15 1 Thermal Energy Sources Most animals live in habitats that vary in temperature As Ta changes Tb will change unless the animal does something to prevent it If Ta Tb the animal will lose heat produced by the catabolic exothermic reactions in its cells This heat may be transferred to the environment by conduction convection radiation or evaporation Some animals poikilotherms allow their Tb to change with Ta and have adaptations to tolerate these changes in Tb Other animals either live in environments where Ta does not change or they have adaptations that prevent Tb from changing Either way these animals have a stable body temperature homeotherms Poikilothermy and homeothermy describe changes in Tb or lack thereof Regardless of whether an animal has a fluctuating body temperature or stable body temperature it may have mechanisms to control its Tb In general homeotherms have more of these mechanisms and use them more extensively than poikilotherms but not always The mechanisms an animal uses to change its Tb can be grouped into endothermic strategies and ectothermic strategies Endothermic strategies of Tb regulation control how much heat is produced inside the body by controlling metabolism Ectothermic strategies of Tb regulation control how much heat is transferred to or from the environment How body temperature is maintained endo vs ectothermy may vary within a single animal depending on the time of day time of year or part of body This is referred to as heterothermy A temporal heterotherm relies heavily on endothermic processes for some periods but not as a general rule An example of temporal heterothermy is a python which is typically ectothermic it does not rely on metabolic heat to control Tb It is also poikilothermic in that Tb changes with Ta But after a python eats a large meal the metabolic processes that catabolize macromolecules and store them are aggressively upregulated The python becomes lethargic and uses these processes to keep itself warm while digesting so it relies more on endothermic strategies of thermoregulation during this time The graph below shows how an animal may maintain a Tb different from Ta but does so to a lesser degree during the night allowing Tb to dip down closer to the cooler environmental temperature displaying temporal heterothermy An example of regional heterothermy is the billfish Recall from Lesson 5 2 Muscle Diversity that the billfish uses a heater organ near its eye to keep it warmer than other parts of its body allowing it to maintain its visual system even while pursuing prey in cold deep waters Fish are generally ectothermic and unless they live in polar environments generally poikilothermic The modified myocytes of the heater organ have high metabolic rates catabolizing glucose and releasing heat at higher rates than normal cells thus warming the region around the eye through endothermic means The second graph below shows how the thorax and abdomen may be at different temperatures within the body of a single animal In this animal the thorax is maintained at a higher temperature than the abdomen which is closer to Ta Endothermy Heat is released in all exothermic reactions like when macromolecular bonds are broken during digestion or as glucose is broken down to make ATP to fuel muscle contraction This heat will radiate from the animal to the environment If the animal s surroundings are colder than the animal this heat will be transferred via conduction to materials that the animal is in contact with or transferred via convection to the fluid medium around the animal In terrestrial animals some of this heat will be carried by water via evaporation assuming that the air is dry enough for water to follow a concentration gradient away from the animal s body Endothermic animals have mechanisms to increase the efficiency of metabolic heat generation based on body temperature and use the metabolic heat to warm their bodies Three endothermic strategies for controlling Tb are 1 Futile cycling 2 Shivering thermogenesis 3 Non shivering thermogenesis Insects are poikilothermic allowing Tb to fluctuate with Ta However they do have mechanisms for regulating Tb that allow their bodies to maintain its functions Prior to flight many insects use endothermic mechanisms to warm their bodies They contract antagonistic pairs of flight muscles at the same time which expends energy and produces heat without moving the wings They also flutter their wings at a frequency and angle that generates heat but does not generate lift Lastly some flying insects are able to employ futile cycling where two opposing enzymatic pathways are activated simultaneously ATP production and ATP hydrolysis This generates heat from two sets of exothermic reactions as carbohydrates are broken down via glycolysis to make ATP and ATP is simultaneously broken down and carbohydrates are rebuilt via gluconeogenesis Thus heat is produced without any net change in the levels of substrates or products These are endothermic processes that allow insects to warm their bodies in preparation for flight Birds and mammals possess a unique endothermic strategy for warming their bodies shivering thermogenesis Myofibers in antagonistic skeletal muscles rapidly contract such that the muscle as a whole does not contract As actin myosin crossbridges are rapidly formed and broken the muscle uses its store of ATP converting this chemical energy to kinetic energy contraction and heat The heat radiates outward from the muscle and warms the animal s tissues Generating heat via shivering is not a long term solution as the myofibers will rapidly deplete their stores of ATP and become exhausted Shivering also represents a trade off in that the skeletal muscles that are twitching cannot be used effectively for locomotion preventing the animal from foraging or avoiding predators Mammals are also able to generate heat using a type of fat tissue called brown adipose tissue BAT so named due to its higher content of mitochondria relative to other adipose tissue which is white due to its high lipid content BAT generates heat via non shivering thermogenesis and is particularly important in small mammals and newborn mammals where it is concentrated around the back and shoulders Cells that comprise BAT express a cold inducible gene for thermogenin which is also called uncoupling protein UCP The graph below shows expression of one of the UCP genes UCP 1 being induced in the nucleus by exposure to cold In the electron
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