Why Elevate and Regulate Tb above Ambient Temperature

Catalytic or enzymatic rates are temperature dependent according to the Boltzmann's constant (Qi0 is used to describe the effects of a 10 °C temperature difference on reaction rate). A constant Tb allows enzymes to always be at their optimal temperature for catalysis and overcomes the need for having isoenzymes adapted to operate at different temperatures. Interestingly, across the taxa,

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Figure 8 Body temperature in a monotreme, the echidna, collected over 1 year in Tasmania. During the winter months (April-August) the animal enters torpor, characterized by a lowering of the thermal set-point and subsequent marked decline in Tb together with characteristic test arousals. The characteristic large circadian variation in Tb of echidnas (more easily seen in the upper panel) is a function of the activity cycle. Departures from this daily Tb cycle are associated with a decrease in the activity pattern as a result of the animal spending time in the burrow following egg laying (indicated by shaded bar). Data provided by SC Nicol and modified from Nicol SC and Andersen NA (2006) Body temperature as an indicator of egg-laying in the echidna, Tachyglossus aculeatus. Journal of Thermal Biology 31: 483-490.

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Figure 8 Body temperature in a monotreme, the echidna, collected over 1 year in Tasmania. During the winter months (April-August) the animal enters torpor, characterized by a lowering of the thermal set-point and subsequent marked decline in Tb together with characteristic test arousals. The characteristic large circadian variation in Tb of echidnas (more easily seen in the upper panel) is a function of the activity cycle. Departures from this daily Tb cycle are associated with a decrease in the activity pattern as a result of the animal spending time in the burrow following egg laying (indicated by shaded bar). Data provided by SC Nicol and modified from Nicol SC and Andersen NA (2006) Body temperature as an indicator of egg-laying in the echidna, Tachyglossus aculeatus. Journal of Thermal Biology 31: 483-490.

homeotherms have tended toward Tb's of 30-40 °C. This may be an attempt to minimize the effects of temperature on enthalpy and entropy required for activation energy for biochemical reactions.

A warm constant Tb means that rate processes such as metabolism, digestion, neural function, muscle force, and speed of contraction, growth, and other processes dependent on temperature can be maximized and maintained independent of ambient temperature. In turn, greater and more sustainable levels of activity and various processes enable expansion of both the temporal niche (i.e., animals can forage for longer) and thermal niche (i.e., animals are freed from fluctuations in ambient temperature).

Warm Is Not Always Better

In endothermic homeotherms the major downside to maintaining a warm Tb in the cold is the high energy expenditure for thermogenesis. Obviously, energetic savings can be made through a reduction in thermal set-point and the associated hypometabolism. For all animals a disadvantage of a higher Tb is that the effect will increase demand for energy. Some animals optimize the costs of a higher set-point through behavioral adjustments. In the case of the dogfish, a squaliform shark, it forages in warm water but lowers energy costs by resting and digesting in cooler water, thereby increasing bioener-getic efficiency and growth rate.

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