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.
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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. Consequently, the new terms of ectothermy and endothermy are used. An ectotherm has a high rate of thermal conductance and a low rate of heat production; the body temperature is therefore determined by the environment. However, endotherms are capable of raising the temperature of their tissues above that of the environment, due to the heat from metabolism.
Heat can be transferred by a number of means (Adams & Iampietro, 1968):
* “Conduction…thermal energy exchange through a medium or between objects in a physical contact by the transfer of intramolecular energy, not involving the transfer of material.”
* “Convection…route of thermal energy flow depending on the movement of a fluid over a surface which is at a different temperature.”
* “Radiation…heat transfer by the exchange of electromagnetic energies between facing surfaces…”
* “Evaporation…depends upon thermal transfer in the conversion of a material from a liquid to a gas phase.”
These different factors are utilised by both endotherms and ectotherms, but ectotherms depend on them more than the endotherms (as the endotherms can control their internal temperature).
It is believed that endotherms evolved from ectotherms, but the intermediate steps are not clear. Endotherms are an inverse of ectotherms; where ectotherms rely upon thermal conductance to increase body temperature, endotherms control their body temperature by heat produced from metabolism, and they have a low thermal conductance to retain this heat. Endotherms have up to five times more mitochondria than ectotherms, and in addition, Akhmerov found that in certain endotherms oxidative phosphorylation in the mitochondria was uncoupled, thereby producing heat and not ATP (1986). It has been proposed that insulation evolved at first, as this would have limited a fluctuation in body temperature (Crawshaw et al., 1982), aiding specialisation of biochemical processes.
The majority of animals are ectothermic, and their activity is determined by the prevailing environmental temperature. The metabolic rate of ectotherms is relatively low and as previously mentioned, they lack mechanisms for conserving heat. As a result, aquatic vertebrates such as fish usually have a body temperature which is at thermal equilibrium with that of the water. Fish cannot maintain a temperature below that of the water but may in some cases, such as that of the tuna, retain heat by means of a countercurrent heat exchanger system. This can raise the temperature of the ‘red’ swimming muscle to about 12?C above that of the sea water.
Terrestrial ectotherms have to contend with greater temperature fluctuations than those of the aquatic ectotherms, but they have the benefit of living at higher environmental temperatures. This allows them to be more active, and show a variety of complex behavioural patterns based upon prevailing temperature conditions. Many species are capable of maintaining temperatures slightly above or below that of the air, and thereby avoid extremes. The relatively poor thermal conductivity of air reduces the rate of heat loss from organisms, whilst water loss by evaporation may be used to cool the organism.
The key feature to the success of the ectotherms is their low energy lifestyle. All their characteristics require a minimum amount of energy for their survival, enabling them to exploit many more niches than the endotherms (and is a reflection of their high number). For ectotherms to remain active they must maintain a certain body temperature, enabling necessary biochemical reactions to continue. This has been perfected in some species, to such an extent that their body temperature only varies by a few degrees throughout the day; body temperature doesn’t fluctuate by any more than 10?C in most ectothermic species. The range of body temperatures at which a species carries out its normal activities is the “activity temperature range” (ATR). The ATR is an “ecological optimum” which integrates internal and external forces acting on a species (Cowles & Bogert, 1944). By limiting the range of activity temperatures, an organism is able to adapt its physiological and biochemical processes to this limited temperature range, increasing its efficiency (ie an optimum rate for enzyme reactions to occur).
The driving force for the evolution of ectotherms was, as with every species, to survive. The most important specialisation for this purpose is their low energy requirements – as illustrated by the heart and vascular system. Ectotherms circulatory system is facilitated by low fluid resistance. This low blood pressure also reduces the oxygen transport ability of the blood of ectotherms; between 25%-50% lower than in endotherms. This does not inhibit their normal activities, as sufficient oxygen is still supplied to the tissues. An additional advantageous feature of ectotherms is their ability to withstand complete and prolonged deoxygenation of their blood. As an escape mechanism, some ectotherms able to remain under water for several hours (eg turtles); or to regulate their body temperature – many lizards are capable of burying themselves entirely in sand.
Another advantage of ectotherms is the way in which their energy is managed. Solar energy is used to heat the body, leaving ingested food to be used to maintain high rates of growth and reproduction. This will in turn rapidly increase a species population, illustrating its success.
Another element of their success, which stems from their low energy demands, is their ability to colonise many niches, which can not support endotherms. Such zones are characterised by a limited resource supply; the limitation may be a shortage of food, water or oxygen. Ectotherms have taken advantage of all of these.
The effect of limited food is exemplified by the size of many ectotherms. Adult body masses of less than or equal to 1 g are characteristic of 8% of lizards and 20% of salamanders; this is only possible due to their low energy requirements, but confers several advantages. Small size requires little energy input, and also enables the organism to evade predators more successfully (hiding in under rocks). Similarly, the shape of ectotherms is also adapted. Reptiles and amphibians have the greatest variation in length/diameter ratios among all tetrapods (Gans, 1978). This enables some animals, such as the boa constrictor, to be the shape they are; there are few constrictions on ectotherm shape, compared to the endotherm that tries to conserve energy, and is thus restricted in shape. For the same reasons water is required in smaller amounts; so small in the case of some ectotherms, that sufficient water can be obtained by diffusion from the air or the soil.
The method of exercise employed by ectotherms is extremely energy efficient once again – clearly another advantage. Relying upon an anaerobic metabolism gives them the ability to generate energy rapidly for brief periods without the costs of maintaining high resting metabolic rates (as in endotherms).
Many lizards are able to change their peripheral circulation. When they are hot the dermal blood vessels dilate to increase the blood flow close to the skin so that heat is lost and when they are cold the opposite occurs. Other species pant and sweat to lose heat. Ectotherms are frequently able to temporarily relax homeostasis – allowing physiological variables to fluctuate more widely than usual – so that they can survive in hostile environments. One of the most extreme cases of this is the ability of many amphibians and fish to supercool which allows them to survive at temperatures below freezing. 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.
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. During this warming process, the locust will be extremely visible, thus increasing the possibility of being seen by a predator. In addition to this many ectotherms are seen as an attractive source of food – lacking fur, they are more easily digestible than many small rodents, and are also easier to capture due to their rapid exhaustion.
The moist skin of amphibia provides an ideal mechanism to enable heat to be lost by evaporation. This water loss, however, cannot be regulated physiologically as in mammals. As a result, amphibia must find moist shaded conditions where the rate of evaporation can be limited, thus limiting their activity.
Birds and mammals are endothermic, and their activity is largely independent of prevailing environmental temperatures. In order to maintain a constant body temperature, which is normally higher than that of the ambient air temperature, endotherms need to have a high metabolic rate and an efficient means of controlling heat loss from the body surface. Endotherms are able to adjust the production of metabolic heat to equal the heat loss from their bodies under different environmental conditions. Metabolic heat is produced from the obligatory basal metabolic rate, the heat increment of feeding, muscular heat and from nonshivering thermogensis. Because endotherms usually live under conditions in which ambient temperatures are lower than their regulated body temperatures, heat loss to the environment is more usual than heat gain, although heat gain can be a major problem in deserts. Hence, different mechanisms have evolved that provide insulation (or radiate heat effectively, in the case of deserts).
The key advantage of endotherms is their high metabolic rate, but there are many other factors that contribute to their success too. The high metabolic rate enables a constant internal temperature to be maintained by homeostasis. Homeostasis enables the body temperature to be maintained within a degree in most cases, which has then led to the evolution of more efficient biochemical pathways; and these pathways can function at all times during the day.
Homeostasis is a very complex regulatory system present in organisms, used to greater accuracy by endotherms to regulate their temperature. Mechanisms include insulation, sweat glands, vasoconstriction and dilation – these are controlled by the hypothalamus of the brain, so are not consciously controlled (unlike behavioural thermoregulation) (Hardy et al., 1971). Hence, the organism does not have to concern itself with its current temperature, enabling it to do something else (ie predate). There are a variety of types of insulation employed by endothermic vertebrates:
* Hair or fur – capable of being raised to trap air next to the skin, acting as additional insulation, usually to decrease the loss of heat from the body.
* Feathers – again trap air next to the skin, increasing thermal resistance to cold. An exception is the ostrich, which can erect its feathers increasing their thickness from three to ten centimeters (Crawford & Schmidt-Nielson, 1967) – this serves to maintain a lower body temperature.
* Adipose tissue – especially brown adipose tissue that has the ability to release large amounts of heat, whilst also insulating.
Sweating again decreases body temperature by evaporation of water, which is in contact with the skin. This is utilised by some ectotherms, but there is not the same degree of control. The numerous blood capillaries in the dermis can be shunted through the surface of the skin, or deeper in the body, thus varying loss of heat; again a similar system is employed by a few lizards, but not with the same degree of control. These different mechanisms of thermoregulation are so useful because they do not require the animal to alter any of its daily routines. Whilst an ectotherm must devote part of its daily routine to warm its body, endotherms are active and searching for food.
The high metabolic rate also enables the endotherms to lead very active lives, enabling them to predate or travel. This increased mobility (in comparison to ectotherms) is taken advantage of by some species of birds. During the winter, to avoid the cold weather, some birds migrate to warmer climes, thus avoiding the need to be sufficiently adapted to combat the cold.
Endotherms are also able to inhabit more extreme environments successfully. In colder climes, extra insulation is used, and in the case of penguins extra feathers can be found on the head than those from tropical climates (Stonehouse, 1967). Another advantageous mechanism found in birds and mammals is that of the counter-current heat exchange mechanism, which again reduces heat loss from extremities.
Allen’s rule, that the size of extremities increase with environmental temperature, has been proven to be true, and hence confers further advantage, especially to the endotherms. Weaver and Ingram showed that pigs reared at 35?C for approximately two and a half months had longer tails, larger ears, less hair, longer bodies, and were less stocky than the animals kept at 5?C (1969). The ability to adapt to the environment is a characteristic of endotherms that will surely have influenced the success of the group.
To combat cold, another mechanism some endotherms utilise is to decrease the internal body temperature. This in turn decreases the temperature gradients and associated heat fluxes between the animal and its surroundings. Many animals achieve this by entering a torpid state. This is extremely advantageous during the winter months, as if they were active more energy would be needed to maintain a constant body temperature, yet there is less food available.
In response to heat, and in accordance to Allen’s rule, extremities are generally increased in size. The African bat-eared fox has large ears containing many blood vessels, leading to effective radiation and convection of heat from the body, without excessive loss of water by evaporation.
Endotherms in deserts have had to adopt methods of behavioural thermoregulation. Smaller endotherms can survive in burrows underground during periods of intense heat (ie during midday), and emerge when the ambient temperature is suitably cool. However, for larger animals different strategies must be employed. Some species, due to their insulation, are able to absorb the radiation from the sun during the day, and then release it during the night. Camels are able to survive the high temperatures by relaxing their limits of homeostasis; and to survive dehydration the camel stores energy as fat, which can be broken down to produce metabolic water. By this method, camels are taking advantage of characteristics exhibited by both endotherms and ectotherms, and are sometimes described as heterotherms.
The primary disadvantage to endotherms is their high metabolic rate, because it consumes so much energy, and this energy must be supplied by ingestion of food. As a result, all endotherms (in the wild) spend most of their life searching for food. This active lifestyle also leaves them open to attack from predators, as much time will be spent in view on the terrain.
Although there is a wide range of sizes of endotherms, their size is limited at the lower end of the scale. As body mass decreases, the surface/volume ratio increases, so smaller animals lose energy faster. There is a point where endotherms are simply too small to survive, because the mass specific cost of living is too high. 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.
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.