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 (V . O∑) using the equation of Bartholomew et al. (1981). V . O∑ 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, V . O∑ 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 V . O∑ that decreased as Tb dropped, exhibiting a Q10 of approximately 2. Based on the slope of the relationship between V . O∑ and Tb during Phase II, we were able to estimate maximum V . O∑ 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 V . O∑ 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.
The Journal of Experimental Biology
Ricklefs, Robert and Williams, Joseph, "Metabolic Responses of Shorebird Chicks to Cold Stress: Hysteresis of Cooling and Warming Phases" (2003). Biology Department Faculty Works. 46.