Endothermic Animals

Endothermic vertebrate animals have a fundamentally different relationship between Ta and Tb than ectothermic animals and plants as a consequence of ther-moregulatory thermogenesis. Thermal and energetic consequences of torpor are therefore more complex for endotherms because at low Ta their MR is normally increased above basal (BMR) by metabolic thermogenesis that maintains Tb constant (normothermia). During torpor, there is a profound decrease in MR, typically to 1% or even less of normothermic MR, and a concomitant decrease in Tb often close to Ta (Figure 4). Entry into torpor appears to be a controlled physiological process, not simply an inability to thermoregulate. During torpor at moderate to high Ta's, Tb declines to nearly Ta and MR declines exponentially with Tb. This is the same pattern as for ectotherms, and indicates a state of nonthermore-gulation. However, if Tb decreases below a species-specific set point at lowered Ta, then Tb is regulated at that set point by the onset of thermogenesis; this is the same as the normothermic thermoregulatory response except that the Tb set point is lower than for normother-mia. For many single-day torpidators the torpor set point is about 20 °C, but it is generally much lower for

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Figure 4 A daily torpor cycle in a typical single-day torpidator, the dunnart Sminthopsis macroura, showing the decline in Tb and MR during entry into torpor, a short period of sustained torpor, and then the increase in Tb and MR during arousal from torpor. Data from F. Geiser, unpublished data.

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Time of day

Figure 4 A daily torpor cycle in a typical single-day torpidator, the dunnart Sminthopsis macroura, showing the decline in Tb and MR during entry into torpor, a short period of sustained torpor, and then the increase in Tb and MR during arousal from torpor. Data from F. Geiser, unpublished data.

hibernators (about 0-5 °C, but as low as —5 °C for arctic ground squirrels).

Two mechanisms contribute to the marked decline in MR of endotherms during torpor. Defense of normal body temperature is relaxed and so the thermogenic increment in MR above BMR is eliminated. As a consequence, heat production is less than heat loss and Tb declines to close to Ta and so there is a further decline in MR due to the Q±0 effect. However, if Ta decreases below the torpor set point where Tb is again defended, then MR increases for thermogenesis. For most endotherms, the decline in MR during torpor is accounted for by the elimination of the thermogenic MR increment and the decline in Tb and MR with a typical Qj0 (~2.5).

Intrinsic metabolic depression is a third possible mechanism contributing to MR reduction during torpor. However, the contributions of the elimination of thermogenic MR increment and the decline in Tb and Qi0 effect are so great that the contribution of intrinsic metabolic depression, should it occur in an endotherm, would be a relatively minor absolute energy saving.

Arousal from torpor is typically a physiologically driven event requiring considerable thermogenesis by shivering (skeletal muscle thermogenesis) or metabolism of specialized brown adipose tissue (in placental mammals but not marsupials or birds). There is also increasing evidence that many species use passive rewarming (e.g., basking) to arouse, since it greatly reduces the metabolic cost of arousal. Long-term hibernation by mammals is not necessarily a continuous period of prolonged inactivity. It is periodically broken by a short period of arousal, then re-entry into hibernation (e.g., Figure 2). The reason for these periods of arousal and re-entry is not clear. There appears to be some physiological 'need' to periodically arouse. It has been suggested that perhaps some accumulated metabolite needs to be eliminated by urination, which only occurs if the animal is normothermic.

The beneficial energy savings of torpor are clearly evident from the difference between the high normother-mic MR and the greatly reduced torpid MR, even after accounting for the metabolic cost of arousal. For daily torpor, the energy saving depends on the length of the torpor bout and the depth of torpor; for a dunnart, the daily energy saving is about 36% for 13 h of torpor (e.g., Figure 4). For hibernation, the daily energy saving is greater because metabolic rate is low for typically 24 h per day; for a hibernating ground squirrel, the energy saving is about 85% over 6 months.

There is a complex pattern of single-day torpor, multiday hibernation, and multiday estivation amongst mammals and birds (Table 1) that partly reflects phylogeny but also body mass. Torpor is more advantageous for small than large species. Small species have a higher mass-specific metabolic rate and therefore benefit more from the energetic saving associated with torpor. The rate of entry into and arousal from torpor is strongly dependent on body mass. Small species enter torpor quicker because of their higher thermal conductance (higher surface-to-volume ratio) and they also arouse quicker because of their higher mass-specific MR and lower thermal inertia. In contrast, larger species cool and rewarm slower, so the energy savings are less, especially for daily torpor.

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