Lecture 9: Temperature Regulation

Reading:  None.

Responses to Temperature

Most organisms cannot tolerate temperatures greater than 45°C because protein denaturation begins at this and higher temperatures.  Exceptions are photosynthetic cyanobacteria which may tolerate 75°C, and some thermophilic bacteria that exist in hot springs at temperatures close to 100°C.

Cold tolerance is limited by ice crystal formation. Solutes in water depress the freezing point and inhibit ice formation. Glycerol and glycoproteins may be produced specifically to depress the freezing point of body fluids.

Metabolic rate responds to temperature, as temperature increases, chemical reaction rates increase. Typically, there is a 2x - 4x rate change for each 10°C change. Temperature can influence protein conformation so functional conformation changes with temperature.

Temperature Regulation: Terminology

Classification
          Source of Body Heat

	Endothermy - internal heat source
	Ectothermy - external heat source

Constancy of Body Temperature

	Homeothermy - constant within a range of environmental temperatures
	Poikilothermy - changing with environmental temperature

Ectothermy (functional - adaptation definition)

	Body temperature varies with environmental conditions
	Metabolic rate is low (one-tenth of endotherms), low body temperature
	Body temperature not a function of internal heat production in most cases
	Body temperature not maintained at rest         Examples: fishes, amphibians, reptiles, all invertebrates

Endothermy (functional - adaptation definition)

	Body temperature constant
	Metabolic rate is high, high body temperature (35°C - 40°C)
	Body temperature maintained by metabolic (endogenous) heat production
	Body temperature maintained at rest (birds and mammals only)

Ectothermy Compared to Endothermy

Efficiency of Biomass Conversion         Ectotherms are more efficient than endotherms. Among endotherms,         most of assimilated energy goes to heat (data from Pough, 1980).

Energy Costs of Maintenance

Comparing mammals and reptiles of the same adult body mass, the energy required for maintenance is 10x - 13x greater for a mammal than a reptile. This differential cost of maintenance (resting metabolism cost) is often termed the cost of endothermy.

Variable "endothermy" or heterothermy is a catch-all term for a wide variety of phenomena in which body temperature is constant only under certain conditions.
This term is loosely applied to both functional endotherms, birds and mammals, and to functional ectotherms which cannot maintain constant body temperature at rest.

Variation in true functional endotherms:

To conserve energy: hummingbirds (daily cycles)

    animals in hibernation (seasonal cycles)

    shrews (daily cycles)

To conserve water:  camels (daily cycles with ambient temperature)

Variation in functional ectotherms:

Pseudoendothermy results from active metabolism (muscle activity)

Temporal variation in elevated constant body temperature

	flying insects
	some snakes (pythons)
	some fishes (mackerel)

Spatial variation within the body, hot core, cold extremities

	mako sharks
	tuna
	flying insects

Responses to Environmental Temperature and Temperature Regulation

Endothermic body temperature regulation

	control of insulation
	muscular heat production
	evaporative heat loss
	control of heat exchange by conduction and convection

Body temperature and metabolic rate as a function of ambient temperature for typical endotherms. Figure symbols: TNZ = thermal neutral zone (the range of ambient temperatures at which metabolic rate is minimal and the animal is at its basal metabolic rate), LCT = lower critical temperature (the low end of the TNZ), UCT = upper critical temperature (the upper end of the TNZ), LLT = lower lethal temperature (temperature at which lethal hypothermia occurs), ULT = upper lethal temperature (temperature at which lethal hyperthermia occurs). (after Eckert and Randall, 1983, p 723, Fig. 16-28)

Outside the ambient temperature range of the TNZ an animal must expend energy to maintain a constant body temperature.

Within the TNZ, constant body temperature is maintained by regulating heat loss:

	Behavioral control (body orientation and habitat choice)
	Insulation control
	Vasoconstriction/dialation

At ambient temperatures greater than the upper critical temperature, evaporative cooling becomes important.

There is considerable variation among endotherms in the width and limits of the TNZ. In some endotherms, the TNZ is a point rather than a range of temperatures (data from Prosser, 1973).

Insulation and surface area:volume ratio influence rates of heat loss in endotherms living at relatively cold ambient temperatures. The LCT and the rate of metabolic rate change with decreasing ambient temperature are both a function of heat loss rates. The position of the LCT, and slope of typical low temperature response curves are shown for well insulated and poorly insulated animals. (after Eckert and Randall, 1983, p 727, Fig. 16-32)

Ectothermic body temperature changes with ambient temperature

Regulation of body temperature is very limited and metabolic rate changes as body temperature changes. The optimal temperature for a given ectotherm is defined as the temperature at which metabolic rate is at a maximum.

Body temperature and metabolic rate as a function of ambient temperature for typical ectotherms. Figure symbols: OT = optimal temperature, LLT = lower lethal temperature (temperature at which lethal hypothermia occurs), ULT = upper lethal temperature (temperature at which lethal hyperthermia occurs). The shape and position of this typical temperature response curve can change for a given animal living under different thermal conditions, and varies between species.

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