Thermal Set Point Load Error and Temporal Variation

Body temperature is regulated through multiple temperature sensors (core and periphery) that fall into two groups, one responding with increasing activity to rising temperature and the other responding with increasing activity to falling temperature (Figure 5). Centrally, the coordinated control of Tb takes place mainly in the pre-optic/anterior hypothalamus. The load error is the minimal deviation of Tb from the thermal set-point that is tolerated by the system. Deviation from the thermal setpoint triggers a response from multiple effectors, adjusting heat gain or loss, as appropriate, to restore equilibrium. When the signal rates from the warm- and cold-responding sensors balance each other, Tb is at its thermal set-point and load error is zero. Load error is determined in part by the delay of the temperature sensors and the gain of the response by the various effectors. Any regulatory change in heat gain or loss is

20 25 30 35 40 Ambient temperature (°C)

Figure 4 Body temperatures as a function of ambient temperature during flight in three representative insects. Note that the temperature of the thorax (dark symbols) and, to a lesser extent, the head (light shading) are relatively independent of ambient temperature while that of the abdomen (open symbols) is not. Dashed line Tb = Ta. Drawn from data taken from May ML (1995) Simultaneous control of head and thoracic temperature by the green darner dragonfly Anax Junius (Odonata: Aeshnidae). The Journal of Experimental Biology 198: 2373-2384; Borrell BJ and Medeiros MJ (2004) Thermal stability and muscle efficiency in hovering orchid bees (Apidae: Euglossini). Journal of Experimental Biology 207: 2925-2933; and Hegel JR and Casey TM (1982) Thermoregulation and control of head temperature in the sphinx moth, Manduca Sexta. Journal of Experimental Biology 101: 1-15.

20 25 30 35 40 Ambient temperature (°C)

Figure 4 Body temperatures as a function of ambient temperature during flight in three representative insects. Note that the temperature of the thorax (dark symbols) and, to a lesser extent, the head (light shading) are relatively independent of ambient temperature while that of the abdomen (open symbols) is not. Dashed line Tb = Ta. Drawn from data taken from May ML (1995) Simultaneous control of head and thoracic temperature by the green darner dragonfly Anax Junius (Odonata: Aeshnidae). The Journal of Experimental Biology 198: 2373-2384; Borrell BJ and Medeiros MJ (2004) Thermal stability and muscle efficiency in hovering orchid bees (Apidae: Euglossini). Journal of Experimental Biology 207: 2925-2933; and Hegel JR and Casey TM (1982) Thermoregulation and control of head temperature in the sphinx moth, Manduca Sexta. Journal of Experimental Biology 101: 1-15.

Behavior Heat transfer Heat production

Set point +

Load

Thermal disturbance

Control elements

Heat gain Control action Heat loss

Thermal disturbance

Heat gain Control action Heat loss

Control

system

+ Feedback elements

- Feedback

elements

Figure 5 Dual controller model for the regulation of body temperature with a thermal set-point. One group of feedback elements (sensors) responds with increased activity to rising temperature while the other group responds with increased activity to falling temperature. By comparing the activity of both groups of sensors the load error is generated (the difference between set-point and Tb) and the control elements are activated in proportion to the load error. An increase in Tb results in dominance of warm sensor activity and the control elements (e.g., changes in behavior, heat transfer properties, or heat production) restore equilibrium by increasing heat loss. When the activity from warm and cold sensors equals each other the load error is zero and Tb is at its set-point. For Tb to be regulated the system requires Tb to be displaced from set-point. How far the system can be displaced from equilibrium establishes the load error that is tolerated for the system.

proportional to the load error and is generated by comparing the signals from both groups of sensors. In endothermic homeotherms the load error is generally small; deviations in Tb of a fraction of a degree elicit responses to return Tb toward set-point. Ectothermic homeotherms, on the other hand, often tolerate much larger variations but are still capable of good regulatory control of Tb. For example, varanid lizards, a highly

40 30 20

40 30 20

10 Rosenberg's goanna

60 120 180 240 300 360 420 Time (min)

10 Rosenberg's goanna

60 120 180 240 300 360 420 Time (min)

40 30 20 10

40 30 20 10

Figure 6 Body temperature (red line) in relation to sources of heat input as a function of time in the varanid lizard, Rosenberg's goanna. In (a) under constant cool conditions (Ta = blue line) but the animal was free to shuttle in and out from underneath a radiant heat source (black bar) and in (b) under natural conditions that occur in summer and winter. When provided with an appropriate radiant source of heat from the sun Rosenberg's goanna can quickly elevate Tb above Ta in the shade, achieved through dark skin coloration and adjustments in peripheral circulation that favor heat gain. Once the desired (set-point) temperature is achieved the lizard can regulate Tb through behavioral adjustments that affect heat transfer. (a) From Clark TD, Butler PJ, and Frappell PB (2006) Factors influencing the prediction of metabolic rate in a reptile. Functiona Ecology 20: 105-113. (b) Drawn from data obtained and modified from Christian KA and Weavers BW (1996) Thermoregulation of monitor lizards in Australia - An evaluation of methods in thermal biology. Ecologica Monographs 66: 139-157.

active group of reptiles, are able to behaviorally regulate their Tb between 30 and 36 °C on cold days when provided with an appropriate radiant source of heat (Figure 6).

Some ectothermic homeotherms have a smaller range in Tb. For example, ectothermic fish are not usually considered homeothermic, but many species spend 75% of their time within ±2 ° C of their preferred temperature. It has also been demonstrated that ectothermic sharks possess thermoreceptors that are capable of temperature resolution to 0.001 °C and can occupy narrow thermal niches.

Load error should not be confused with temporal adjustments that may occur in thermal set-point. Even in endotherms, Tb oscillates by as much as several degrees over nycthemeral and circadian cycles (Figure 7). The circadian rhythm of activity can accentuate the daily oscillations of Tb (Figure 8), though elevation of Tb by activity or exercise is not always indicative of a change in set-point. The thermal set-point is also known to change in accord with other cycles. During the human ovarian cycle, for example, the thermal set-point is about 0.5 °C higher in the luteal phase. In circannual cycles (e.g., seasons) the

3 4 Days

Figure 7 Daily variation in body temperature in four mammalian species reveals a range of average Tb and variation about the mean. The acrophase (time of peak) for the nocturnal rat was much later than that for the diurnal species. Light and dark phases are indicated. Reproduced from Refinetti R and Piccione G (2005) Intra- and inter-individual variability in the circadian rhythm of body temperature of rats, squirrels, dogs, and horses. Journal of Thermal Biology 30: 139-146.

3 4 Days

Figure 7 Daily variation in body temperature in four mammalian species reveals a range of average Tb and variation about the mean. The acrophase (time of peak) for the nocturnal rat was much later than that for the diurnal species. Light and dark phases are indicated. Reproduced from Refinetti R and Piccione G (2005) Intra- and inter-individual variability in the circadian rhythm of body temperature of rats, squirrels, dogs, and horses. Journal of Thermal Biology 30: 139-146.

set-point is lowered and some animals display a marked reduction in metabolic rate and an associated regulated decline in Tb (e.g., torpor, Figure 8). Finally, stressful situations such as infection result in an increase in thermal set-point (fever), whereas injury, hypoxia, starvation, and other situations can result in a decrease in the thermal set-point (anapyrexia).

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