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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
We developed a protocol for determining the maximum rate of oxygen
consumption of shorebird chicks (Scolopacidae and Charadriidae) in response to
cold challenge. We first subjected the chicks to gradually decreasing
temperatures until their metabolism peaked and began to decrease. We ended the
cooling phase of a trial when a chick's body temperature
Tb had declined typically to 32–34°C. After this
point, we gradually increased the temperature in the metabolism chamber until
normal Tb values and thermoneutral resting metabolism were
restored. We refer to this cycle as the down–up (DU) protocol. We
estimated instantaneous oxygen consumption
(
O2) using the
equation of Bartholomew et al.
(1981).
O2 and
Tb were monitored continuously during the trials.
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
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  K. Phillips CHICKS ARE COOL ABOUT STAYING WARM J. Exp. Biol., August 15, 2003; 206(16): 2711 - 2711. [Full Text] [PDF]
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