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First published online February 15, 2006
Journal of Experimental Biology 209, 938-944 (2006)
Published by The Company of Biologists 2006
doi: 10.1242/jeb.02063
Respiration by buried echidnas Tachyglossus aculeatus
School of Integrative Biology, University of Queensland, Brisbane, Australia 4072
* Author for correspondence (e-mail: d.booth{at}uq.edu.au)
Accepted 22 December 2005
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Summary |
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O2) in five
individuals under similar conditions, in two substrates with different
air-filled porosities (fa). A theoretical diffusion model
indicated that diffusion alone was insufficient to account for the flux of
oxygen required to meet measured rates of
O2. However, it
was noticed that echidnas often showed periodic movements of the anterior part
of the body, as if such movements were a deliberate effort to flush the tidal
air space surrounding their nostrils. These `flushing movements' were
subsequently found to temporarily increase the levels of interstitial oxygen
in the soil around the head region. Flushing movements were more frequent
while O2 was
higher during the burrowing process, and also in substrate with lower
fa. We conclude that oxygen supply to buried echidnas is
maintained by diffusion through the soil augmented by periodic flushing
movements, which ventilate the tidal airspace that surrounds the nostrils.
Key words: monotreme, burrowing, respiration, gas exchange, oxygen consumption, echidna
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Introduction |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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) and
remaining there, holding fast against attempts to dislodge them, for long
periods. This behaviour poses questions about how echidnas manage their
respiratory gas exchange, cut off from a free flow of gases and threatened by
the risk of inhaling particles of soil.
It is clear that echidnas are frequently exposed, at least for short
periods, to increased hypoxia and hypercapnia, either when digging into soil
or within their burrows (Augee et al.,
1971). Previous studies have demonstrated that echidnas are
physiologically well suited for burrowing. Augee et al.
(Augee et al., 1971) covered
echidnas with soil to simulate natural conditions and found them to be very
tolerant of high carbon dioxide (CO2) and low oxygen
(O2) under these conditions.
However, no studies have explored the processes by which O2
supply is maintained while an echidna is buried, completely surrounded by
soil, without any tunnel to facilitate the convective movements of gas.
Presumably echidnas re-breathe the interstitial gas around them while buried.
The diffusive transport of O2 to a buried mammal was explored in
the Namib Desert golden mole Eremitalpa granti namibensis
(Seymour and Seely, 1996),
which survives long periods buried in sand. By comparing measurements of the
PO2 gradient away from the snout of a buried
mole to the PO2 gradient predicted from a
mathematical model of gaseous diffusion through sand, Seymour and Seely
(Seymour and Seely, 1996)
explained that golden moles can survive being buried in sand by utilizing
O2 diffusing through sand into the tidal air space that surrounds
the snout. However, their model predicted an upper size limit of approximately
200 g for resting mammals able to continue respiration in this manner.
Echidnas are commonly 2–4 kg and can reach 7 kg, so the model used to
explain sub-sand respiration by the golden mole cannot by itself explain the
submergence capabilities observed in echidnas.
In this study we took a similar approach to that of Seymour and Seely
(Seymour and Seely, 1996). We
measured the PO2 gradient away from the snouts
of submerged echidnas and
O2 in separate
experiments on five individuals under similar conditions, in two media with
differing porosity fa values, with the hope of determining
how echidnas can remained buried for long periods of time.
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Materials and methods |
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), and the
Texas area, 50 km SW of Stanthorpe in SE Queensland (latitude
28°43'S, longitude 151°28'E). The animals were held in a
free-range enclosure at the University of Queensland's Pinjarra Hills
Veterinary Farm.
Modelling the diffusive exchange of respiratory gases
The diffusion model (Seymour and Seely,
1996) assumes that the buried animal is surrounded completely by a
medium that permits O2 to diffuse radially towards it from all
directions. This assumption is supported by empirical data
(Seymour and Seely, 1996;
Wilson and Kilgore, 1978;
Withers, 1978). In a steady
state, the amount of O2 diffusing radially through a given
spherical shell is equal to the rate at which it is consumed. As O2
diffuses radially toward the animal, the volume through which it passed
decreases and, therefore, the changes in ambient
PO2 shell by shell, with decreasing distance to
the animal, can be calculated from the equation
(Seymour and Seely, 1996):
 | (1) |
O2 is the
rate of oxygen consumption (cm3 min–1),
KO2 is the diffusion coefficient of oxygen in
substrate (cm2 min–1 kPa–1),
Po and Pi are the oxygen partial
pressures (kPa) at the outer (ro) and inner
(ri) radii of a given spherical shell (cm). The following
additional assumptions were made for the model: (1)
KO2 was the product of the binary diffusion
coefficient of oxygen in air (DO2 = 12.1
cm2 min–1 at 25°C)
(Nobel, 1983), the
O2 capacitance of air ßO2 =
–0.0098 cm3 cm–3 kPa–1
(Seymour and Seely, 1996) and
the air-filled porosity coefficient=f 1.5 a
(Marshall, 1959;
Seymour and Seely, 1996).
KO2 was therefore taken as 0.032 cm2
min–1 kPa–1 in coarse sand of
fa=0.42, and 0.053 cm2 min–1
kPa–1 in kitty litter (a water absorbent granular substance
made from recycled newspaper) of fa=0.580 (see later). (2)
Po was assumed in the earlier study
(Seymour and Seely, 1996) to
be atmospheric at ro=100 cm. In our case, the echidna was
in a large plastic bin and the surface through which diffusion was possible
was therefore constrained. The environment of this experiment is therefore
more inimical to gaseous diffusion than that on which the model is based, and
that needs to be kept in mind when interpreting the results. (3) The internal
radius was the radius of a sphere of sand containing a volume of interstitial
gas equal to the tidal volume of the animal's breath
(Seymour and Seely, 1996).
This assumption is based on the premise that a volume of interstitial air in
the sand, equal to tidal volume, was constantly being rebreathed and mixed by
the animal. The radius of this `tidal space' was calculated using the equation
(Seymour and Seely, 1996):
 | (2) |
),
VT=8.96 ml kg–1.
To apply the model to echidnas,
O2 values and
PO2 gradients away from buried animals were
measured separately, and the assumption made that the
O2 measured
while submerged was indicative of the
O2 during
measurement of the PO2 gradient, as was done
previously with the golden mole (Seymour
and Seely, 1996).
Measurement of O2
O2 was
measured using a flow-through respirometry system, at an ambient temperature
of 25°C. Each animal was placed in an air tight, 50 cm deep and 40 cm in
diameter, cylindrical chamber 
-filled with a test medium into which
the echidnas could burrow. Gas entered the chamber into an air space above the
burrowing medium and was extracted from below the burying medium through a
wire mesh-covered hole at the base of the chamber. Two media with differing
fa values were used and each animal was exposed to each
medium. The fa of each of the two media was measured by
filling a 1000 ml graduated cylinder with the medium and slowly adding it to
1000 ml of water in a 2000 ml cylinder, avoiding any bubbles
(Seymour and Seely, 1996). The
kitty litter, which would otherwise have absorbed water, was first sprayed
with a water repellent spray (Motortech, Balchan International, Australia).
While this method discounted any porosity of the kitty litter itself, we
consider this to have been negligible.
The lid of the respirometry chamber was transparent, so lung ventilation
movements could be directly observed during these experiments even when the
animal was completely buried (the surface of the substrate moved slightly with
each breath). The behaviour of an animal on introduction to the respirometry
chamber, whether for
O2 or
PO2 gradient measurements, was to burrow
immediately vertically downward until submerged completely in the substrate, a
behaviour identical to that of echidnas burying themselves in the wild.
O2 was measured
while the animal remained buried. Resting metabolic rate was considered to
have been reached when the animal had been submerged and resting for 4 h and
the fractional concentration of O2 in the excurrent air was stable.
Air from the chamber was passed through small diameter tubing to a
CO2 absorbent (Soda Lime) and then a desiccant (DrieriteTM)
before passing into a mass flow meter (MFS-1; Sable Systems, Las Vegas, NV,
USA). The mass flow meter pulled air though the chamber and its exhalent air
was sampled via an oxygen analyser (Sable Systems PA-1B) calibrated
with oxygen-free gas (0.00% O2) and room air (20.95%
O2). The flow rate through the respirometry chamber was 750 ml
min–1 in all cases. Body temperature Tb
was monitored throughout the measurement period using the implanted
temperature transmitter and a radio receiver connected to a pulse meter. The
output voltage from the O2 analyser and pulse meter were fed into
PowerLab hardware (ADInstruments, Sydney, NSW, Australia) connected to a
computer running Chart5 software (v5.0.1. ADInstruments).
Measurement of PO2 at different distances from the snout in buried echidnas
Measurements of PO2 within the substrate
surrounding buried echidnas were performed in a large cylindrical plastic bin
(measurements given above) at an ambient temperature of 25°C. Experiments
were commenced by placing the animal on top of the substrate and, in all
cases, echidnas burrowed immediately until they were completely submerged.
Gas samples from the snout region were collected through silicone tubing (1 mm i.d.) after flushing dead space from the tubing into 3 ml plastic syringes. The tip of this tube was secured onto the nose of the animal above the nostril using a combination of medical glue (collodian) and micropore tape. To measure the PO2 gradient within the medium away from the snout region, further lengths of silicon tubing were attached at 2 cm intervals along the initial tubing, up to 10 cm away from the snout tip.
Gas samples (2 ml) were analysed for O2 content with a thermally stabilised Clarke Oxygen Electrode (DOX, Analytical Sensors, Inc., Sugarland, TX, USA), connected to a Radiometer PHM73 gas analyser (Copenhagen, Denmark), calibrated with outside air (20.95% O2) and oxygen-free gas.
Gas samples were taken immediately on submergence of the echidna, and further samples were taken at 15 min intervals for a 5 h period. Each animal was measured individually in each of the media. Tb was also monitored throughout these experiments using the implanted temperature transmitter.
Measurement of PO2 and movement
During trial measurements of O2 tensions around buried echidnas,
the animals would move periodically. Such movements would be followed by a
rise in PO2 levels in the substrate, seemingly
as a result of these movements. To determine if there was a causal
relationship between the movements of echidnas in different substrates and the
PO2 measured in the snout region while
submerged, a piezoelectric movement sensor (Sigma Delta technologies, Perth,
Western Australia) was attached to the echidnas. This sensor was wrapped in
electrical tape and glued to a dorsal spine on the shoulder region of the
animal. The output voltage from the movement sensor was recorded,
simultaneously with Tb and measured
PO2 samples, using PowerLab hardware connected
to a computer running Chart5 software (v5.0.1. ADInstruments). Respiration
rate and larger `flushing movements' were detected by the movement sensor
(Fig. 1).
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Statistics
All results are presented as means ± s.d. Differences between
empirical and theoretical values of PO2 in
substrate surrounding buried echidnas were tested using paired
t-tests. Significance was assumed at P<0.05.
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Results |
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O2 of each
echidna, the PO2 in the snout region, and
movement and activity level while submerged in two different substrates,
coarse sand (fa=0.42) and kitty litter
(fa=0.58).
VO2 measurements while submerged
O2
measurements were taken at a chamber temperature of 25°C. As would be
expected, O2 was
greater during burrowing than resting, and substrate type did not influence
resting rate of
O2
(Table 1).
Tb and respiration rate always decreased during the course
of O2
measurements. Tb of echidnas at the start of the
experiments ranged from 27.0 to 34.5°C and, on average, decreased
2.2±1.3°C (N=5) during a trial. Respiration decreased from
12.0±0.5 to 4.6±2.2 breaths min–1 over the 5 h
measurement period.
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PO2 gradient in substrate while submerged
PO2 measurements were taken at an ambient
temperature of 25°C. The mean atmospheric
PO2 was 19.7 kPa (range 19.3–19.9 kPa) in
water-saturated air. The PO2 level in each
substrate, without an echidna, was atmospheric.
PO2 levels of gas samples from near the snout
of echidnas buried in the medium were always below the atmospheric level, even
immediately after burial. The mean minimum PO2
values at the tip of the echidna's snout during burrowing in each substrate
were 12.1±1.4 kPa in coarse sand (fa=0.42,
N=5) and 12.3±1.4 kPa in kitty litter
(fa=0.58, N=5). Gas samples taken at a series of
distances away from the snout revealed a PO2
gradient within the substrate (Table
2).
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Tb and respiration rate always decreased over time, independent of medium, during experiments that measured PO2 gradients. Tb of echidnas at the start of the experiments ranged from 28.0 to 35.0°C and, on average, decreased 1.2±0.5°C (N=5) during a trial. Respiration decreased from 12.0±0.5 to 4.7±3.3 breaths min–1 over the 5 h measurement period.
Comparison of measured PO2 values with those predicted by modelling diffusive exchange of oxygen
Measured O2
data and published values for tidal volume
(Bech et al., 1992) were used
to generate theoretical diffusion gradients away from the snout for each of
the substrates, using Eqn 1. These theoretical
PO2 values at distances away from the snout
were compared with empirically measured PO2
values around buried echidnas (Figs
2,
3).
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In the natural substrate, coarse sand (fa=0.42) the
theoretical calculated values were significantly different from the measured
values (Fig. 3). In coarse
sand, supply of oxygen by diffusion alone was apparently insufficient to
account for the measured rates of
O2.
PO2 in relation to body movements
A behavioural pattern was observed in the experiments that measured
PO2 profiles in the substrate surrounding
buried echidnas. PO2 at any measurement
distance from the snout did not remain constant, it fluctuated in a cyclic
manner and mean values tended to increase as the experiment progressed
(Fig. 4). When first introduced
to the burrowing medium, echidnas immediately dug down until completely
encased in the substrate. At this stage, animals had a relatively high
O2,
Tb and respiration rate, and the minimum
PO2 near the snout was always lowest during
this early phase of an experiment. The minimum
PO2 rose over time as
O2 fell to a
resting rate (Fig. 4). Periodic
movements of the anterior body were more common early in these experiments
when oxygen consumption rates were higher, and these movements were followed
soon after by an increase in PO2 close to the
snout and further away from the snout (Fig.
4) indicating that such `flushing movements' caused the convective
movement of oxygen from the atmosphere to the snout area.
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The ability of echidnas to respire while submerged in different media
The frequency of flushing movements varied between kitty litter and course
sand (Fig. 4). In kitty litter
there were approximately two movements h–1 for the first hour
and then one movement h–1 for the rest of the experiment
(Fig. 4A), while in course sand
there were on average 6.2 movements h–1 for the first hour,
4.6 movements h–1 for the second hour, and two movements
h–1 for the rest of the trial
(Fig.
4B).
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Discussion |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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O2 and published
values for tidal volume (Bech et al.,
1992) were used to generate theoretical
PO2 profiles away from the snout region, with
which measured PO2 values could be compared. In
kitty litter, at rest, there was no significant difference between the
measured and the predicted gradients, suggesting that diffusion was sufficient
to meet the O2 requirements when buried in this high porosity
medium, as for golden moles buried in fine sand
(Seymour and Seely, 1996).
Even in this high porosity medium, however, resting echidnas chose to make
periodic movements, albeit infrequently.
In the natural substrate, empirically measured and theoretical
PO2 values were significantly different. The
actual mean minimum PO2 values at the snout
region and along the gradient were significantly greater than predicted by the
model, suggesting that diffusive oxygen transport through the sand alone was
insufficient to account for the measured
O2. The higher
than predicted PO2 values appear to result from
the periodic flushing movements during submergence, which caused temporarily
increased interstitial O2 levels closer to the snout.
The effect of periodic movement on the PO2 profiles surrounding buried echidnas
Periodic movements were more frequent in coarse sand, particularly in the
earlier phase of an experiment when
O2 was highest.
The higher frequency of `flushing' movements at the beginning of the burying
trials enabled the echidna to stay submerged while experiencing higher
O2 demand, but as O2 demand decreased later in trials,
the frequency of flushing movements decreased. Once a resting rate of
O2 was achieved,
a steady state was achieved and flushing movements were regular but less
frequent and the PO2 profiles became more or
less constant.
VO2 and Tb of buried echidnas
Monotremes are characterized by metabolic rates and Tb
that are lower than those typical of eutherian mammals
(Bech et al., 1992;
Griffiths, 1978;
Schmidt-Nielsen et al., 1966)
and this was confirmed in our study. Basal metabolic rates of echidnas have
been reported to be only 25–50% of that predicted for eutherian mammals
(Bech et al., 1992;
Dawson et al., 1978;
Dawson et al., 1979;
Schmidt-Nielsen et al., 1966).
However, echidnas are notoriously difficult subjects in which to measure
resting O2,
usually being very restless and attempting to escape while in classical
respirometry chambers. This is the first study to measure
O2 in echidnas
buried in substrate. Indeed, further work in our laboratory has shown that
providing echidnas with even a small quantity of material in the respirometer,
into which they can bury their head, makes it much easier to achieve
measurements at apparently resting levels (P. H. Brice, G. C. Grigg and L. A.
Beard, unpublished observations). Accordingly, we think that buried echidnas
are more relaxed than echidnas in a respirometer without anywhere to `hide'
and are more likely to provide good data on resting metabolic rates.
The O2
measured from echidnas in this study were also somewhat more variable than
might be expected for metabolic rate measured in a typical mammal at rest. The
variability may be explained in terms of their heterothermy. Echidnas have the
advantages of endothermy, including the capacity for impressive homeothermic
endothermy during incubation (Grigg et
al., 2004). The modal Tb of active echidnas is
32°C (Grigg et al., 2004).
However, they are very relaxed about using thermoregulatory mechanisms to
maintain a stable Tb and periods of rest are typically
accompanied by a drop in Tb. Accordingly, cyclic changes
in daily Tb of 3–6°C are routine. They also show
both short- and long-term torpors (Grigg
et al., 2004). In our study, Tb at the start
of a trial differed between echidnas and this is likely to account for much of
the variability in measured
O2. The declines
in Tb during each experimental trial reflect the expected
drops, which occur in echidnas at rest after a period of activity.
The magnitude of the decrease in ambient PO2 surrounding burrowed echidnas
The PO2 of the immediate O2
environment of the burrowing echidna showed a decrease in ambient oxygen to
about 11 kPa, substantially below atmospheric (21 kPa). This is a much greater
drop than was found in golden moles buried in sand
(Seymour and Seely, 1996),
where ambient PO2 values were about 20 kPa.
Kuhnen (1986) summarized
published data on the burrow O2 and CO2 levels for 13
species of burrowing mammals. Typically, these species were exposed to
PO2 values between 17–20 kPa, but values
down to 10 kPa have occasionally been recorded
(Kuhnen, 1986). The results
from the present study showed that the immediate O2 environment of
buried echidna near the snout was 12.3±0.2 kPa while active and
14.9±0.2 kPa while resting. A PO2 of 11
kPa was recorded (Augee et al.,
1971) in an echidna encased in substrate for a period of 4 h at a
depth of 20 cm, which is in good agreement with our study. Bentley et al.
(Bentley et al., 1967) found a
PO2 of 13.9 kPa in an echidna completely
encased in crushed corncobs at a depth of 30–60 cm. The O2
environment tolerated by buried echidnas seems to put them at the extreme end
among fossorial and semi-fossorial mammals.
Behavioural adaptations of echidnas while submerged in different substrates
O2 supply is one aspect of survival under soil, but the
mechanical problem of breathing in loose material also needs to be explored.
The porosity of coarse sand was low due to a discontinuity of pore spaces and
the smaller particles filling in the void spaces between the larger particles.
The echidna apparently used this discontinuity to stay submerged and was able
to use its snout to create a small air space and prevent the inhaling of soil
particles into the nose.
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Acknowledgments |
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References |
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TOPSummaryIntroductionMaterials and methodsResultsDiscussionReferences |
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Augee, M. L., Elsner, R. W., Gooden, B. A. and Wilson, P. R. (1971). Respiratory and cardiac responses of a burrowing animal, echidna. Respir. Physiol. 11,327 -334.[Medline]
Bech, C., Nicol, S. C. and Andersen, N. A. (1992). Ventilation in the echidna, Tachyglossus aculeatus. In Platypus and Echidnas (ed. M. L. Augee), pp. 134-139. Sydney: The Royal Zoological Society of NSW, Sydney.
Bentley, P. J., Herrid, C. F. and Schmidt-Neilsen, K.
(1967). Respiration of a monotreme, the echidna, Tachyglossus
aculeatus. Am. J. Physiol.
212,957
-961.
Brice, P. H., Grigg, G. C., Beard, L. A. and Donovan, J. A. (2002). Patterns of activity and inactivity in echidnas (Tachyglossus aculeatus) free-ranging in a hot dry climate: correlates with ambient temperature, time of day and season. Austr. J. Zool. 50,461 -475.[CrossRef]
Burrell, H. (1926). The burrowing habits of Tachyglossus aculeatus. Austr. J. Zool. 4, 197-198.
Dawson, T. J., Fanning, D. and Bergin, T. J. (1978). Metabolism and temperature regulation in the New Guinea monotreme Zaglossus bruijni. Austr. Zool. 20, 99-103.
Dawson, T. J., Grant, T. R. and Fanning, D. (1979). Standard metabolism of monotremes and the evolution of homeothermy. Austr. J. Zool. 27,511 -515.[CrossRef]
Frappell, P. B., Franklin, C. E. and Grigg, G. C. (1994). Ventilatory and metabolic responses to hypoxia in the echidna, Tachyglossus aculeatus. Am. J. Physiol. 267,R1510 -R1515.
Griffiths, M. (1978). The Biology of the Monotremes. London: Academic Press.
Grigg, G. C., Beard, L. A. and Augee, M. L. (2004). The evolution of endothermy and its diversity in mammals and birds. Physiol. Biochem. Zool. 77,982 -997.[CrossRef][Medline]
Kuhnen, G. (1986). O-2 and CO-2 concentrations in burrows of euthermic and hibernating golden-hamsters. Comp. Biochem. Physiol. 84A,517 -522.[Medline]
Marshall, T. J. (1959). The diffusion of gases through porous media. J. Soil Sci. 10, 79-82.[CrossRef]
Nobel, P. S. (1983). Biophysical Plant Physiology and Ecology. San Francisco: W. H. Freeman.
Parer, J. T. and Hodson, W. A. (1974). Respiratory studies of monotremes. 5. Normal respiratory functions of echidnas and ventilatory response to inspired oxygen and carbon-dioxide. Respir. Physiol. 21,307 -316.[Medline]
Schmidt-Nielsen, K., Dawson, T. J. and Crawford, E. C. (1966). Temperature regulation in the echidna (Tachyglossus aceuleatus). J. Cell. Physiol. 67, 63-72.[Medline]
Seymour, R. S. and Seely, M. K. (1996). The respiratory environment of the Namib Desert golden mole. J. Arid Envir. 32,453 -461.[CrossRef]
Wilson, K. J. and Kilgore, D. L. (1978). Effects of location and design on diffusion of respiratory gases in mammal burrows. J. Theor. Biol. 71, 73-101.[Medline]
Withers, P. C. (1978). Models of diffusion-mediated gas-exchange in animal burrows. Am. Nat. 112,1101 -1112.[CrossRef]
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