–Ta), with metabolic heat
production (MHP) calculated from MR using the oxycalorific coefficient [J ml
O2–1 (Withers,
1992b)]. Cdry (J g–1
h–1 °C–1) was calculated as
(MHP–EHL)/(Tb–Ta), with
evaporative heat loss (EHL; J h–1) determined from EWL from
the latent heat of vaporisation [2.4 J mg H2O–1
(McNab, 2002)].
Body mass of a possum for a particular measurement was calculated as the
mean of masses before and after an experiment. The effects of ambient RH and
Ta on
O2, EWL,
Tb, Cwet and Cdry
were determined by linear least squares regression, and ANOVA with
Student–Newman–Keuls (SNK) post hoc tests. Values are
presented as means ± s.e.m. (N=6) unless otherwise stated. All
statistical analyses were accomplished using statistiXL V1.7
(Nedlands, Western Australia).
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RESULTS |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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WVP (mg H2O h–1
Torr–1),
logEWL=0.745(±0.045)logM–0.605(±0.119), was also
highly significant (F1,25=276, P<0.001,
R2=0.917, residual mean square=0.053;
Fig. 1). An F-test of
the residual mean squares for the regressions indicates that the regression
with EWL data corrected for WVP was significantly more variable than
that for uncorrected data (F25,25=2.32,
P=0.020).
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Brushtail possums
The mean body mass of the brushtail possums over all experiments
(N=6 individuals, measured 10 times each, so total N=60),
was 1847±22.3 g (range 1450–2186 g). Observations of possums in
the metabolic chamber indicated that they settled after about 30 min, and then
remained resting for the duration of the experiment, curled up in a sphere at
25°C and stretched out on their backs at 30°C
(Fig. 2). There was no evidence
of the possums spreading saliva on their fur or paws to enhance evaporative
cooling in any of the experiments. Excurrent chamber RH during experiments was
26±1.3%, 43±0.9%, 63±0.9%, 82±1.0% and
93±1.2% at Ta=25°C, and 26±1.0%,
44±3.0%, 60±1.2%, 79±0.5% and 93±0.8% at
30°C.
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A two-way (Ta and RH) multivariate
(Tb,
O2, EWL,
Cwet and Cdry) ANOVA revealed a highly
significant effect of both Ta
(F6,45=108.5, P<0.001) and RH
(F24,158=6.6, P<0.001) on these physiological
variables for brushtail possums. In the light of a significant
Ta–RH interaction (F24,158=2.0,
P=0.006), we further examined the physiological variables separately
with univariate one- and two-way ANOVA and linear regression to determine
specifically the effects of Ta and RH on EWL,
Tb, MR and C.
There was a highly significant effect of both Ta
(F1,50=100.4, P<0.001) and RH
(F4,50=59.2, P<0.001;
Fig. 3A) on EWL by two-way
ANOVA, with EWL significantly lower at 25°C than at 30°C, and lower at
high RH. There was also a highly significant interaction between
Ta and RH (F1,59=6.2,
P<0.001). At Ta=25°C, the effect of RH on
EWL by one-way ANOVA was highly significant (F4,25=20.24,
P<0.001); there was no significant difference in EWL at RH values
of 25%, 43% and 63% (SNK P
0.426), but these values were
different to those at RH of 82% and 92% (SNK P<0.001). There was a
significant linear regression (F1,28=34.3,
P<0.001, R2=0.55, residual sum of
squares=0.071) for RH and EWL at 25°C. A regression of EWL with RH from
63% to 92% was also highly significant (EWL=–0.0040RH+0.518;
F1,16=26.7, P<0.001,
R2=0.63; residual sum of squares=0.029), but regression of
RH from 26% to 63% was not significant (EWL=0.000RH+0.287;
F1,16=0.0, P=0.983,
R2<0.001, residual sum of squares=0.03). The sum of the
residual sum of squares for two separate regressions, 26–43% and
63–92%, of 0.062 was less than that of the overall regression (0.071),
indicating a break point at RH=63%.
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At Ta=30°C, the effect of RH on EWL by one-way
ANOVA was highly significant (F4,25=44.3,
P<0.001). There was a significant difference in EWL at each RH
(SNK P
0.038) except 43% and 62%. There was a highly significant
relationship between RH and EWL for all 30°C experiments;
EWL=–0.0042RH+0.602 (F1,28=34.3,
P<0.001, R2=0.80). A regression between EWL
and RH from 63% to 92% was also highly significant
(F1,16=52.5, P<0.001,
R2=0.77), as was the regression for EWL and RH from 26% to
63% (F1,16=11.7, P<0.001,
R2=0.42).
The slopes of the EWL regression relationships at 25°C and 30°C at RH 63–92% were statistically indistinguishable (F1,40=0.02, P=0.887; common slope=–0.0042), but the relationship at 30°C had a significantly higher elevation (F1,33=19.8, P=0.001). However, at low RH (26–63%) there was a significant difference between slopes at Ta=25 and 30°C (F1,41=19.1, P<0.001). The relationship between EWL and RH over the range 26–92% at Ta=30°C had an x-axis intercept of 144% RH, while at Ta=25°C (over the RH range 63–92%) the x-axis intercept was 129% RH.
A univariate two-way ANOVA indicated that Tb was influenced by Ta (F1,50=5.6, P=0.021), being higher at 30°C than at 25°C, but was not influenced by RH (F4,50=0.98, P=0.425; Fig. 3B). Further analysis (one-way ANOVA) of the effect of RH separately at Ta=25 and 30°C confirmed that Tb was independent of RH at both Ta values (F4,25=0.40, P=0.809; F4,25=1.14, P=0.359).
O2 was
influenced by both Ta (univariate two-way ANOVA;
F1,50=4.7, P=0.001) and RH
(F4,50=4.7, P=0.035;
Fig. 3C).
O2 was generally
higher at Ta=25 than 30°C. One-way ANOVAs examining
the effect of RH on
O2 at each
Ta indicated that at Ta=30°C the
O2 at 25% RH was
significantly higher than at all other RH values
(F4,25=7.58, P<0.001, with SNK
P0.017) but at Ta=25°C the
O2 was
independent of RH (F4,25=1.63, P=0.199). The
linear regression between RH and
O2 at 30°C
was highly significant;
O2=–0.001(±0.0003)RH+0.35(±0.017)
(F1,28=30.8, P<0.001,
R2=0.52). The regression at 25°C was not significant
(F1,28=2.77, P=0.107,
R2=0.09).
There was a highly significant effect of both Ta
(univariate two-way ANOVA; F1,50=101.3,
P<0.001) and RH (F4,50=22.1,
P<0.001) on the ratio of EWL to
O2 (evaporative
quotient, EQ). For RH, one-way ANOVA indicated that it significantly
influenced EQ at both Ta=25°C
(F4,25=7.5, P<0.001) and 30°C
(F4,25=14.8, P<0.001). EQ was constant at low
humidities at both Ta, but decreased at higher RH (SNK
P<0.05; Fig. 4). At
25°C, EQ was 0.90–0.98 at RH of 26–62%, and declined to 0.71
at 82% and 0.58 at 92%. At 30°C, EQ was higher than at 25°C, being
1.44–1.53 at RH of 26–62%, and declining to 1.09 at 82% and 0.85
at 92%.
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DISCUSSION |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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Allometry of EWL in marsupials
The allometric relationship for marsupial EWL presented here is derived
from 14 separate studies, that used a range of different measurement
techniques, and WVP within the metabolic chamber during these measurements
varied considerably (Table 1).
We expected that variation in chamber WVP would contribute substantial
variability to the marsupial EWL data set, due to the influence of chamber RH
on EWL demonstrated for other endotherms
(Lasiewski et al., 1966;
Proctor and Studier, 1970;
Baudinette, 1972;
Christian, 1978;
Edwards and Haines, 1978;
Webb et al., 1995).
Consequently, it was surprising that correcting EWL for WVP within the
metabolic chamber significantly degraded the allometric relationship for EWL,
by accounting for less of the variability in EWL. The influence of ambient RH
on EWL (and other physiological variables) that we measured for a medium-sized
marsupial, the brushtail possum, suggests why WVP correction of
marsupial EWL did not reduce variability in the marsupial EWL dataset.
Brushtail possum EWL and RH
Our EWL values for brushtail possums were lower than previously published
values (Dawson, 1969) at
RH25%, at both 25°C (0.28 compared with 0.45 mg H2O
h–1) and 30°C (0.49 compared with 1.5 mg H2O
h–1). These lower values presumably result from a combination
of longer experiment durations (Cooper and
Withers, in press), measurement of minimum EWL by continuous
hygrometric vs time-averaged gravimetric measurement, and allowing
the possums to adopt their preferred posture in the metabolic chamber.
At 30°C, the relationship between ambient RH and EWL for the brushtail
possum was the expected inverse relationship, consistent with the general
relationship found for other small endotherms [rodents, bats and birds
(Lasiewski et al., 1966;
Proctor and Studier, 1970;
Baudinette, 1972;
Christian, 1978;
Edwards and Haines, 1978;
Webb et al., 1995)]. When EWL
was expressed as a function of WVP, which in theory is the driving
force for evaporation, the expected positive linear relationship was observed
at Ta=30°C (Fig.
5). As RH in the metabolic chamber increased, WVP between
the possum and the ambient air decreased, reducing evaporation from the skin
and/or respiratory surface.
|
63%;
the slope of the relationship (–0.0040 mg H2O%
RH–1) was the same as that observed at
Ta=30°C (–0.0042). Expressing EWL as a function
of WVP to remove the Ta effect resulted in the
expected positive linear relationship below 10 Torr WVP, as at 30°C
(Fig. 5). Unexpectedly, EWL was
independent of WVP above 10 Torr at Ta=25°C. At
25°C the possums were tightly curled into a sphere
(Fig. 2). In this posture, they
would humidify the airspace within the fur around their nose and mouth with
expired air. Re-breathing this humid air therefore reduces the WVP
between the respiratory surfaces and the inspired air, reducing respiratory
water loss below that expected from the overall chamber RH. As the majority of
EWL for possums at the Ta values examined is respiratory
(Dawson, 1969), a reduction in
respiratory water loss in this manner would reduce total water loss. Cutaneous
water loss will also be somewhat reduced in a spherical posture as the
animal's ventral surface and limbs are tucked up, reducing the evaporative
surface area and allowing the build-up of a boundary layer of moist air, which
will be further humidified by respiratory water from the nose and mouth. So,
we suggest that the possum's posture at Ta=25°C
modifies its immediate microclimate and eliminates the RH effect at low
chamber RH. At 30°C the resting possums stretched out on their backs,
presumably to maximise heat loss, and in this posture would not `re-breathe'
humidified air from within their fur and the majority of the body surface
along with the respiratory passages would be exposed to chamber RH. This is
presumably why EWL was directly related to RH at
Ta=30°C but not at 25°C.
Brushtail possum EQ
The EQ at 30°C and low RH values for brushtail possums (1.49 mg
H2O ml O2–1) is less than EQ values for
other marsupial species [mean 2.7 mg H2O ml
O2–1, calculated from previously published data
(Withers et al., 2006)] at BMR
and similarly low RHs. This reflects our lower EWL for the brushtail possums
rather than a higher
O2. EQ values
for brushtail possums at 25°C were constant at RH65% because EWL and
O2 were
constant. The decline in EQ at high RH reflected the decrease in EWL while
O2 remained
constant. In effect, rebreathing air within the sphere they created by curling
added a diffusive resistance [cf. figures 16–22 of Withers
(Withers, 1992b)]. At
Ta=30°C, EQ was higher, but showed a similar pattern
with RH, but for a different reason. Both EWL and
O2 changed with
RH, but the increase in
O2 was less at
lower RH than that of EWL, so EQ at Ta=30°C also
flattened out at RH65%.
EWL and chamber RH
Most studies have found a highly significant effect of chamber RH (or
WVP) on EWL (Lasiewski et al.,
1966; Proctor and Studier,
1970; Coulombe,
1970; Baudinette,
1972; Bernstein et al.,
1977; Christian,
1978; Edwards and Haines,
1978; Webb et al.,
1995), consistent with the expected negative relationship with RH
or positive relationship with WVP. However, there are exceptions.
Non-reproductive little brown bats Myotis lucifugus
(Proctor and Studier, 1970) at
Ta=28 and 33°C have a reduced EWL at low RH, and a
highly positive relationship at Ta=37°C (presumably
reflecting compromised thermoregulatory capacity at high
Ta). Bernstein et al.
(Bernstein et al., 1977)
measured the expected inverse relationship of EWL over a range of ambient WVPs
for pigeons (Columba livia) at Ta=40°C, as
was observed for possums at 30°C, but EWL of pigeons was independent of
WVP at 20°C, as for possums at 25°C and <62% RH. They did not
discuss these findings in detail, but suggested that `adjustments of
respiratory ventilation or in temperature of exhaled air may account for the
independence of EWL at 20°C'. No observations of posture, such as the head
being tucked under a wing, were reported.
The unexpected independence of EWL from low ambient RH at 25°C for brushtail possums presumably explains why our WVP correction for marsupial EWL increased, rather than decreased, the residual variability of the allometric relationship. Clearly there is not necessarily a direct relationship between ambient RH (or WVP) and EWL. Correcting EWL for RH is spurious if there is no effect, or if the effect is not understood (i.e. not the assumed linear relationship). Postural adjustments may have a significant impact on EWL, by altering the animal's immediate microclimate. Thus, although the impact of chamber RH on EWL needs to be considered in comparative studies, a `correction factor' for this problem is not straightforward. A simplistic approach based on the theoretical relationship between EWL and WVP will not account for behavioural modification of the animal's immediate surroundings and thus the potential independence of EWL from chamber RH. We suggest that enabling the animal to adopt its preferred posture during experiments, and maintaining a low chamber RH will probably result in the most meaningful and realistic measure of EWL for comparative studies.
EWL at 100% RH
Christian (Christian, 1978)
suggested that EWL should theoretically be 0 at 100% RH, but a WVP
remains for both cutaneous and respiratory water loss if skin temperature
(Tskin) and/or Tb is
>Ta, even if the ambient RH is 100% at
Ta, due to the greater capacity of air to hold water
vapour at higher Ta. Thus when
Tb>Ta and
Tskin>Ta, the warmer air in the
respiratory passages and near the body surface will continue to drive EWL.
However, this water loss would presumably condense in the metabolic system
because the air at Ta would be saturated, and would not
actually be measured as EWL (measured EWL would plateau at the value
calculated for saturation water vapour density of ambient air). The magnitude
of EWL at 100% RH will depend on the body core to skin thermal gradient and
the extent of nasal counter-current heat and water exchange
(Schmidt-Nielsen et al.,
1970). We calculate from the regression relationship for EWL and
RH at Ta=30°C that a theoretical RH of 144% would be
required for zero EWL. A RH of 144% at Ta=30°C (WVP=45
Torr) is equivalent to a RH of 100% at 36.6°C, calculated using
hygrometric equations (Parish and Putnam,
1977). We consider 36.6°C as the effective evaporative
temperature (Tee) of the possums at
Ta=30°C, and it closely approximates the possums'
actual Tb at Ta=30°C and RH=92% of
36.1°C. This close approximation of Tee and
Tb suggests that the possums were not recovering any water
via nasal counter-current heat and water exchange, and did not have a
large body core to skin thermal gradient at Ta=30°C.
However, it is likely that nasal counter-current heat and water exchange
mechanisms were operating at Ta=25°C. We calculate
from the regression relationship for EWL and RH (from 62% to 92% RH) at
Ta=25°C that a theoretical RH of 129% would result in
zero EWL at Ta=25°C. A RH of 129% at
Ta=25°C (WVP=29.6 Torr) is equivalent to a RH of 100%
at only 29°C calculated from hygrometric equations
(Parish and Putnam, 1977);
this Tee is substantially lower than the actual
Tb of 35.0°C at 25°C and 93% RH. Thus nasal
counter-current heat and water exchange and/or a large body core to skin
thermal gradient were reducing EWL below that expected if the
Tee approximated Tb at
Ta=25°C. This may have contributed to the independence
of EWL from RH at RH62%, together with postural adjustments.
Effect of RH on Tb, MR and C
Body temperatures of the brushtail possums measured in this study were
somewhat lower than those previously measured
(Dawson, 1969) at both
Ta=25°C and Ta=30°C
(34.9±0.08°C compared with 35.9°C, and 35.4±0.18°C
compared with 36.5°C, respectively).
O2 at both
Ta=25°C and Ta=30°C (means
over all RH values at each Ta were 0.29±0.01 and
0.27±0.01 ml O2 g–1 h–1,
respectively; N=30) was slightly lower than previous measurements
(Dawson and Hulbert, 1970;
0.31 ml O2 g–1 h–1 at
Ta=27°C). Long experiment durations
(Cooper and Withers, in press),
allowing possums to adopt their preferred posture within the metabolic
chamber, and possible intraspecific differences between possum populations
presumably account for these differences.
The effect of ambient RH has rarely been examined for physiological
variables other than EWL. RH has not previously been shown to affect MR or
Tb (Baudinette,
1972; Ewing and Studier,
1973; Edwards and Haines,
1978), although Kay (Kay,
1975) suggested that RH may have an effect on
Tb at high Ta
(>Tuc). Cwet must be affected by RH
if EWL is, as evaporative heat loss is a substantial component of
Cwet, especially at higher Ta. Thus
the significant relationship between Cwet and RH at
30°C but not at 25°C is consistent with the relationship between EWL
and RH that we observed at these Ta values.
O2 was
significantly related to RH at Ta=30°C (but not at
Ta=25°C), increasing with decreasing RH. As
Tb of possums was independent of RH, they had to adjust
their MR and/or Cdry to account for the effect of RH on
EWL and thus evaporative heat loss (and Cwet) at
Ta=30°C. Cdry was independent of
RH at both Ta values, and therefore
O2 (and thus
metabolic heat production) had to vary with RH to counteract the effect of RH
on evaporative heat loss to maintain a constant Tb. We
observed that the pattern of
O2 change with
RH mirrored that of EWL (Fig.
3A,C), as expected. However, at Ta=25°C,
where possums can maintain EWL independent of RH, the effect of RH on
Cwet and therefore
O2 was not
significant.
Our finding that the MR of brushtail possums within the thermoneutral zone can be influenced by RH has important implications for the measurement of standard physiological variables such as BMR, Tb and Cwet. If valid intraspecific and interspecific comparisons are to be made, then it is essential that comparative standards (e.g. BMR) are measured under comparable environmental conditions. Although the effect of chamber RH has been considered for the comparison of standard EWL data, little attention has been paid to the possible confounding effect of chamber RH on commonly measured variables such as BMR. Here we demonstrated that under conditions where RH has a significant effect on EWL, other interrelated variables such as BMR, Tb and Cwet may also be influenced. Thus chamber RH should be considered when defining standard conditions for measurement of physiological variables such as EWL and BMR.
LIST OF ABBREVIATIONS
CO2
O2
WVP
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Acknowledgments |
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References |
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TOPSummaryINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONReferences |
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