endothermy and activity in vertebrates. what is endothermy and homeothermy? endothermy is the...

Endothermy and Activity Endothermy and Activity in Vertebrates in Vertebrates

What is Endothermy and What is Endothermy and Homeothermy?Homeothermy?• Endothermy is the maintenance of a high and Endothermy is the maintenance of a high and

constant body temperature by a metabolic constant body temperature by a metabolic means. means.

• Homeothermy is the maintenance of a constant Homeothermy is the maintenance of a constant body temperature in endothermic (warm-body temperature in endothermic (warm-blooded) animals, by the use of chemical blooded) animals, by the use of chemical processes to compensate for heat loss or gain processes to compensate for heat loss or gain when external temperatures change. Such when external temperatures change. Such processes include generation of heat by the processes include generation of heat by the breakdown of food and the contraction of breakdown of food and the contraction of muscles, and loss of heat by sweating, panting, muscles, and loss of heat by sweating, panting, and other means. and other means.

Endothermy Cont.Endothermy Cont.

• Endothermic animals expend great quantities Endothermic animals expend great quantities of energy to regulate and maintain internal of energy to regulate and maintain internal thermal conditions and functional processes thermal conditions and functional processes over wide range of environmental over wide range of environmental temperatures. temperatures.

• In contrast to ectotherms, they are generally In contrast to ectotherms, they are generally warmer than their environment and are often warmer than their environment and are often more active animals. more active animals.

• Only in mammals and birds is endothermy Only in mammals and birds is endothermy maintained under resting conditions. maintained under resting conditions.

HypothesisHypothesis

• Their hypothesis is that homeothermy Their hypothesis is that homeothermy evolved in two steps.evolved in two steps.

• The first enabled the mammals to The first enabled the mammals to invade temporally wide nocturnal invade temporally wide nocturnal niche that had been previously not niche that had been previously not been exploited fully by insectivores. been exploited fully by insectivores.

• The second enabled them to invade a The second enabled them to invade a diurnal niche and evolved the diurnal niche and evolved the acquisition of higher body acquisition of higher body temperatures and metabolic rates.temperatures and metabolic rates.

Evolution of HomeothermyEvolution of Homeothermy

• The first homeotherms lived in nocturnal The first homeotherms lived in nocturnal niches where their body temperature was niches where their body temperature was ten degrees Celsius lower than what is ten degrees Celsius lower than what is found in present day mammals (38C-40C).found in present day mammals (38C-40C).– This allowed them to maintain this This allowed them to maintain this

temperature without having as high of a temperature without having as high of a metabolic rate as mammals today.metabolic rate as mammals today.

– This eventually evolved into having higher This eventually evolved into having higher metabolic rates (higher body temp) which metabolic rates (higher body temp) which allowed them to become diurnal animals. allowed them to become diurnal animals. Some mammals have kept this low body Some mammals have kept this low body temperature because they use less energy.temperature because they use less energy.

Evolution of HomeothermyEvolution of Homeothermy

• The body temperature that these mammals acquired was The body temperature that these mammals acquired was set to as low as possible. Because the metabolic rate set to as low as possible. Because the metabolic rate increases 2 to 3 times with each increase of 10 C, the increases 2 to 3 times with each increase of 10 C, the animal would have to spend more time finding food to animal would have to spend more time finding food to survive.survive.

• The body temperature also can not be too hot because if it The body temperature also can not be too hot because if it is above the ambient temperature then the animal has to is above the ambient temperature then the animal has to use energy to evaporate their water to cool the body. This use energy to evaporate their water to cool the body. This would cause the small animals to have to constantly drink would cause the small animals to have to constantly drink water to compensate.water to compensate.

Evolution of HomeothermyEvolution of Homeothermy• Many believe that temperature causes restriction Many believe that temperature causes restriction

on activity such as rate of muscle contraction or on activity such as rate of muscle contraction or movement, however when looking at the movement, however when looking at the metabolic rates of Antarctic fish (0C-5C) they are metabolic rates of Antarctic fish (0C-5C) they are the same as the metabolic rates of tropical fish the same as the metabolic rates of tropical fish (25C-30C).(25C-30C).

• The rate of muscle contraction is limited by body The rate of muscle contraction is limited by body mass and size. As an animal increases in size, mass and size. As an animal increases in size, the speed decreases.the speed decreases.

• Muscles of a 30g mouse contracts three times Muscles of a 30g mouse contracts three times more rapidly than a 500kg horse. If temperature more rapidly than a 500kg horse. If temperature limited the speed of contraction, then the horse limited the speed of contraction, then the horse would have the same contraction speed as the would have the same contraction speed as the mouse with a body temperature about 10C lower.mouse with a body temperature about 10C lower.

SpeedSpeed

• Animals of the same size with Animals of the same size with different body temperatures can reach different body temperatures can reach almost the same top running speeds. almost the same top running speeds. – This proves the popular theory that body This proves the popular theory that body

temperature affects speeds of mammals temperature affects speeds of mammals is wrong. is wrong. •This theory was based on the fact that when This theory was based on the fact that when

reptiles have low body temperatures their reptiles have low body temperatures their movements are slow and sluggish.movements are slow and sluggish.

Body Temperature EvolutionBody Temperature Evolution

• The first nocturnal animals must have The first nocturnal animals must have maintained a constant body temperature maintained a constant body temperature of 30C. This would keep it from having a of 30C. This would keep it from having a metabolic rate of three to five times faster.metabolic rate of three to five times faster.

• Diurnal animals would have needed a body Diurnal animals would have needed a body temperature of 40C which would force temperature of 40C which would force them to have the higher metabolic rates to them to have the higher metabolic rates to maintain the higher temperatures then the maintain the higher temperatures then the nocturnal animals would have at night.nocturnal animals would have at night.

Examples of nocturnal Examples of nocturnal mammals todaymammals today

• Tenrecidae and Erinaceidae are two Tenrecidae and Erinaceidae are two families of insectivores that descended families of insectivores that descended from animals which have remained the from animals which have remained the nocturnal insectivore niche.nocturnal insectivore niche.– The retain many of the same The retain many of the same

characteristics and when the temperature characteristics and when the temperature of of Setifersetosis Setifersetosis was taken when they was taken when they emerged from their burrows it averaged emerged from their burrows it averaged 29.5C when ambient temperature was 29.5C when ambient temperature was 21C and 24C. 21C and 24C.

EnergeticsEnergetics

• The behaviors and temperature of The behaviors and temperature of these animals are the same as these animals are the same as reptiles, but what about their reptiles, but what about their energetics?energetics?– The measurements are variable with no The measurements are variable with no

actual answer for any specific animal. actual answer for any specific animal. For example, the Ethiopian hedgehog For example, the Ethiopian hedgehog has reptilian-like energetics while the has reptilian-like energetics while the European does not. European does not.

EnergeticsEnergetics

• To avoid the variability, the metabolic To avoid the variability, the metabolic rates of running lizards and mammals are rates of running lizards and mammals are compared.compared.– The rates of oxygen consumption of these The rates of oxygen consumption of these

animals of the same body weights increased at animals of the same body weights increased at the same rate with increasing speeds. the same rate with increasing speeds. However, the metabolic rate of the lizard was However, the metabolic rate of the lizard was lower consistently throughout the experiment. lower consistently throughout the experiment. • The difference was about the same as was thought to The difference was about the same as was thought to

be between the resting metabolic rates, so those of be between the resting metabolic rates, so those of the exercising animals were used. the exercising animals were used.

ExperimentExperiment

• The animals chosen were three European The animals chosen were three European hedgehogs, three tenrecs, three setfers, two hedgehogs, three tenrecs, three setfers, two opossums, and two echidnas and were trained opossums, and two echidnas and were trained to run on treadmills while oxygen consumed to run on treadmills while oxygen consumed was measured and temperature was being was measured and temperature was being taken.taken.

• These animals were trained for four months These animals were trained for four months with variations in their run being less then 5%.with variations in their run being less then 5%.

• The results were compared to those of the The results were compared to those of the exercising lizards and mammals before. exercising lizards and mammals before.

ResultsResults

• The volume of oxygen increased linearly The volume of oxygen increased linearly with speed among all of the animals (figure with speed among all of the animals (figure 2).2).

• Each of the insectivores (tenrecs, setifers Each of the insectivores (tenrecs, setifers and hedgehogs) have reptilian like and hedgehogs) have reptilian like energetics and are nocturnal.energetics and are nocturnal.

• The opossum and echidna have mammalian The opossum and echidna have mammalian type energetics, but have changed from type energetics, but have changed from their ancestors to become nocturnal as well. their ancestors to become nocturnal as well.

Were mammal like reptiles Were mammal like reptiles homeotherms??homeotherms??• Homeothermy was an adaptation which Homeothermy was an adaptation which

opened up a nocturnal niche for opened up a nocturnal niche for insectivores, however many try to argue insectivores, however many try to argue that mammal like reptiles were that mammal like reptiles were homeotherms before the mammals.homeotherms before the mammals.– One way in which mammals and mammal like One way in which mammals and mammal like

reptiles are similar is because of their bone reptiles are similar is because of their bone structure. Both have well vascularized structure. Both have well vascularized fibrolamellar containing primary osteons and fibrolamellar containing primary osteons and highly developed Haversian systems.highly developed Haversian systems.• This causes rapid growth rate, high rates of freeing or This causes rapid growth rate, high rates of freeing or

fixing phosphocalcic salts and a high metabolic rates fixing phosphocalcic salts and a high metabolic rates which can be considered evidence of homeothermy which can be considered evidence of homeothermy because ectotherms have the opposite of these.because ectotherms have the opposite of these.

Were mammal like reptiles Were mammal like reptiles homeotherms?? Contd….homeotherms?? Contd….

– Since passerine birds and small Since passerine birds and small mammals lack the typical mammalian mammals lack the typical mammalian bone histology is an indicator of overall bone histology is an indicator of overall activities and growth rate, however activities and growth rate, however there is no link between bone histology there is no link between bone histology and homeothermy.and homeothermy.

Were mammal like reptiles Were mammal like reptiles homeotherms?? Contd….homeotherms?? Contd….• Another scientists suggests that mammal-like Another scientists suggests that mammal-like

reptiles were homeotherms because of the reptiles were homeotherms because of the ectothermic predator/prey ratio which were ectothermic predator/prey ratio which were measured in the Late Permian and Early Triassic measured in the Late Permian and Early Triassic fossil deposits.fossil deposits.– This predator/prey ratio is greater then the endothermic This predator/prey ratio is greater then the endothermic

ratio, which suggests a higher rate of activity of ectotherms.ratio, which suggests a higher rate of activity of ectotherms.– Also, the tooth crown and jaw movement suggest high rates Also, the tooth crown and jaw movement suggest high rates

of processing food. It may have been possible to have high of processing food. It may have been possible to have high rates of activity without homeothermy, just as it is possible rates of activity without homeothermy, just as it is possible to have low activity in animals who are homeotherms.to have low activity in animals who are homeotherms.• However, neither rate of activity or resting metabolic rates are However, neither rate of activity or resting metabolic rates are

not solid proof of homeothermy. not solid proof of homeothermy.

Evolution of EndothermyEvolution of Endothermy

• The evolution of endothermy in mammals and The evolution of endothermy in mammals and birds has been the subject of considerable birds has been the subject of considerable speculation and debate has also been raised speculation and debate has also been raised concerning metabolic thermoregulation among concerning metabolic thermoregulation among the dinosaurs.the dinosaurs.

• These discussions are generally based on the These discussions are generally based on the assumption that the endothermic condition is the assumption that the endothermic condition is the end product of selection for a high and stable end product of selection for a high and stable body temperature.body temperature.

• Attention has been centered upon the Attention has been centered upon the advantages of a relatively high and constant body advantages of a relatively high and constant body temperature. temperature.

Evolution of Endothermy Evolution of Endothermy Cont.Cont.• Arguments for the evolution of endothermy based Arguments for the evolution of endothermy based

on thermoregulatory considerations alone tend to on thermoregulatory considerations alone tend to be ad hoc rather than uniform explanation be ad hoc rather than uniform explanation equally applicable to all groups. equally applicable to all groups.

• Bennett and Ruben suggest that Bennett and Ruben suggest that thermoregulation was not the sole selective thermoregulation was not the sole selective advantage behind the evolution and was, advantage behind the evolution and was, perhaps, not even the initial factor that began perhaps, not even the initial factor that began evolution. They believe that this process was evolution. They believe that this process was directly linked with the development of high directly linked with the development of high activity that was sustained by aerobic activity that was sustained by aerobic metabolism. metabolism.

Cost of EndothermyCost of Endothermy

• Hypotheses based only on Hypotheses based only on thermoregulatory considerations fail to thermoregulatory considerations fail to justify in selective terms the great justify in selective terms the great increments in energy consumption increments in energy consumption demanded by endothermy. demanded by endothermy.

• The rate of resting metabolic The rate of resting metabolic expenditure is similar among all groups expenditure is similar among all groups of ectothermic animals, including the of ectothermic animals, including the lower vertebrates.lower vertebrates.

Cost of Endothermy Cont.Cost of Endothermy Cont.

• Resting and maximal levels of oxygen Resting and maximal levels of oxygen consumption of endothermic vertebrates consumption of endothermic vertebrates exceed those of ectotherms by an average of exceed those of ectotherms by an average of five-to tenfold. five-to tenfold.

• Endotherms have a much broader range of Endotherms have a much broader range of activity that can be sustained by this activity that can be sustained by this augmented aerobic metabolism.augmented aerobic metabolism.

• Ectotherms are more reliant upon, and limited Ectotherms are more reliant upon, and limited by, anaerobic metabolism during activity. by, anaerobic metabolism during activity.


• The higher rate of heat production (by the The higher rate of heat production (by the mammal or bird), coupled with a low thermal mammal or bird), coupled with a low thermal conductance, is the basis of endothermy and it conductance, is the basis of endothermy and it exists even when the differences in body exists even when the differences in body temperature between ectotherms and temperature between ectotherms and endotherms are eliminated. endotherms are eliminated.

• Thus, high body temperature alone does not Thus, high body temperature alone does not establish equivalent metabolic rates. establish equivalent metabolic rates.

• Under most circumstances, however, as ambient Under most circumstances, however, as ambient temperature decreases, the body temperature of temperature decreases, the body temperature of the endotherm remains constant and that of the the endotherm remains constant and that of the ectotherm decreases, thus increasing the ectotherm decreases, thus increasing the metabolic differential between the animals. metabolic differential between the animals.


• Depending on environmental temperature, Depending on environmental temperature, an endotherm expends a great deal of an endotherm expends a great deal of energy for thermoregulation alone, an energy for thermoregulation alone, an amount that has been estimated to be as amount that has been estimated to be as high as 80-90% of total energy intake of a high as 80-90% of total energy intake of a rodent active under natural conditions. rodent active under natural conditions.

• If an animal attempted to increase its food If an animal attempted to increase its food intake by 5-to 20 fold it would at the same intake by 5-to 20 fold it would at the same time increase the danger. time increase the danger.

Aerobic CapacityAerobic Capacity

• As physical activity increases, power As physical activity increases, power requirements are met by augmented aerobic requirements are met by augmented aerobic metabolism. After an initial lag phase, metabolism. After an initial lag phase, organismal oxygen consumption rises in organismal oxygen consumption rises in proportional demand. proportional demand.

• If energetic demand exceed the support If energetic demand exceed the support capacity of aerobic systems, anaerobic capacity of aerobic systems, anaerobic metabolism is used to supplement energy metabolism is used to supplement energy input.input.

• In vertebrates, anaerobic metabolism involves In vertebrates, anaerobic metabolism involves principally the production of lactic acid. principally the production of lactic acid.

Aerobic Capacity Cont.Aerobic Capacity Cont.

• Among the ectothermic vertebrates, maximal Among the ectothermic vertebrates, maximal oxygen consumption and aerobic scope are oxygen consumption and aerobic scope are proportionately as low as resting oxygen proportionately as low as resting oxygen consumption in comparison to the aerobic consumption in comparison to the aerobic capacities of endotherms.capacities of endotherms.

• During maximal activity, oxygen consumption in During maximal activity, oxygen consumption in the lower vertebrates can be increased an average the lower vertebrates can be increased an average of five-to tenfold at any single body temperature. of five-to tenfold at any single body temperature. Thus a body temperatures characteristic of Thus a body temperatures characteristic of mammals and birds, levels of oxygen consumption mammals and birds, levels of oxygen consumption in maximally active ectotherms are equal resting in maximally active ectotherms are equal resting levels of oxygen consumption in the former groups. levels of oxygen consumption in the former groups.


• Mammals and birds during activity are also Mammals and birds during activity are also capable of increasing oxygen consumption capable of increasing oxygen consumption by an average of five to tenfold. The levels by an average of five to tenfold. The levels of aerobic metabolism achieved by of aerobic metabolism achieved by endotherms are thus considerably in excess endotherms are thus considerably in excess of those attained by lower vertebrates. of those attained by lower vertebrates.

• There appears to be a consistent linkage There appears to be a consistent linkage between resting and maximal levels of between resting and maximal levels of oxygen consumption in the vertebrates. oxygen consumption in the vertebrates.


• When an animal is in any given physiological When an animal is in any given physiological state, oxygen consumption may increase an state, oxygen consumption may increase an average of only five to tenfold. This generalization average of only five to tenfold. This generalization is true not only for animals active at normal body is true not only for animals active at normal body temperatures but also for ectotherms over a temperatures but also for ectotherms over a range of body temperatures; low levels of resting range of body temperatures; low levels of resting oxygen consumption are associated with low oxygen consumption are associated with low levels of maximal oxygen consumption. levels of maximal oxygen consumption.

• Hibernating mammals are unable to achieve Hibernating mammals are unable to achieve normothermic rates of maximal oxygen normothermic rates of maximal oxygen consumption until normally active body consumption until normally active body temperatures have been resumed. temperatures have been resumed.

Consequences for Locomotion Consequences for Locomotion and Sustained Activityand Sustained Activity• The differential between maximal aerobic power The differential between maximal aerobic power

input in vertebrate endotherms and ectotherms input in vertebrate endotherms and ectotherms has direct consequences for the level and has direct consequences for the level and duration of activity which these animals can duration of activity which these animals can sustain.sustain.

• Anaerobic metabolism provides the additional Anaerobic metabolism provides the additional energy required at speeds in excess of that at energy required at speeds in excess of that at which oxygen consumption becomes maximal. which oxygen consumption becomes maximal.

• Thus, two animals have similar costs of Thus, two animals have similar costs of locomotion, the animal with the higher aerobic locomotion, the animal with the higher aerobic scope will be able to attain and sustain greater scope will be able to attain and sustain greater speeds aerobically. speeds aerobically.

Consequences for Locomotion Consequences for Locomotion and Sustained Activityand Sustained Activity• In comparison to aerobic potential, the cost of In comparison to aerobic potential, the cost of

locomotion in terrestrial vertebrates is high, and locomotion in terrestrial vertebrates is high, and even modest levels of activity quickly outstrip the even modest levels of activity quickly outstrip the aerobic scopes of the lower vertebrates. aerobic scopes of the lower vertebrates.

• Any activity greater than a slower walk in Any activity greater than a slower walk in terrestrial ectotherms entails anaerobic terrestrial ectotherms entails anaerobic metabolism and the production of lactic acid.metabolism and the production of lactic acid.

• The metabolic power that overshadows the The metabolic power that overshadows the aerobic abilities of most lower vertebrates, and aerobic abilities of most lower vertebrates, and moderate or high levels of activity in these moderate or high levels of activity in these animals appear to be attainable only with the animals appear to be attainable only with the activation of anaerobiosis.activation of anaerobiosis.

Consequences for Locomotion Consequences for Locomotion and Sustained Activityand Sustained Activity

• As the article points out, the activity of the As the article points out, the activity of the Cnemidophorus murinus (lizard) Cnemidophorus murinus (lizard) illustrates illustrates the behavioral and locomotor the behavioral and locomotor consequences of this pattern of metabolic consequences of this pattern of metabolic support. support.

• The lizard belongs to the Teiidae family, The lizard belongs to the Teiidae family, one of the most active groups of lizards. It one of the most active groups of lizards. It is highly alert and weighs approximately is highly alert and weighs approximately 100 g as an adult. 100 g as an adult.


• This is a species that might be expected to This is a species that might be expected to maximize aerobic potential. However, the maximize aerobic potential. However, the oxygen consumption becomes maximal at oxygen consumption becomes maximal at only 0.3 km/hr. At greater speeds anaerobic only 0.3 km/hr. At greater speeds anaerobic metabolism is activated, and the lizard metabolism is activated, and the lizard reaches exhaustion in less than 5 minutes reaches exhaustion in less than 5 minutes when walking at only 0.5 km/hr.when walking at only 0.5 km/hr.

• These speeds are slow, and speeds of These speeds are slow, and speeds of 3.2km/hr can be maintained for 1 minute 3.2km/hr can be maintained for 1 minute with bursts up to 8.2 km/hr over short with bursts up to 8.2 km/hr over short distances. distances.


• Thus, although aerobic abilities and Thus, although aerobic abilities and stamina are limited, a substantial stamina are limited, a substantial increase in activity can be obtained increase in activity can be obtained anaerobically.anaerobically.

• This activity can not be sustained, This activity can not be sustained, and anaerobic metabolism during and anaerobic metabolism during activity can be supplement, but not activity can be supplement, but not supplant, aerobic metabolism. supplant, aerobic metabolism.

Selective Factors in the Selective Factors in the Evolution of EndothermyEvolution of Endothermy

• The high levels of resting metabolism of The high levels of resting metabolism of endotherms are associated with high endotherms are associated with high levels of maximal oxygen consumption.levels of maximal oxygen consumption.

• The selective advantages of increased The selective advantages of increased activity capacity are not subtle but rather activity capacity are not subtle but rather central to survival and reproduction. central to survival and reproduction.

• An animal with greater stamina has an An animal with greater stamina has an advantage that is readily comprehensible advantage that is readily comprehensible in selective terms. in selective terms.

Selective Factors in the Selective Factors in the Evolution of EndothermyEvolution of Endothermy• It can sustain greater levels of pursuit or It can sustain greater levels of pursuit or

flight in gathering food of avoiding becoming flight in gathering food of avoiding becoming food. food.

• It will be superior in territorial defense or It will be superior in territorial defense or invasion. invasion.

• It will be more successful in It will be more successful in mating/courtship.mating/courtship.

• These advantages appear to be worth These advantages appear to be worth increased energetic costs, particularly, since increased energetic costs, particularly, since the enhanced capacities give their the enhanced capacities give their possessor the ability to increase energy possessor the ability to increase energy intake and to meet new energy demands.intake and to meet new energy demands.

Enhancement of Aerobic Enhancement of Aerobic CapacitiesCapacities

• How might the evolution of increased How might the evolution of increased aerobic capacity have occurred?aerobic capacity have occurred?

• An examination of the diversity of An examination of the diversity of modern reptiles can suggest modern reptiles can suggest adjustments possible within the adjustments possible within the reptilian framework.reptilian framework.

• The most active species may be The most active species may be expected to have maximized their expected to have maximized their potential for oxygen transport. potential for oxygen transport.


• Further increments in oxygen Further increments in oxygen processing ability would appear to processing ability would appear to require energetic investments in require energetic investments in morphological structures associated morphological structures associated with oxygen uptake and transport—for with oxygen uptake and transport—for example, development of an even example, development of an even more complex lung and ventilation more complex lung and ventilation system, the subdivision of the heart system, the subdivision of the heart and a great increment in blood flow and a great increment in blood flow and pressure.and pressure.


• Skeletal muscle, the tissue with the greatest Skeletal muscle, the tissue with the greatest aerobic scope, is the principal consumer of aerobic scope, is the principal consumer of oxygen during activity.oxygen during activity.

• Skeletal muscle has a relatively low metabolic Skeletal muscle has a relatively low metabolic rate at rest and does not constitute a major rate at rest and does not constitute a major thermogenic organ in resting mammals.thermogenic organ in resting mammals.

• Endothermy is not based on muscle Endothermy is not based on muscle metabolism alone but involves increments in metabolism alone but involves increments in the metabolic rates of many organs.the metabolic rates of many organs.


• The greater metabolic rate of the endotherm The greater metabolic rate of the endotherm appears to be due to the greater appears to be due to the greater concentrations of mitochondria in its tissues. concentrations of mitochondria in its tissues.

• If the catalytic capacity of the mitochondrial If the catalytic capacity of the mitochondrial enzymes represented a major limitation on enzymes represented a major limitation on the oxygen-processing ability of animals the oxygen-processing ability of animals evolving higher metabolic rates, this capacity evolving higher metabolic rates, this capacity would have bee increased by increasing would have bee increased by increasing mitochondrial volume.mitochondrial volume.

endothermy and activity in vertebrates. what is endothermy and homeothermy? endothermy is the...

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