Metabolic responses of shorebird chicks to cold stress: hysteresis of cooling and warming phases
1 Department of Biology, University of Missouri-St Louis, 8001 Natural
Bridge Road, St Louis, MO 63121-4499, USA
2 Department of Zoology, Ohio State University, 1735 Neil Ave, Columbus, OH
43210, USA
* Author for correspondence (e-mail: ricklefs{at}umsl.edu)
Accepted 9 May 2003
![]() |
Summary |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Here, we illustrate typical temperature and metabolism dynamics of the DU
protocol by describing several trials in detail, and we discuss the
implications of these results for the control of metabolism and regulation of
Tb. Chicks subjected to the DU protocol exhibited three
distinct phases of metabolic response to ambient temperature
(Ta). In Phase I,
O2 increase was
directly related to the gradient between Tb and
Ta, consistent with a Newtonian response to cooling.
During Phase II, chicks sustained a maximum level of
O2 that
decreased as Tb dropped, exhibiting a
Q10 of approximately 2. Based on the slope of the
relationship between
O2 and
Tb during Phase II, we were able to estimate maximum
O2 at a
standardized high Tb. Phase II continued until chick
Tb began to rise as a result of the gradually increasing
Ta. During Phase III, the Tb-adjusted
rate of oxygen consumption decreased from the maximum level at low
Tb to the resting level at high Tb in
the thermoneutral zone. Further trials with faster and slower rates of chamber
cooling showed that
O2 during Phase
I varied in proportion to the difference between Tb and
Ta (
T), whereas during Phase III it
responded to Tb.
Even though chicks may be capable of generating enough heat to regulate Tb during the early part of Phase I of the DU protocol, the constantly decreasing Ta created a time lag between Ta and the chick's metabolic response, leading to body cooling. The hysteresis observed between Phase I and Phase III suggests that chicks rewarm passively while being brooded following the decrease in Tb experienced during active foraging. The results of the DU protocol suggest that Tb should be measured continuously during measurements of maximum oxygen consumption, and that peak values should be adjusted by Tb to make them comparable with other studies.
Key words: body temperature, brooding, Charadriidae, hysteresis, maximum metabolic rate, peak metabolic rate, Q10, Scolopacidae, shorebird, temperature regulation
![]() |
Introduction |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Shorebird chicks are precocial and self-feeding from hatching; however,
their capacity to generate heat for temperature regulation at this time is
poorly developed. The ability of a chick to generate heat in response to cold
stress depends on the relative size of its skeletal muscles, especially in
later development the pectoral muscles, and the metabolic intensity of muscle
tissue, which increases as muscles mature
(Choi et al., 1993;
Hohtola and Visser, 1998
;
Marjoniemi and Hohtola, 1999
;
Krijgsveld et al., 2001
).
Smaller species can often generate more heat per gram of muscle tissue than
larger species (Krijgsveld et al.,
2001
), but their unfavorable surface- to-volume ratios result in
rapid heat loss and body cooling, even under mild environmental temperatures
(Chappell, 1980
;
Visser and Ricklefs, 1993
). As
a result, young shorebird chicks alternate their foraging, when they cool if
the ambient temperature is low, with brooding in association with a parent,
when they rewarm (Norton,
1973
; Chappell,
1980
; Beintema and Visser,
1989
).
Here we describe a protocol for simultaneously measuring metabolism and
Tb in shorebird chicks through phases of decreasing and
then increasing Ta. The protocol was designed to mimic the
natural cycle of cooling and warming experienced by chicks. It begins by
allowing a chick to achieve a thermal and metabolic equilibrium at a
thermoneutral Ta, followed by a period during which
Ta decreases at a rate of approx. 0.5°C
min-1, until Tb falls to about
3234°C. The protocol is then concluded by a period of rewarming
under increasing Ta. The protocol ends when
Tb returns to 3840°C, which is typical of birds
under thermoneutral conditions. A small number of trials in this study
involved variations on this protocol, in which the rate of decrease in
Ta was slowed or accelerated to examine how rate of change
in temperature affected the metabolic response to cold challenge. Finally,
several chicks were maintained under mild cold stress for up to 2 h to test
metabolic endurance. In all trials, continuous records of
Ta, Tb and instantaneous oxygen
consumption (O2)
were used to examine the relationship between rate of metabolism and
Tb, rate of change in body temperature
(
Tb), and the gradient between
Ta and Tb (
T).
We conducted this study to determine suitable conditions for measuring
resting and maximum cold-induced metabolic rates in shorebirds as an index to
the functional capacity of their skeletal muscles to produce heat through
shivering thermogenesis (Hohtola and
Stevens, 1986; Choi et al.,
1993
; Koteja,
1996
; Hohtola and Visser,
1998
). We were concerned that protocols that expose
thermoneutrally equilibrated chicks to cold temperatures might produce
measurement biases. For example, a chick might fail to elevate its metabolic
rate quickly enough to track rapid changes in Ta. We were
also concerned that slower cooling protocols might lead to physiological
exhaustion or to a decrease in Tb before peak metabolic
rate is achieved. Metabolic rate and Tb must be measured
simultaneously, in order to determine the influence of Tb
on estimated maximum metabolic rate (MMR). We found that estimated MMR depends
strongly on Tb, and should be corrected for
Tb to provide a standardized measurement for comparison.
Analysis of the response of metabolism to temperature under different
protocols also provided information on the sensory inputs used by chicks to
respond to cold challenge.
Finally, the results of these trials demonstrated a clearly defined hysteresis in the response of metabolic rate to cold stress. As we shall show, chicks defend their body temperatures metabolically when they are cooling, but warming is a passive process. In this article, we illustrate typical temperature and metabolism dynamics of the DU protocol by describing several trials in detail, and we discuss the implications of these results for the control of metabolism and regulation of Tb. Comparisons of the metabolic responses of shorebird chicks between species and as a function of age will be published elsewhere. The outcomes of this study have been to (1) provide some of the first continuous recordings of both metabolism and body and air temperatures in cooling trials, (2) establish a more sound basis for comparative observations of `maximum' metabolic rate and metabolic scope, (3) discover a pattern of hysteresis not suspected previously, and (4) develop several novel speculations concerning stimuli for thermogenesis and potential `economic' benefits of passive warming in precocial chicks.
![]() |
Materials and methods |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Metabolic measurements
Metabolism trials were conducted in aluminum metabolism chambers of
internal volume 3375749 ml, being 109±78 ml g-1 chick
mass (mean ± S.D., range 28573). The inside surfaces
of the chambers were painted flat black to reduce reflected radiation
(Porter, 1969). Within the
chambers, chicks were placed in wire mesh baskets to reduce activity and
prevent contact with the walls of the chambers. Each chamber was surrounded by
a metal jacket through which coolant was circulated from a Thermo NESLAB
(Portsmouth, NH, USA) Model RTE-4 or LT-50 refrigerated water bath. Incurrent
air entered the chambers at one end and excurrent air was drawn from the
chambers at the other end. Comparisons of chamber volumes, determined by
filling with water and by washout curves of CO2, indicated that air
was well mixed within the chambers. Chamber temperature was measured with a
30-gauge calibrated thermocouple.
Incurrent air coursed through columns of Drierite®, soda
lime and Drierite® again to absorb water and carbon dioxide
before passing through a Mykrolis (Billerica, MA, USA) Tylan® mass flow
controller (FC-260; 03000 ml min-1) calibrated against a
1000 ml bubble meter (Levy,
1964). Flow rates were considered to be accurate to 1% and to have
a precision of at least 1%. Flow rates were adjusted to ensure that oxygen
concentration did not decrease below 19.3%.
Excurrent air passed through a General Eastern (Woburn, MA, USA) Model Hygro M4 dew point hygrometer and then through tubes packed with silica gel, soda lime and silica gel, respectively. The dry, CO2-free excurrent air line then passed through an Ametek (Paoli, PA) S-II-A oxygen analyzer.
Tb was monitored continuously by means of a 3638-gauge thermocouple inserted 12 cm into the cloaca, the depth depending on the size of the chick. The thermocouple was passed through a hole in a small plastic disk to the desired length and fixed in place with cyanoacrylate glue. The thermocouple was then inserted into the cloaca and feathers surrounding the cloaca were folded over, and glued to, the outside edge of the disk. In most cases, this arrangement held the thermocouple in place throughout metabolism trials lasting 40 min to 2 h.
Electrical outputs from the mass flow controller, dew point hygrometer, oxygen analyzer and thermocouples were monitored in real time by a Campbell Scientific (Logan, UT, USA) CR10 or CR21 data logger. Data were acquired every 1 or 2 min throughout each trial.
Calculations
We estimated instantaneous oxygen consumption from the equation of
Bartholomew et al. (1981):
![]() | (1) |
We calculated rate of oxygen consumption by equation 4a of Withers
(1977):
![]() | (2) |
We smoothed values of oxygen consumption by calculating moving averages based on windows of 5 values (5 or 10 min). Considering the length of the runs, the window of 5 measurements gave excellent smoothed results, but was also sensitive to rapid changes in oxygen consumption at the beginning and ends of runs.
![]() |
Results |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Fig. 1 presents the results
of a downup (DU) metabolism protocol for a 5-day old dunlin
Calidris alpina chick weighing 15.6 g on July 4, 1996. Chamber
temperature (Ta), body temperature
(Tb), and estimated instantaneous oxygen consumption
[O2(eq)] are
plotted as a function of time. This record is typical and representative of
chicks showing a moderate capacity to regulate Tb, and it
will be used to illustrate basic characteristics of the DU metabolism records.
In this particular case, the chick was able to maintain Tb
as Ta decreased to approximately 20°C, at which point
metabolic rate reached a peak and the body began to cool with further decrease
in Ta. As Tb continued to decrease,
metabolism also slowed. The rewarming phase of the DU protocol was initially
accompanied by continued body cooling in response to the still large gradient
between chamber and Tb. As Ta rose
further, the chick's Tb began to increase and metabolism
also began to increase, but not to the levels attained during the cooling
phase of the protocol.
|
We explored the dynamic nature of the relationship between
Ta, Tb and metabolism by plotting
ln(O2)
versus Tb (Fig.
2), which allowed us to determine the logarithmic relationship
(Q10) between maximum metabolic rate (MMR) during cooling
and Tb. During the cooling phase, metabolism reached an
upper limit that decreased with declining Tb. A straight
line fitted to this curve describes the temperature dependence of the
relationship, which can be converted to a Q10 value. In
Fig. 2A, the line fitted to the
linearly decreasing portion of the data (filled symbols) represents the
Tb-adjusted MMR. This line has the equation
ln(
O2)=-1.95+0.073Tb.
The slope (b) of the line (0.073) is equivalent to a
Q10 of e10b, or 2.08. Having
determined the temperature dependence of MMR, we calculated all values of
O2 relative to
this value to give
O2(adj)
(Fig. 2B). In this case,
O2(adj) is
approximately 1 for maximum metabolism and about 0.5 for resting metabolism at
thermoneutral Ta (RMR).
|
The relationships plotted in Fig.
2A,B suggest three phases of response during the DU protocol
regime. Phase I is a graded increase in oxygen consumption that is directly
related to the increasing temperature gradient and also to decreasing
Tb. Phase I continues until metabolism reaches a maximum
level. Phase II pertains to the period during which metabolism is maintained
at maximum
O2(adj) as the
chick cools. Once Ta during the rewarming phase approaches
Tb, Phase II ends and Phase III begins, with
O2(adj)
decreasing from peak to the resting level even though the chick's
Tb remains well below normal during this phase.
Two additional relationships shown in
Fig. 3 for the 5-day old dunlin
chick characterize different aspects of the metabolic response in the DU
protocol. These are the adjusted
O2 plotted as a
function of the temperature gradient
(
T=Tb-Ta) in
Fig. 3B and the rate of change
in body temperature (
Tb) in
Fig. 3C. The relationship of
O2(adj) to
(Tb) (Fig.
2B) is repeated in Fig.
3A for comparison. The relationship between
O2(adj) and
T is approximately linear through the range of
T values during the cooling part of the protocol
(Fig. 3B). The relationship
between
O2(adj)
and Tb, however, exhibits a marked decrease in slope below
Tb=39°C (Fig.
3A). Peak
O2(adj) is
reached at about the maximum
T
(Fig. 3B), suggesting that
O2(adj) is
responsive to the temperature gradient rather than to body temperature.
|
As the chick's body continues to cool, the temperature gradient decreases,
and the rate of body cooling at first increases (Tb
more negative) and then decreases (Fig.
3C). The transition between Phase II and Phase III occurs at the
point at which
T=0 (Fig.
3B) and Tb begins to increase
(Fig. 3A). Absolute oxygen
consumption begins to increase during the rewarming phase when
T is between 0 and 5°C
(Fig. 1). However, because the
chick's body temperature is beginning to increase at this time,
O2(adj) actually
decreases continually through Phase III. Metabolism during rewarming never
achieves the level seen during cooling. This hysteresis occurs whether
metabolism is portrayed with respect to Tb,
T or
Tb
(Fig. 3).
A second example, that of a 7-day old lesser yellowlegs Tringa
flavipes chick weighing 26.1 g on 02 July 1996, shows a similar
hysteresis in the relationship between
O2 and
Tb (Fig.
4). During Phase I, the chick attempted to defend
Tb, and
O2(adj)
increased in proportion to the temperature gradient
(Fig. 4C).
Tb decreased slowly (about 0.03 °C min-1)
during stage I Fig. 4D).
Nonetheless, although the chick appeared to have sufficient metabolic capacity
to maintain a constant Tb through much of the cooling
phase of the protocol, it did not do so. It is possible that because
Ta decreased constantly during the cooling period, the
chick's metabolic response may have lagged behind. This explanation would
apply if metabolic rate were adjusted in response to the
T
(=Tb-Ta) and not
Tb. As in the dunlin chick,
O2(adj)
increased in response to decreasing Tb
(Fig. 4B). In both cases, the
increase in metabolism with respect to Tb slows below
approx. Tb=39°C as
O2(adj)
approaches the MMR (Figs 2B and
4B). This suggests that
metabolic rate is not directly responsive to body temperature. In contrast,
O2(adj) is
linear with respect to
T throughout the cooling phase
(Fig. 4C). During Phase I, the
metabolic rate continued to increase in parallel with the increasing
temperature gradient in the chamber until the maximum
O2(adj) was
reached. At that point, the bird entered Phase II, which is a period of
maximum, but inadequate, metabolic heat production.
|
During Phase II, Tb dropped at an increasing rate, and
maximum metabolic rate decreased in accord with the Q10.
The slope of the
ln(O2)
versus Tb relationship during Phase II (which is generally
brief), is 0.0274, which is equivalent to a Q10 of approx.
1.32 (Fig. 4A). After
Tb decreased to approx. 32°C, the chamber temperature
was increased, and the rate of decrease in Tb slowed and
then gradually increased towards 0. At this point, the chick still appeared to
be in Phase II, hence at maximum metabolism. When Ta
became high enough, however, the gradient between Tb and
Ta decreased and the chick's metabolism was sufficient to
cause an increase in Tb
(
Tb>0). At this point, the bird entered Phase
III, the warming phase.
During Phase III, metabolism gradually declined towards the resting rate.
O2(adj) varied
as a linear function of Tb during this phase, decreasing
to RMR as Tb approached the level seen in chicks under
thermoneutral conditions, that is, at the beginning of the DU protocol (Figs
3A,
4B). During this period of
rewarming, the chick appeared to adopt a conservative strategy of energy
expenditure. Temperature gradients
(
T=Tb-Ta) were close
to 0 (Fig. 4C) and probably of
little use for estimating the appropriate metabolism for returning
Tb to the control point.
A model of metabolic control
The foregoing examples suggest that the level at which a chick defends
Tb during Phase I depends on the temperature gradient and
can be evaluated by the relationship
![]() | (3) |
The results of DU protocols, which exhibit the patterns shown above in all
young shorebird chicks, lead us to propose the following scenario. In response
to cold challenge, a chick increases its metabolism to a peak value dependent
on Tb, which is maintained until Tb
begins to increase again. The metabolic rate during initial exposure to cold
(Phase I) apparently responds to the difference between Tb
and Ta. As Tb decreases below
3738°C, metabolism remains at a maximum level as long as
Tb continues to decline at least to 32°C (Phase II).
During the rewarming phase (III), temperature-adjusted metabolism decreases to
resting level so that rewarming is largely passive. This difference between
the cooling (I) and warming (III) phases creates a hysteresis in the
relationship between both
O2 and
O2(adj) and
Tb.
This pattern raises a number of questions about the dynamics of metabolic
responses of shorebird chicks to Ta. (1) What stimulates
the initiation of Phase I? Metabolism rises rapidly with only a small rate of
decrease in Tb, and the level of metabolism is directly
related to the ambient-body temperature gradient (T). (2) Why
is
O2(adj)
directly related to
T? This might have been fortuitous in our
studies if metabolism and the temperature gradient increased at the same rate.
(3) If a chick defended its Tb, why did it take so long to
increase
O2 to a
maximum level? This required about 34 min in the case of the dunlin chick
portrayed in Figs 1,
2,
3. The cold-challenge
experiments described below suggest that metabolism can increase much more
rapidly than observed in the DU experiments. (4) Why would chicks use a graded
response of metabolism to
T, especially if it is not
sufficient to maintain Tb? Do shorebird chicks employ a
strategy of controlled cooling even when they may be capable of maintaining
Tb? Such a strategy might optimize the rate of body
cooling to prolong feeding time at modest energy expenditure. (5) Does passive
rewarming, which is indicated by the DU metabolism protocol, mean that chicks
are adapted to warming under the brooding parent at low metabolic cost? How
much shorter would the warming period be if they were to keep their metabolism
at peak? The further experiments described below do not answer all these
questions, but they do provide additional insights into a shorebird chick's
metabolic response to temperature.
Cold challenge experiment
For a small number of chicks, after equilibration at a thermoneutral
temperature, the metabolism chamber was placed in a freezer or coolant bath at
ca. -20°C. Under these conditions, the rate of decrease in
Ta initially was approximately 2°C m-1, or
four times faster than the standard DU protocol. The outcomes of regular DU
and cold challenge (CC) experiments are shown for two 5-day-old dowitcher
chicks, both weighing approximately 24 g, in
Fig. 5. The shape of the
metabolismtemperature response curve in the DU trial
(Fig. 5A) is similar to that of
the dunlin and yellowlegs chicks described above, with the three phases
clearly identifiable. In the DU trial involving the dowitcher chick,
Tb was allowed to drop to 27°C and the absolute level
of oxygen consumption dropped to slightly below the resting level at high
Tb, even though the chick was presumably continuing to
defend Tb. During the rewarming phase, metabolic rate for
a particular Tb dropped below that of the cooling phase
and returned gradually to the resting metabolic level.
|
The CC experiment was stopped after Tb began to drop
rapidly and O2
had risen above that of the DU chick. The CC trial showed that a chick could
increase its rate of metabolism more rapidly than observed in the DU protocol
(Fig. 5B). This result was
consistently repeated in other CC trials. Thus, the failure of chicks to
regulate Tb in the face of ambient cooling is not a
consequence of the rate at which metabolism can respond to cold stress. The
relationship between Tb and metabolism differed slightly
between the two chicks (Fig.
5C), but they maintained similar linear relationships between
metabolism and the temperature gradient
(Tb-Ta) regardless of how rapidly
Ta decreased (Fig.
5D). As in other trials, elevation of metabolism above the resting
level was not strongly related to the rate of change in Tb
(data not shown). Thus, the CC trials strengthen the idea that metabolism
during cooling is responsive to the
Tb-Ta gradient.
In most of the CC trials, chicks elevated their metabolic rates to higher
levels than observed in DU trials (data not shown). Development of higher
temperature gradients in the CC trials before chick Tb had
decreased to the maximum seen in DU trials could explain this result.
Accordingly, CC chicks would generate more heat at a given T
because of their higher Tb. However, the comparisons in
Fig. 5C,D do not support this
idea because metabolism at a given Tb or
T
is actually somewhat lower in the CC chick than in the DU chick. The slope of
the
O2(adj)
versus
T regression (a) for three CC
dowitcher chicks aged 5 and 6 days varied between 0.102 and 0.137; this range
includes the value for the 5-day-old DU chick in
Fig. 5 (0.117) and does not
differ from the regression of a versus body mass for a larger sample
of DU trials in several species, mentioned above. Instead, metabolism appears
to increase to a higher level in the CC chick because a larger temperature
gradient is achieved. This occurs because Ta decreases
much faster than Tb in the CC protocol, generating higher
maximum
T even though the minimum Ta did
not vary between the protocols (1.5 versus 2.1°C).
Protracted cooling experiment
To determine the effect of a slow rate of decrease of
Ta on metabolism, we subjected an 8-day-old lesser
yellowlegs chick weighing 24.8 g to a protracted DU protocol. In this case,
Ta decreased at a rate of 0.18 °C min-1
between 30.1 and 13.6°C and the cooling period lasted 94 min, compared to
less than 50 min for normal DU trials (Fig.
6). The chick is compared to two 7-day old yellowlegs chicks
weighing 22.5 and 26.1 g, which achieved maximum metabolic rates in DU
protocols of 2.55 and 3.01 ml min-1 at Tb=37.35
and 37.20°C and T=29 and 32°C, respectively. Several
aspects of the protracted cooling trial are noteworthy. (1) The metabolic
response of this chick was erratic, with large swings in metabolism occurring
at Tb close to 36°C. (2) The data indicated no obvious
maximum metabolic rate. However, MMR occasionally approached that of the two
chicks in the DU trials even though
T did not exceed 21°C.
(3) During the cooling phase, the response of
O2 to the
temperature gradient was consistent with that observed during faster cooling,
whereas the response to Tb was not. Specifically,
a was 0.121 and 0.125 for the two DU chicks, which compared favorably
with the value of 0.157 obtained in the slow cooling trial. This is consistent
with the hypothesis that chicks elevate metabolic rate during cooling (Phase
I) in response to
T. (4) The rate of decrease in
Tb during Phase II (approximately 0.09°C
min-1) was one-third of the rate typical for chicks of similar age
(approximately 0.28 and 0.34°C min-1) during DU protocols. This
suggests that the metabolic response to the increasing temperature gradient
was more nearly adequate to maintain Tb, perhaps owing to
a reduced time lag with respect to rate of decrease in Ta.
(5) During the warming phase (III), the response of
O2(adj) to
Tb was similar to that during faster warming, whereas the
response to the temperature gradient was not (results not shown). This is
consistent with the hypothesis that chicks adjust metabolic rate during
warming in response to Tb. This particular yellowlegs
chick lacked a distinct Phase II during the slow-cooling protocol, but phases
I and III were similar to those obtained during the normal DU protocol.
|
Cold plunge experiment
To further explore the dynamics of cold-induced metabolism, we attempted to
separate the effects of Tb and the temperature gradient by
placing a chick directly into a pre-cooled metabolism chamber, where it would
experience a high temperature gradient before Tb decreased
substantially. The trial involved a 3-day-old dunlin chick weighing 12.4 g and
it is compared to a DU trial for a similarly aged chick weighing 14 g
(Fig. 7). The
Tb of the cold-plunge chick had declined to about 36°C
by the time the metabolism chamber had equilibrated, but its metabolic rate
was nonetheless higher than that of the DU chick at the same
Tb. In this case, the difference was associated with a
higher T in the cold-plunge experiment
(Fig. 7B).
|
Endurance experiment
One of the characteristics of the DU protocol is that chicks tend to cool
continuously through trials even before reaching maximum metabolic rate. This
apparently reflects a lag in the metabolic response to heat loss, which
results in a chick not generating enough heat to replace losses even though it
is metabolically capable of doing so. To test this idea and also to determine
whether a chick could sustain a high level of metabolic activity, we subjected
two 1-day old Hudsonian godwit Limosa haemastica chicks to a modified
cooling protocol. In this protocol, Ta was decreased to a
level that would have stimulated about 60% of peak metabolism (ca. 25°C)
and was maintained for up to 2 h (Fig.
8). 1-day-old godwit chicks are capable of increasing their
metabolism in DU trials to about 50% above resting metabolic rate in the
thermoneutral zone. The initial phase of the endurance experiment mimicked a
DU trial and the chicks responded in typical fashion. After
Ta had stabilized, both chicks maintained
Tb reasonably well, albeit with variation.
Tb of the lighter chick continued on a downward trend
until it appeared to level off at approximately 33°C
(Fig. 8A). The heavier chick
maintained its Tb at 3738°C throughout the
experiment (Fig. 8B). In both
trials, metabolism fluctuated widely despite the maintenance of a relatively
constant temperature gradient. Nonetheless, both chicks achieved levels of
metabolism (1.01.2 and 0.81.0 ml min-1, for the
heavier and lighter chicks, respectively) similar to the maximum rate of
godwit chicks of similar age and size in the DU protocols. Indeed, the heavier
chick was able to increase its oxygen consumption substantially when
Ta was reduced at the end of the endurance trial after
nearly 2 h at ca. 25°C (Fig.
8B).
|
![]() |
Discussion |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
The DU protocol produced a temperaturemetabolism response with three
phases: (I) cooling, (II) maximum metabolism, and (III) rewarming. During the
cooling phase, metabolism increases in direct proportion to the temperature
gradient between the chick's body and its environment. Even though the chick
has additional metabolic capacity to generate heat until the end of this
phase, Tb continually drops. The second phase begins when
the chick reaches its maximum metabolic rate (MMR), which then proceeds to
decline with a Q10 of approximately 2 as the body cools
further. The third phase begins as soon as Tb begins to
increase. During this rewarming phase,
O2 remains more
or less unchanged at a low level until the chick's Tb
regains the normal range of approximately 3840°C. When metabolism
is corrected for Tb using an appropriate
Q10,
O2(adj) remains
constant during the second phase, as it must because the temperature
sensitivity of MMR is determined from these data. During Phase III, however,
O2(adj)
decreases steadily with increasing Tb until it drops to
the normal resting metabolic rate when Tb reaches a high
value after rewarming.
The DU protocol reveals a hysteresis in the response of metabolism to Tb and Ta during the cooling (I and II) versus rewarming (III) phases. It is tempting to draw a parallel between the cooling and rewarming phases in the laboratory and the foraging and brooding periods in nature. If this were the case, it would appear that chicks defend Tb against cooling while they are foraging, but warm up passively when they are being brooded. We have not yet undertaken an economic analysis of this as an energy management strategy. Clearly, however, the added foraging time made possible by defending Tb must be compensated by the additional food gathered. Alternatively, passive warming prolongs the brooding spell, but the energy saved by the chick may more than compensate the lost foraging time without undue stress on the time budget of the brooding parent.
The regulation of body temperature and metabolism
To maintain a constant Tb, chicks must replace lost
heat by biochemical thermogenesis, which is thought to be primarily the
product of shivering of skeletal muscles
(Hohtola and Stevens, 1986;
Hohtola and Visser, 1998
). The
most straightforward signal for regulation of Tb would be
the departure of Tb from a set point. An alternative would
be to increase metabolism in proportion to the rate of decrease in
Tb. There is, however, no evidence that shorebird chicks
in the DU protocols adjusted metabolic rate to either Tb
or to rate of change in Tb. We presume that chicks have a
preferred Tb because they maintain relatively constant
Tb under thermoneutral conditions and even mild cold
stress. How they achieve this is less clear, but presumably the mechanism
involves a brain thermostat.
Tb in this study was measured in the cloaca, which is undoubtedly more variable than Tb measured in brain. It is possible that the observed hysteresis might be due to the cloacal temperature increasing less rapidly than that of the brain during the warming phase (III). However, this would require a very large temperature gradient within the body during the warm-up period, indeed much greater than that during the cooling phase of the DU protocol. We feel that this is unlikely.
When exposed to continually decreasing Ta, metabolism
increases in proportion to the difference between Tb and
Ta discounted by the effects of reduced
Tb on physiological processes. This relationship is
apparently the same whether the rate of change in T or
Tb is fast or slow, as shown by comparing the DU, cold
challenge, and slow cooling protocols in this study. Tb
itself does not predict metabolism well. In multiple regressions of metabolism
as a function of both Tb and
T during
Phase I (not shown), Tb is never a significant effect and
its trend is positive rather than negative, reflecting the
Q10 effect of increasing Tb on
metabolism. Results of the DU protocol consistently suggest that metabolism
responds to the perception of the temperature gradient between the body and
the surrounding air, which depends on peripheral temperature sensors
(Calder and King, 1974
).
As explained above, Tb decreases during the DU trials even before chicks reach their maximum metabolic rate. This failure to regulate Tb might derive from the lag between the sensation of a particular temperature gradient and the elevation of metabolism to balance the resulting heat loss. By the time metabolism has increased, Ta has decreased further and the response is therefore not sufficient to prevent a decrease in Tb. When Ta is maintained at a constant level, chicks are able to maintain constant Tb, although this may be considerably below the preferred temperature in thermoneutral conditions.
As Ta decreases and T increases,
metabolism eventually reaches a maximum level, typically twice the resting
level in shorebird chicks (Visser and
Ricklefs, 1993
). With further decline in Ta,
metabolic rate begins to decrease as Tb declines further,
owing to the Q10 effect. If one assumes that metabolism is
maintained at a maximum level during the second phase of the DU protocol, then
it is possible to estimate the magnitude of the Q10 effect
by a nonlinear regression. Among 27 shorebird chicks in this study, the value
averaged about 2, which is typical for physiological processes
(Williams and Ricklefs, 1984
).
Thus, we may interpret Phase II of the DU protocol as a period during which
chicks are stimulated to maximum thermogenesis, which is
Tb-dependent.
Assuming a Q10 of about 2, it is possible to calculate
an adjusted O2
for an arbitrary Tb and examine the course of metabolism
independently of the Tb effect. Because we have estimated
the Q10 primarily from the slope of the maximum metabolism
on Tb,
O2(adj) exhibits
a horizontal plateau through Phase II.
Over the range of Tb values produced in our DU trials,
the transition between Phase II and Phase III is evidently signaled by an
increase in Tb. This is shown quite clearly by comparing
the point of hysteresis of metabolism as a function of Tb
and T (e.g. in Figs
3 and
4). The change in phase is
clearly associated with a reversal of the Tb increment
rather than a particular value of
T. After the onset of Phase
III, metabolism is no longer sensitive to
T, but rather
depends on Tb itself,
O2(adj)
declining in a nearly linear fashion towards the resting metabolic rate as the
preferred Tb is approached. Alternatively,
O2(adj) might
have dropped to the resting level as soon as the chick began to warm up. It is
unclear, however, whether the juncture of Phases II and III represents
`maximum' or `resting' metabolism. At this point, the rate of oxygen
consumption may represent the rate of tissue metabolism in the absence of
shivering thermogenesis, which is then not activated during the warm-up
period.
Estimating maximum metabolism from downup protocols
The original motivation of this study was to develop a protocol for
estimating the maximum metabolic rate under cold stress as an index of the
developmental maturity and size of skeletal muscles. It is clear from the
present analyses that maximum metabolism depends on Tb and
that it should be corrected to a reference Tb for
comparison between species. The reason for this is that different species and
different protocols will produce different patterns of metabolism and
Tb, such that absolute metabolism is difficult to compare
between species, ages and studies.
The proper adjustment of the maximum metabolic rate with respect to Tb can be determined by plotting the relationship between metabolism and Tb during Phase II of the protocol. Assuming that this represents MMR, one can then extrapolate the line to a reference temperature (e.g. 40°C) to determine the Tb-adjusted MMR. Alternatively, one could assume a Q10 of about 2.0 and adjust a single value of MMR accordingly to a reference Tb. The two methods give nearly identical results. Clearly, it is essential that Tb be recorded continuously to obtain proper measurements of MMR. One can see from several of the data plots in this study (e.g. Figs 2A, 4A) that adjusted MMR will exceed the highest measured metabolic rate, often by 20% or more. The adjusted MMR represents the metabolic capacity of the chick at a high Tb. It also represents the amount of heat that the chick is capable of producing to defend that Tb in a constant cold environment. When conductance is known, it is then possible to estimate the lowest Ta at which a chick can maintain a high Tb. Thus, the adjusted MMR represents a physiological benchmark for comparison among species.
Conclusions and recommendations
The results of our experiments have clear implications for the measurement
of maximum metabolic rate, at least in young birds with labile body
temperatures. Reliable estimates of MMR that are comparable among species and
experimental conditions require simultaneous measurement of
Tb to adjust metabolic rate with respect to body
temperature. In our experiments, the difference between measured MMR and the
MMR extrapolated to a normal thermoneutral Tb was as much
as 20%. Because the temporal pattern of exposure of chicks to ambient
temperature differs between natural and experimental conditions, this
variation should be thought of as an error resulting from the particular
experimental protocol.
Our finding of a marked hysteresis in metabolism between the cooling and warming phases of the downup protocol suggests that passive warming may be a key attribute of the energy management of shorebird chicks under natural conditions. Temperature cycles of chicks have not been well characterized, but rates of heating during the brooding phase of the foraging cycle might provide an indication of the rate of heat accumulation by the chick and the sources of this heat. Such measurements would require telemetry of body temperature and surface temperature gradients and might be attempted initially with chicks under penned conditions using artificial brooders designed to mimic parent birds.
The results of our experiments also raise the issue of how shorebird chicks
sense temperature to adjust their metabolism. The strong correlation between
metabolism and body-ambient temperature gradients during Phase I of the
downup protocol suggests that birds might use peripheral temperature
receptors to detect heat loss before core body temperature decreases, and
adjust their metabolic response according to the temperature gradient. This
hypothesis could be explored by recording the metabolic response to localized
heating and cooling of regions of the skin under different ambient temperature
protocols. Different cooling protocols might also allow the separation of the
effects of T, Tb, and
Tb
statistically. Ultimately, it will be necessary to measure brain temperature
and cloacal and other peripheral temperatures simultaneously to disentangle
the interplay between central and peripheral inputs.
![]() |
Acknowledgments |
---|
![]() |
References |
---|
![]() ![]() ![]() ![]() ![]() ![]() ![]() |
---|
Aulie, A. (1976). The pectoral muscles and the development of thermoregulation in chicks of willow ptarmigan (Lagopus lagopus). Comp. Biochem. Physiol. 53A,343 -346.
Aulie, A. and Steen, J. B. (1976). Thermoregulation and muscular development in cold exposed willow ptarmigan chicks (Lagopus lagopus L.). Comp. Biochem. Physiol. 55A,291 -295.[CrossRef]
Bartholomew, G. A., Vleck, D. and Vleck, C. M. (1981). Instantaneous measurements of oxygen consumption during pre-flight warmup and post-flight cooling in sphingid and saturniid moths. J. Exp. Biol. 90,17 -32.
Bech, C., Mehlum, F. and Haftorn, S. (1991). Thermoregulatory abilities in chicks of the antarctic petrel (Thassaloica antarctica). Polar Biol. 11,233 -238.
Beintema, A. J. and Visser, G. H. (1989). The effect of weather on time budgets and development of chicks of meadow birds. Ardea 77,181 -192.
Calder, W. A., III and King, J. R. (1974). Thermal and caloric relations of birds. In Avian Biology, vol. 4 (ed. D. S. Farner and J. R. King), pp. 259-413. London: Academic Press.
Chappell, M. A. (1980). Thermal energetics of chicks of arctic-breeding shorebirds. Comp. Biochem. Physiol. 65A,311 -317.[CrossRef]
Choi, I. H., Ricklefs, R. E. and Shea, R. E. (1993). Skeletal muscle growth, enzyme activities, and the development of thermogenesis: a comparison between altricial and precocial birds. Physiol. Zool. 66,455 -473.
Hohtola, E. and Stevens, E. D. (1986). The relationship of muscle electrical activity, tremor and heat production to shivering thermogenesis in Japanese quail. J. Exp. Biol. 125,119 -135.[Abstract]
Hohtola, E. and Visser, G. H. (1998). Development of locomotion and endothermy in altricial and precocial birds. In Avian Growth and Development. Evolution within the AltricialPrecocial Spectrum (ed. J. M. Starck and R. E. Ricklefs), pp. 157-173. New York: Oxford University Press.
King, J. R. and Farner, D. S. (1961). Energy metabolism, thermoregulation and body temperature. In Biology and Comparative Physiology of Birds, vol. II (ed. A. J. Marshall), pp. 215-288. New York: Academic Press.
Koteja, P. (1996). Measuring energy metabolism with open-flow respirometric systems: which design to choose. Funct. Ecol. 10,675 -677.
Krijgsveld, K. L., Olson, J. M. and Ricklefs, R. E. (2001). Catabolic capacity of the muscles of shorebird chicks: maturation of function in relation to body size. Physiol. Biochem. Zool. 74,250 -260.[CrossRef][Medline]
Levy, A. (1964). The accuracy of the bubble meter method for gas flow measurements. J. Sci. Instrument. 41,449 -453.[CrossRef]
Marjoniemi, K. and Hohtola, E. (1999). Shivering thermogenesis in leg and breast muscles of galliform chicks and nestlings of the domestic pigeon. Physiol. Biochem. Zool. 72,484 -492.[CrossRef][Medline]
Marsh, R. L. and Wickler, S. J. (1982). The role of muscle development in the transition to endothermy in nestling bank swallows, Riparia riparia. J. Comp. Physiol. B 149,99 -105.
Norton, D. W. (1973). Ecological Energetics of Calidrine Sandpipers Breeding in Northern Alaska. Fairbanks, Alaska: University of Alaska.
Olson, J. M. (1994). The ontogeny of shivering
thermogenesis in the red-winged blackbird (Agelaius phoeniceus).
J. Exp. Biol. 191,59
-88.
Porter, W. P. (1969). Thermal radiation in metabolic chambers. Science 1661,115 -117.
Ricklefs, R. E. (1974). Energetics of reproduction in birds. In Avian Energetics (ed. R. A. Paynter, Jr), pp. 152-292. Cambridge, Massachusetts: Nuttall Ornithological Club.
Scholander, P. F., Hock, R., Walters, V., Johnson, F. and Irving, L. (1950). Heat regulation in some arctic and tropical mammals and birds. Biol. Bull. 99,237 -258.
Visser, G. H. (1998). Development of temperature regulation. In Avian Growth and Development. Evolution within the Altricial-Precocial Spectrum (ed. J. M. Starck and R. E. Ricklefs), pp. 117-156. New York: Oxford University Press.
Visser, G. H. and Ricklefs, R. E. (1993). Development of temperature regulation in shorebirds. Physiol. Zool. 66,771 -792.
Williams, J. B. and Ricklefs, R. E. (1984). Egg temperature and embryo metabolism in some high-latitude procellariiform birds. Physiol. Zool. 57,118 -127.
Withers, P. C. (1977). Measurements of
O2,
CO2 and
evaporative water loss with a flow-through mask. J. Appl.
Physiol. 42,120
-123.
Related articles in JEB: