Coping with variation in temperature II. Heat Exchange in Animals

Learning goals:
* be able to determine whether an organism's temperature is increasing or decreasing due to changes in its energy balance
* understand the basic differences in ectothermic and endothermic animals and their heat balances
* be able to explain how torpor benefits small endotherms, and what is required for animals to rely on it as a strategy for heat and energy balance

Energy exchange is similar to plants, with one exception- some animals (endotherms) can generate heat internally
Majority of animals rely on exchange of external sources of energy for temperature regulation = ectotherms
General equation:

∆ (change in)H_animal= SR + IR_in- IR_out + H_conv + H_cond – H_et + H_met (eq. 4.2)

where H_met is metabolic heat
H_et includes sweating, panting in dogs and other animals, and licking of the body by some marsupials

Behavior of ectothermic animals, coupled with mobility allows them to seek out sites that facilitate keeping body temperatures near optimal

Ratio of surface area: volume determines the gain and loss of energy for ectotherms; As the size of the organism increases, its surface area: volume ratio decreases; this will constrain the upper size of ectotherms

Ectotherms exhibit greater tolerance to temperature variation than endotherms; As a group, ectotherms are active with body temperatures between -2 to 45 °C; endotherms limited to 30 to 45 °C.
Exposure to sub-freezing temperatures in temperate and polar climates a serious problem for ectotherms – avoidance of body freezing may not be possible; Minimizing ice crystal formation- using special chemical protectants.
Some animals that are primarily ectothermic exhibit some degree of internal heat generation

Evolution of endothermy allow animals to be somewhat independent from the external climate, greatly expanding their geographic ranges, especially in cold climates
* endothermy is found in birds and mammals
* requires additional energy (food) for heat generation- metabolic rates of endotherms higher than ectotherms

Metabolic rate varies with environmental temperatures and rate of heat loss
* lower temperatures require greater heat generation
* higher surface area: volume ratio results in greater heat loss; contributes to smaller endotherms having higher metabolic rates
* insulation needed- fat, fur, feathers; insulative value (e.g. thickness, dead air space) influences metabolic rate
thermoneutral zone= basal metabolic rate maintains constant body temperature
lower critical temperature = temperature threshold for increase in metabolic heat to maintain constant body temperature
* lower critical level is related to animal’s habitat
* increase in metabolic rate varies according to level of insulation

Small endotherms face challenges in cold climates
* high surface area: volume
* short fur
* higher basal metabolic rates (greater demand for energy/ food)
* low capacity to store energy (fat)
One solution to the problem of cold climates is to temporarily abandon endothermy, and allow body temperature to drop to near ambient = torpor
Torpor results in energy savings of 50-90% relative to using endothermic heat generation
Torpor occurs primarily in small and medium sized mammals and a few birds (e.g. hummingbirds); lacking in lagomorphs (rabbits) like Pika
Long-term torpor is possible only in mammals with enough energy reserves (e.g. Marmots); torpor usually occurs as repeated bouts with intermittent arousal