Figure 1. Variation in energy requirement to keep a body at a fixed body temperature (T_b) as a function of ambient temperature. Theoretical energy requirements are shown in (a) for a body that has high thermal conductance (red line) and a body with low thermal conductance (blue line). These lines cross the abscissa at T_b. Endothermic animals are able to modulate their surface conductance by altering the properties of their pelage. The effect on energy requirements depends on the manner in which thermal conductance is modulated with changes in ambient temperature. Hence in (b) the animal decreases its thermal conductance as temperature declines from i to ii. The resultant line (thick green line) tracing the relation between energy requirement and ambient temperature follows a shallower curve than the lines for fixed thermal conductance and this line also extrapolates to a much higher temperature on the abscissa than T_b. In (c) the animal modulates its thermal conductance in the opposite direction – increasing conductance as temperature declines from i to ii. The resultant line relating energy requirement to ambient temperature is steeper (thick green line) and intersects the abscissa below T_b. Panel (d) illustrates the interaction between basal metabolic rate (BMR) and the thermal conductance. Where basal metabolism intersects the thermal conductance line, the animal is in energy balance (requirements met by demands) without the need to expend further energy on thermoregulation. These intersection points, called the lower critical temperatures (T_lcs), are at lower ambient temperatures when BMR is higher and when thermal conductance is lower.

Figure 2. Relationships between energy requirements and ambient temperature for animals regulating their body temperatures (T_b) at different levels (red line: T_b regulated at euthermic levels; blue line: torpid T_b regulated at 20°C; and orange line: torpid T_b regulated at 5°C). In all cases the patterns of variation in energy requirements as a function of temperature are similar, reflecting the fact that torpor is a regulated state of energy metabolism. The metabolic rate of torpid animals (TMR) reaches a minimum at an ambient temperature (T_a) below the point where T_a = T_b. Higher and lower ambient temperatures result in higher metabolic rates. Lower energy requirements follow the lines defined by torpid conductance. Abbreviation: T_lc, lower critical temperature.