Discuss the Advantages and Disadvantages of being Ectothermic and Endothermic for Vertebrates.
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Discuss the Advantages and Disadvantages of being Ectothermic and Endothermic for Vertebrates. Vertebrates can be found all over the world, from the freezing poles to the hot deserts. The normal air temperature in these regions varies from -40?C to 50?C respectively. The majority of living organisms exist within confined limits of temperature, (approximately 10-35?C), but various organisms show adaptations enabling them to exploit geographical areas at both extremes of temperature. Temperature indicates the amount of heat energy in a system, and is a major factor determining the rate of chemical reactions. The most important reactions which are inhibited by inappropriate temperature are those that are catalysed by enzymes. Below freezing point, cells may freeze, and the cell structure destroyed by formation of ice crystals. Above 45?C enzymes usually become denatured, ceasing to function; in both of these cases, the organism dies. Therefore, if vertebrates did not regulate their body temperature they would be unable to survive outside a narrow range of temperatures. All animals derive heat from two sources: the external environment and from the release of chemical energy within their cells. The extent to which animals are able to generate and conserve this heat depends upon physiological mechanisms associated with their phylogenetic position. The possible evolution of the endotherms will be discussed later. Until recently, animals were classified as poikilotherms or homeotherms; reference to their being respectively cold or warm-blooded animals. The word poikilothermic (Greek poikilos = changeable) refers to the fact that the temperature of a cold-blooded animal fluctuates with that of its surroundings. For example, a fish has the temperature of the water it swims in. However, a deep-sea fish that spends its entire life in water that has barely measurable temperature fluctuations is really an animal with a constant body temperature - hence it would be fair to describe it as a homeotherm, a term reserved for birds and mammals.
Other ectotherms use antifreeze agents in their tissues to lower the freezing point of the water in their cells. Certain species of frogs can actually freeze for periods of a few weeks. Spadefoot toads in Arizona are active only during the summer rains and then retreat underground for nine to ten months. Such tolerance would be impossible for endotherms, even those that hibernate, because even during hibernation they do not relax homeostasis entirely. These are all examples of low cost methods of surviving at times when conditions are extreme. Disadvantages Clearly, such dependence on the environment is not advantageous. Although the weather in a hot desert or cold pole is fairly consistent, there are some countries (such as UK) where the weather is extremely variable from day to day; this means that on some occasions sufficient heat will be able to be obtained to raise the body temperature, and on others a cloudy sky will prevent it. As a result, on those days where there is not enough solar radiation, ectotherms must remain relatively inactive. Ectotherm metabolism allows only for brief periods of high activity, because the low rate of aerobic respiration in ectotherms leads to the quick development of an 'oxygen debt' due to anaerobic respiration (often anaerobic respiration can account for 50%-98% of energy produced during respiration). This peak activity can only be maintained for three to five minutes, and may require several hours to return their tissues to resting levels of lactic acid (Gratz & Hutchinson). This leaves ectotherms vulnerable to predators, not only when trying to evade them, but also when recovering from physical exertion. This is not the only time ectotherms are vulnerable to predators. To maintain a fairly constant body temperature it has already been explained that the animal must position itself relative to the sun. For example, the desert locust (Schistocerca) aligns itself at right-angles to the sun to absorb most energy, re-orientating itself parallel to the sun's rays when warm enough.
The smallest endotherms are about three grams, and they must eat continually to live (or hibernate over night). The shape of endotherms is also limited, as certain shapes lose energy more than others. For example, the weasel loses twice as much energy at low ambient air temperature than wood rats of the same body mass - the surface/volume ratio is normally kept to a minimum to reduce heat loss. Although migration of a species is an advantage, it is also a disadvantage as it involves the expenditure of huge amounts of energy. Hibernating is another very effective way of surviving, although it does leave the animal exposed to predation (even when very well camouflaged). Sweating was also seen as an advantage, yet it requires the prerequisite of copious amounts of water. Evaporation is a very effective method of cooling, but requires much water to be lost from the body. Hence, if sweating is to be effective, water must be drunk as fast as it is lost. In the case of animals which live in hot climates, there is unlikely to be a suitably large supply of water, so they must adapt accordingly by limiting their activity. Conclusion Endothermy and Ectothermy represent a dichotomy affecting far more than just body temperature. The implications of both forms of existence extend to such areas as activity, physiology and behaviour. Simplistically, ectotherms would appear to be the most well suited animals: they have lived longer, and only require only 4% of the energy required by an ectotherm (Bennett & Nagy, 1977). They occupy many niches, more than the endotherms, but this may just have been due to the later evolution of the endotherms. Mammals and birds have much more complex thermoregulatory systems, which is required if they are to maintain a constant temperature - and it was also needed to colonise the remaining niches. Both endotherms and ectotherms are designed such that they are successful in their particular niche, and by their survival, they have exemplified this fact.
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