Metabolic responses of shorebird chicks to cold stress: hysteresis of cooling and warming phases.

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 T(b) had declined typically to 32-34 degrees C. After this point, we gradually increased the temperature in the metabolism chamber until normal T(b) values and thermoneutral resting metabolism were restored. We refer to this cycle as the down-up (DU) protocol. We estimated instantaneous oxygen consumption ((O(2))) using the equation of Bartholomew et al. (1981). (O(2)) and T(b) 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 T(b). Chicks subjected to the DU protocol exhibited three distinct phases of metabolic response to ambient temperature (T(a)). In Phase I, (O(2)) increase was directly related to the gradient between T(b) and T(a), consistent with a Newtonian response to cooling. During Phase II, chicks sustained a maximum level of (O(2)) that decreased as T(b) dropped, exhibiting a Q(10) of approximately 2. Based on the slope of the relationship between (O(2)) and T(b) during Phase II, we were able to estimate maximum (O(2)) at a standardized high T(b). Phase II continued until chick T(b) began to rise as a result of the gradually increasing T(a). During Phase III, the T(b)-adjusted rate of oxygen consumption decreased from the maximum level at low T(b) to the resting level at high T(b) in the thermoneutral zone. Further trials with faster and slower rates of chamber cooling showed that (O(2)) during Phase I varied in proportion to the difference between T(b) and T(a) (DeltaT), whereas during Phase III it responded to T(b). Even though chicks may be capable of generating enough heat to regulate T(b) during the early part of Phase I of the DU protocol, the constantly decreasing T(a) created a time lag between T(a) 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 T(b) experienced during active foraging. The results of the DU protocol suggest that T(b) should be measured continuously during measurements of maximum oxygen consumption, and that peak values should be adjusted by T(b) to make them comparable with other studies.