Figure 1. System for recording the metabolism of free-moving mice under controlled ambient temperature.

(a) A block diagram of the thermoregulatory system in mammals when the animal is not moving, therefore assumed to exert no external work. The upper loop and the lower loop represent the heat loss and heat production loop, respectively. The time derivative of body temperature (T_B) is derived from the difference of heat production (Q_in) and heat loss (Q_out) divided by the thermal capacity (C). Q_out is derived from the difference of ambient temperature (T_A) and T_B multiplied by heat conductance (G). Q_in is derived from the difference of theoretically defined set-point temperature (T_R) and T_Bmultiplied by H, which is the open-loop gain of the thermoregulatory feedback system. (b) A system for evaluating the metabolism of free-moving mice. The temperature-controlled animal chamber (left panel) and the inside of the chamber (middle panel), in which four mice can be recorded at once, are shown. Each animal had an intraperitoneally implanted body-temperature transmitter (right panel, top). Each animal was housed in a metabolic chamber and the VO₂ were recorded by gas mass spectrometry. (c) A representative recording of mouse metabolism for three consecutive days. The animal was placed in the chamber, and the T_A was maintained at 16 °C. Once the mouse was placed in the metabolic chamber, there was no physical contact with researchers during the recording period. Note the clear circadian rhythm seen in the T_B, VO₂ and locomotion. Yellow shading shows the light-on period. (d) A representative recording of metabolism during fasting-induced daily torpor. The mouse was placed in the chamber for three days; food was removed on the second day (filled triangle). The T_A was maintained at 16 °C. Daily torpor started during the latter half of the second day. The mouse returned to a euthermic state immediately after the food was returned to the chamber (unfilled triangle). Yellow shading shows the light-on period.