Discuss the Advantages and Disadvantages of being Ectothermic and Endothermic in the Vertebrates.
Discuss the Advantages and Disadvantages of being Ectothermic and Endothermic in the Vertebrates.
Introduction:
Vertebrates occupy a wide variety of habitats all over the world from the freezing poles to the heat of hot desserts. In order to survive in these areas all vertebrates have adapted to regulate their body temperature, because temperature affects biochemical reactions. Organisms have been described as "bags of chemicals catalysed by enzymes". Although narrow, this emphasises that organisms are highly influenced by the rate at which chemical reactions, and hence vital life processes, occur. At low temperatures the rate of diffusion can be so low that essential functions cease and the organism dies. Below freezing point the cells may freeze and the cell structure destroyed by ice formation. Above 45oC enzymes become denatured and cease to function, again the organism dies. Therefore, if vertebrates did not regulate their body temperature they would be unable to survive outside a narrow range of habitats. There is one final point: most chemical reactions double or triple in rate for every rise of 10oC, it is therefore in an organism's interest to increase its body temperature if it wants to move faster, or react quicker, and so on.
At first, the thermoregulatory modes of animals were classified according to the stability of their body temperatures, as poikilothermic or homeothermic. A poikilothermic animal has a relatively variable body temperature; a homeothermic one has a relatively constant one. These terms, while useful, have become less appropriate as our knowledge of the temperature-regulatory capacities of animals has increased, because they communicate no information about mechanism. For example, mammals have been called homeotherms and fish poikilotherms. But some mammals become torpid at night or in winter, whereas many fish live in water of constant temperature. These inconsistencies led to the development of a second level of classification - endotherms and ectotherms.
All living creatures metabolise and none are a hundred percent efficient. A large fraction of metabolic energy therefore appears as heat, which is released as a by-product. Many animals have such high rates of thermal conductance and low rates of heat production that the heat is lost to the environment. Consequently the body temperatures of these animals are independent of the heat produced by their metabolism and are determined exclusively by the external environment (because of their high rates of thermal conductivity). Such animals are described as ectothermic. In a few groups, however, the metabolic heat production coupled with low thermal conductivity is sufficient to raise the temperature of the tissues above that of the environment. These animals are called endothermic. Endotherms produce extra metabolic heat because they have up to five times more mitochondria than ectotherms. In addition Akhmerov (1986) found that in certain endotherms oxidative phosphorylation in the mitochondria was uncoupled, thereby producing heat and not ATP.
These mechanisms are not mutually exclusive. Many endotherms will make extensive use of external sources of heat, while many ectotherms generate metabolic heat at certain times: for example brooding snakes. Endotherms and ectotherms can also be described as homeothermic and poikilothermic. Man is a homeothermic endotherm, while some fish are homeothermic ectotherms.
Heterotherms are capable of varying degrees of endothermic heat production, but they generally do not regulate body temperature within a narrow range. Temporal heterotherms are generally facultative endotherms who relax their metabolic production of heat at certain times, for example during hibernation. Some ectotherms are regional heterotherms, because they can achieve high core (i.e. deep-tissue) temperatures through muscular activity, for example in the skull cavity of some fish.
Ectothermy: the low-cost approach to life.
Overview of ectothermy:
Since ectotherms are unable to produce extra metabolic heat they must regulate their body temperature using external sources for heating or cooling as required. An organism can gain or lose heat from many different pathways. Many ectotherms, for example lizards, absorb solar radiation, particularly infrared light, by standing in a sunny spot. All objects on earth radiate energy as infrared light: this can either lead to heat gain (e.g. from surroundings) or loss depending on the relative temperature of the animal's body surface. Organisms also gain or lose energy from convection currents, conductive heat exchange, evaporation and metabolic heat production.
Organisms have adapted to utilise these energy sources through behavioural thermoregulation. This is practised by endotherms, but ectotherms have become very highly specialised at this form of internal regulation. The mechanisms are relatively simple: for example the movement of lizards forwards and backwards between sun and shade. The amount of heat radiation absorbed can be altered by changing colour (for example in chameleons) or orientation. Many ectotherms live in burrows (controlled microhabitats) and only emerge for a certain period each day when the temperature is warm (or cool) enough for them to be active, the remaining time they spend in a torpid state. Ectotherms also regulate temperature through physiological means. They do not have insulatory fat or scales, for example, because these would interfere with heat gain (or loss). 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 9 to 10 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.
Advantages of Ectothermy: Low energy costs:
The ectothermal approach to life allows animals to exploit habitats or adaptive zones that would be difficult or impossible for an endotherm to occupy. The key to this is the low energy requirement of ectothermy. Ectotherms do not require the energy to maintain the high body temperature that endotherms do. As a result they can afford to eat less and to have more tolerant tissues. Endotherms have highly specialised tissues adapted to a certain narrow range of temperature and chemical composition, which is maintained through homeostasis. Ectotherms have less specialised tissues, ...
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Advantages of Ectothermy: Low energy costs:
The ectothermal approach to life allows animals to exploit habitats or adaptive zones that would be difficult or impossible for an endotherm to occupy. The key to this is the low energy requirement of ectothermy. Ectotherms do not require the energy to maintain the high body temperature that endotherms do. As a result they can afford to eat less and to have more tolerant tissues. Endotherms have highly specialised tissues adapted to a certain narrow range of temperature and chemical composition, which is maintained through homeostasis. Ectotherms have less specialised tissues, which are able to tolerate greater changes in temperature and chemical composition. The high tolerance of ectotherms has allowed them to colonise the deep oceans (where it is very cold), hot desserts and environments that seasonally freeze. As a result of this tolerance they frequently have longer life spans than many equivalent sized endotherms (for example the life span of some tortoises can be in excess of 100 years) because of the greater strain endotherms place on their bodies by their active lifestyles.
This tolerance of ectotherms is shown by their many physiological adaptations. Because they require less oxygen than endotherms their circulatory system transports considerably less. It is much reduced with fewer, larger blood vessels, resulting in a reduced resistance against which the heart works. One other result of this is that they can tolerate appreciably lower levels of oxygen than endotherms, especially at maximum exercise (when most of the ectotherm's respiration is anaerobic) because their blood has a lower affinity for oxygen. This low affinity of the blood for oxygen leads also to the speedy repayment of oxygen debt. The ability of many ectotherms to survive for substantially periods of activity by anaerobic respiration is also a preadaptation to aquatic life (for example turtles can survive for hours under water). Similarly many lizards are able to bury themselves entirely in the sand. This is not possible for endotherms because their high oxygen requirement cannot be satisfied entirely by diffusion.
The second key point about the energy requirements of ectotherms is what they do with this energy once they have it. Endotherms expend more than 90 percent of the energy they take in to produce heat to maintain their high body temperature. Less than 10 percent, and often as little as 1 percent, is available for increasing the species' biomass by growth of an individual or production of young. Ectotherms rely on 'free' energy for heating so most of the energy they ingest is converted into biomass for their species (between 30 and 90 percent). As a result ectotherms are able to maintain very high rates of growth and reproduction, far higher than endotherms, and are in many ecosystems the most important source of secondary production because of the high amounts of biomass they produce. Many endotherms are born highly developed, but because they have such low rates of growth it can be a very long period of time before they reach sexual maturity and are able to breed, compared with ectotherms (e.g. humans). Another result of the high rate of ectothermic growth is that they are able to quickly store up energy in their bodies during their brief periods of activity (the high tolerance level of their tissues contributes to this: e.g. they are able to ingest more salts than endotherms).
This low cost lifestyle means that Ectotherms can also be much smaller than endotherms because the cost of endothermy is very high at small body sizes. As body mass decreases the surface to volume ratio increases: so smaller animals lose heat faster. There is a point where endotherms are simply too small to survive, because the mass specific cost of living (energy per gram) is too high. The smallest endotherms weigh about 3 grams, and they must eat constantly to live (or hibernate overnight). By contrast the smallest vertebrate ectotherms weigh about 0.3 grams. Ectotherms, of course, are not constrained by problems of heat loss since their body temperature follows the ambient temperature. These small animals occupy key positions in terms of energy flow through an ecosystem. Their small size and high conversion rates (of energy into biomass) allow them to eat small insects, packaging them into a form that can be eaten by larger endotherms. Often these species are competing with insects for food. Because of these high conversion rates the total biomass (and numbers) of ectotherms is often higher than that of equivalent endotherms, so they occupy a very important part of certain trophic levels.
Similar energy considerations to those affecting size also affect shape. It is unlikely that an endotherm could cope successfully with the heat loss that would result from having a very large surface to volume ratio, such as a snake's body form (e.g. a weasel loses twice as much energy at low ambient temperatures as do wood rats of similar body mass). Ectotherms, by contrast, can often be elongated and very flexible in body form allowing them access to many different habitats, because of their freedom from heat-conserving restraints.
Since ectotherms require less food than endotherms they can spend more time quietly avoiding predators. They also need less water, since they lose less by evaporation, and need not be massive for the purpose of reducing the surface to volume ratio. Small ectotherms can gain sufficient water by diffusion from the air or soil.
The consequence of thermoregulation by ectothermic vertebrates is relative stenothermy - reduction in the range of body temperatures they experience during activity on both a daily and a seasonal basis. Often with lizards the body temperature may only vary by a few degrees. The "activity temperature range" (ATR) is the range of body temperatures at which a species carries out its normal activities. The advantage of limiting body temperature to a narrow range during activity is that it simplifies the integration of internal processes. This is an important benefit, which can be reached by ectothermy or endothermy. In both, for example, activity within the ATR yields a maximisation of oxygen consumption and a plateau in resting oxygen consumption (i.e. resting metabolic rate does not vary with body temperature within the ATR, whereas active metabolic rate is at a maximum). This increases energy efficiency. However in endotherms the ATR is narrower and the integration of biochemical processes hence more advanced.
The ectothermic way of life is not, therefore, inferior to that of endotherms, it simply represents a slower low-energy-flow approach that has its own distinct advantages.
Disadvantages of Ectothermy:
However there are certain costs to the ectothermic approach. Ectotherms can only regulate their body temperature if the environment is permissive of such regulation. If it is not sunny lizards cannot warm up and must remain inactive. The ectothermic metabolism allows only brief periods of high activity, because the low rate of aerobic respiration in ectotherms leads to the quick development of oxygen debt due to anaerobic respiration (often anaerobic respiration can account for 50-98% of energy production during activity). Endotherms, by contrast, are able to reach far greater speeds for longer than equivalent sized ectotherms, because they are able to aerobically respire at a much greater rate. As a result many ectotherms rely on camouflage for defence. The low rate of metabolism also restricts the possibility of attaining large size. This is why there are few ectotherms that weigh over 100 kg whereas ten percent of mammals do.
The result is that ectotherms are far more reliant on their physical environment than endotherms. This precludes them from existing in certain environments and filling certain ecological niches (just as endotherms cannot be very small). They are, for example, unable to survive at the poles where it is very cold all year round. Similarly there are no large ectothermic herbivores or medium-sized carnivores because their rate of metabolism is too low. These are the roles that endotherms, among other things, have adapted to fill.
Endothermy: the high-cost approach to life:
Overview of endothermy:
Endothermy has evolved from an ancestral ectotherm at least twice - in birds and in mammals. How this occurred is something of a paradox. The two key features of ectothermy are a low metabolic rate and a high thermal conductivity, whereas the two key features of endothermy are exactly the opposite - namely a high metabolic rate and a low thermal conductivity. For an endotherm to evolve the two must have occurred together: an ectotherm with lots of insulation would be unable to heat up whilst an endotherm with no insulation would lose all its body heat. However it occurred the evolution of endothermy allowed the colonisation of various habitats and ecological niches, in which ectotherms could not survive, because of their ability to maintain a high internal temperature independent of ambient temperature.
Endotherms maintain this body temperature through the action of many physiological processes. Crucially they are able to adjust the production of metabolic heat to equal 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 (from activity or shivering) and from nonshivering thermogenesis. 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. To balance heat loss endotherms, birds and mammals, have developed plumage or hair as a very effective insulation against heat loss. Animals adapted to colder climes have thick layers of insulatory fat, which is a poor conductor of heat energy. Polar Bears, for example, even have hollow hairs, which add increase heat conservation. Endotherms also use vasoconstriction - that is constriction of the dermal capillaries, decreasing the blood flow to the skin surface - to conserve heat (and vasodilation to lose heat). When they are too hot endotherms pant and sweat to cool down.
Advantages of Endotherms: high metabolic rate:
The key advantage of endotherms is their high metabolic rate, which can be six times that of ectotherms. Although this is energetically expensive, there are benefits to regulating body temperature at a constant high level. A constant internal temperature is required to obtain maximum chemical coordination of biochemical enzyme-catalysed reactions. Endotherms have a complex biochemistry highly adapted to the stable internal environment maintained by homeostasis. This means that endotherms are more efficient than ectotherms, because they are adapted to a specific range of conditions whereas in ectotherms the biochemical pathways have to be able to function at a wider range of temperatures and conditions. The reactions in endotherms also occur faster because of the higher temperature (diffusion will occur faster). In addition, the higher the body temperature, the more rapid the response of cells to the organism's needs. The CNS (central nervous system) functions more rapidly at high temperatures, giving faster responses to external and internal stimuli, such as the threat of predators. One reason why is because the diffusion of neurotransmitter substances at the synapses occurs faster. Muscle viscosity declines at high temperatures as well, resulting in a more rapid, forceful contraction and hence faster response times. This accounts for the greater locomotory speed (and stamina) of endotherms compared with ectotherms, particularly at night or when it is cool. Another advantage of this increased metabolism is that endotherms can be larger in size.
Therefore, while endothermy is an energetically expensive way of life, it also confers considerable freedom from the physical environment, especially at low temperatures. In these low temperatures endothermy is remarkably effective; some species of mammals and birds can live in the coldest places on earth whilst maintaining a body temperature 100oC above that of the air. Some endotherms are so well insulated that overheating can be a problem even at modest temperatures (e.g. seals retreat to the water when the temperature reaches 15oC). Evaporative cooling is an effective response to overheating, assuming that a plentiful source of water is available.
The increased mobility available to endotherms as a result of their higher metabolic rate is an important part of the response of large endotherms to both hot and cold environments. Examples include migrations - seasonal movements away from unfavourable conditions - and regular movements between scattered oases that provide water and shade.
Endotherms experience, therefore, the inverse in biological benefits and costs to those experienced by ectotherms. They can be thought of as high-rolling big spenders in energetic terms compared with the more modest ectotherms. Endotherms can do certain things on a bigger, faster scale, but only at a price. This is the requirement that they must take in large amounts of food and often water. Whilst endotherms are able, usually, to maintain a constant elevated internal temperature independent of the physical environment (unlike ectotherms), they are highly dependent on the biological environment for food (again, unlike ectotherms).
Disadvantages of Endothermy: Appetite and success:
Endotherms must eat much more than ectotherms each day simply to remain alive. This means that they can spend less time avoiding predators, and places enormous stress on them when they come to raising a family. In order to feed endotherms must be active for longer periods of time, which is energy-expensive and can lead to increased stress when daylight hours are reduced or it is cold, for example. Many birds migrate only to make use of the increased daylight hours in the polar summers because they need more time to catch food. Very small endotherms must eat continually to stay alive, so there is a lower limit to the size of endotherms. Endotherms also grow and reproduce slowly, compared with ectotherms, because the majority of energy consumed is used to elevate and maintain body temperature.
Another problem encountered by endotherms, because of their high metabolic rate, is that of overheating. The high rate of respiratory gas exchange makes them susceptible to dehydration in hot, dry environments. Evaporative cooling is only really effective as a short-term solution to this problem because of this risk of dehydration. Endotherms have therefore developed another mechanism to prevent overheating, which is also practised by ectotherms - behavioural thermoregulation. But endotherms are unfortunately victims of their own success: because their high insulation, which is so good at maintaining a high body temperature, means that it is also hard to lose heat energy! Many ectotherms are therefore more successful than endotherms in very hot environments. Small endotherms can maintain a constant body temperature by spending the day underground and emerging only when it is cool. However this response does not work for larger animals. The adaptations of camels demonstrate the principle of convergent evolution - that is the evolution of two different species towards a single adaptation. Camels are able to survive in the desert because they relax their limits of homeostasis when confronted by high temperatures or water shortage, just like ectotherms. Their tissues, like those of many ectotherms, are highly resistant to desiccation and temperature change. During the day a Camel's body temperature gradually increases from 34 to 40oC, this change being reversed at night. To meet the problem of dehydration the Camel stores energy as fat, which is then broken down to release metabolic water. It does not sweat, and so its tissues show a remarkable tolerance to desiccation. Because the camel does not maintain a constant body temperature it is not homeothermic, indeed it might be described as a heterotherm.
Some other endotherms are heterothermic. Many small mammals and birds are too small to survive periods of high stress, for example when it is too cold or hot or resources are scarce, and so they enter torpor, a state of adaptive hypothermia called hibernation. Hibernating animals are especially vulnerable, both to predators and to sudden environmental changes, because it can often take many hours to awake. However it is an energy-saving solution to the problem and is practised by many animals from humming-birds to polar bears, at the cost of forfeiting the benefits of endothermy for a while. Periods of torpor can be as brief as a few hours (overnight, such as humming-birds) or it can last for many weeks.
Heterothermy:
Many vertebrates are heterothermic. Examples include the mammals mentioned above. Some ectotherms also use metabolic heat, at times, to maintain an elevated body temperature. One example is the brooding female Indian python that elevates its body temperature with shivering thermogenesis, so as to provide warmth for the group of eggs around which it coils itself. Some tuna have particular adaptations for generating and retaining sufficient heat to raise the core temperature 10oC or more above their surroundings. As a result these fish are able to swim faster and respond quicker than other fish.
Heterothermy is worth mentioning because it represents the possibilities that are produced by combining ectothermy with limited, regional, endothermy (or endothermy with occasional ectothermy). These vertebrates are able to conserve energy (because they are not heating all their body all of the time) and yet reap a considerable reward.
One example of limited endothermy is that of a fever. This is a response many vertebrates initiate in response to an infection. Metabolic heat energy is used to raise the body temperature. Both endothermic and some ectothermic animals (mammals and certain lizards) are known to produce fevers. Experiments have shown that fevers increase the survival rates of certain lizards, but it is not yet known why.
Conclusion:
Endothermy and Ectothermy represent a metabolic dichotomy affecting far more than body temperature. The implications of both modes of existence extend to such areas as activity, physiology and behaviour. Endothermy and ectothermy each have their own strengths and weaknesses, and neither is mechanistically less complex than the other. Some ectotherms are capable of regulating their body temperature around a variable fixed point, depending on the level of activity required. This has the advantage of fuel economy, because it is generally reliant on 'free' sources of energy.
Endothermy and ectothermy also offer animals different advantages in different climates. In the tropics ectotherms can compete successfully with, or even outcompete, mammals both in abundance of species and in numbers of individuals because of the warm climate (that allows ectotherms to be active at night as well as day) and greater energy economy enjoyed by ectotherms. The energy they save from not maintaining an elevated body temperature can be diverted to reproduction and increasing biomass. In moderate to cold climates, however, the ectotherms are slower, are thus less competent as predators and are generally less successful than mammals. Near the poles, for example, there are no ectotherms only endotherms, because the endotherms' tissues are kept warm.
The above examples have shown that there is no real example of a true endotherm or a true ectotherm, only degrees in between the two ideals. A true ectotherm is reliant totally on the physical environment for heat energy, and this environment is therefore to them primarily important. A true endotherm can maintain a constant body temperature totally independent of the physical environment, but at the cost of increased metabolism requiring extra food; to the endotherm the biological environment is primarily important. These two cases demonstrate the particular characteristics and difficulties with the two modes of life. As has been shown, neither is more advantageous than the other, overall, although one may enjoy local benefits.
