Dinosaur Energetics

Australia: The Land Where Time Began

Dinosaur Energetics

There are 2 forms of power production in vertebrates, aerobiosis and anaerobiosis. Aerobiosis, directly using the oxygen absorbed through the lungs to power the functions such as muscles, has an advantage, it can power the functions such as muscles indefinitely, but it is limited by the maximum power output. Animals exercise aerobically when they walk for long distances at moderate speeds. Anaerobiosis is the other form of power production, chemical reactions that don't require oxygen immediately is used to power muscles. An advantage of anaerobiosis is that it can generate about 10 times more power per unit tissue and time, the problem with this method of power production is that it cannot be maintained indefinitely, producing toxins that can lead to serious illness if it is sustained at a rate that is too high for too long. Following a period in which the muscles use anaerobiosis the oxygen debt that has built up must be repaid during a period of recovery. Muscles of an animal running at top speed use anaerobiosis.

Low resting metabolic rates and low aerobic exercise capacity are features of most fish, all amphibians and reptiles - they are bradyenergetic. The result is that even the most aerobically capable monitor lizards, such as goannas, cannot sustain high levels of activity for extended periods of time. In many bradyenergetic animals, such as monitors and crocodilians, they can attain anaerobic burst activity at high levels, as when they suddenly run towards prey. Being bradyenergetic animals with low metabolic rates they largely depend on external heat sources such as the ambient temperature and the sun for their body heat, making them ectothermic. The result of this is that bradyenergetic animals tend to undergo large fluctuations of body temperature - they are heterothermic. Reptiles operate normally at widely varying body temperatures depending on their normal habit. Some have adapted to function optimally at modest temperatures of 12^o C (52^o F). When reptiles live in hot environments they are optimised to function at temperatures of 38^o C (100^o F) or higher, therefore calling reptiles cold-blooded is inaccurate. The higher the body temperature generally the more active the animal can be, though the activity of reptiles is limited even in warm animals.

The resting metabolic rates and aerobic exercise capacity are high in most birds and mammals, so they are tachyenergetic and can sustain high activity levels for extended periods of time. The author suggests the main advantage of being tachyenergetic is probably the ability to exploit oxygen better for power over time. Anaerobic power is also used by tachyenergetic animals for brief periods to reach the highest levels of athletic performance they are capable of, though even over these comparatively short periods they rely on it less than reptiles do and the recovery period is much quicker than in reptiles. Most of the heat produced by tachyenergetic animals is produced internally because they have a high metabolic rate, which makes them endotherms, which allows tachyenergetic animals to maintain body temperatures that are more stable than those of reptiles. As with humans, some tachyenergetic animals are fully homeothermic, able to maintain a constant body temperature at all times, as long as they are healthy. The body temperatures of many birds and animals are allowed to fluctuate to varying degrees on a daily and/or seasonal basis, which makes them heterothermic. Another advantage that comes with a high metabolic rate is the ability to keep the body at, or at least near, the optimal operating temperature. There are a range of normal body temperatures from 30^o C-44^o C (86^o F-105^o F), birds being always at least 38^o C. In tall animals high blood pressures are required that in turn require the cardiac muscles to work hard to achieve sufficient pressures.

The resting metabolic rates and aerobic exercise capacities of mammals and birds are required to be about 10 times higher than those of reptiles, with the energy budgets being even higher. In tachyenergetic animals there is a substantial variation from these norms, some mammals such as monotremes, some marsupials, hedgehogs, armadillos, sloths, and manatees that have modest energy consumption levels and aerobic performance, that in some cases is not much higher than those in the most energetic reptiles. The marsupials are generally less energetic than equivalent placentals, a feature that allows them to thrive in an environment with more limited resources than can sustain the same population of equivalent placentals. as a result of this difference kangaroos are about 1/3 more energy efficient than deer. Big ratites achieve a similar level of energy efficiency as marsupials of a similar size. Extremely high oxygen consumption levels are found in some small birds and small mammals.

According to the author¹ animals have evolved energy systems that vary widely to allow a given species to survive in a particular habitat with a particular lifestyle. It is possible for reptiles to survive, and even thrive, on limited resources because they are so energy efficient. Much higher levels of activity can be maintained by tachyenergetic animals allowing this ability to get the extra food needed to reproduce, the key factor in evolutionary success. Mammals and birds have been allowed to become dominant large animals from the tropics to the poles because of tachyenergy, though in the tropics, and to a lesser extent in temperate regions, reptiles are very numerous and successful.

In spite of the diverse energy systems there are some things that insects can do but vertebrates cannot. The resting metabolic rates of in sects are as low as those of reptiles, but larger insects use oxygen at rates as high as the rates of birds and bats, insects therefore achieving maximum/minimal metabolic ratios that are extremely high, with the result that they are both energy efficient and aerobically capable. The dispersed system of the tracheae to oxygenate the muscles of insects allows them to achieve such high ratios. A very high aerobic exercise capacity and a very low metabolism is not present in any vertebrate, the author¹ suggesting it is probably due to the centralised respiratory-circulatory system requiring the internal organs to work hard even when resting in tachyenergetic vertebrates. This arrangement seen in insects should not be applied to dinosaurs. He also suggests that all the energy systems evolved by vertebrates are unlikely to have survived until the present, so it is necessary to consider the possibility that some or all dinosaurs were exotic forms.

Up until the 1960s it was generally assumed that the energetics of dinosaurs was largely similar to that of reptiles, though it is now believed by most researchers that the power production and thermoregulation of dinosaurs was probably closer to that of mammals and birds, and as there was a considerable range of diverse forms of dinosaur, there was probably also a wide range of energetics among them, as occurs in birds, and to a greater degree than in mammals.

The legs of reptiles are sprawling, non-erect that are suitable for slow waking speeds of 1-2 km/h (0.5-1 mph), and their low aerobic capacity can power such walking over periods of time that can be extended. The author¹ suggests that for reptiles there was an advantage in having sprawling legs as they made it easier for them to drop to their bellies to rest if they became exhausted. There are no bradyenergetic animals that have erect legs, walking always being energy expensive, up to 12 times more expensive than swimming the same distance, so animals that walk faster than 3 km/h are tachyenergetic forms. As with birds and mammals dinosaurs had long, erect legs, favouring the high walking speeds of 3-10 km/h (2-6 mph), the tachyenergetic animals being the only ones that can maintain such speeds for hours at a time. The length of the stride of an animal allows the speed of traveling to be estimated approximately for an animal of a given size. Shorter strides indicate the animal is walking more slowly than when it is moving faster. Speeds of more than 3 km/h is indicated for for the walking speed for many dinosaurs as indicated by the many known trackways. The trackways of prehistoric reptiles indicate they moved much slower. The sustained aerobic capacity of dinosaurs has been estimated from trackways that the sustained aerobic exercise capacity exceeded the maximum for reptiles.

The leg muscles of the fastest of the reptiles are slender as their low capacity respirocirculatory systems are unable to supply enough oxygen to support larger locamotory muscles. The leg muscles of mammals and birds tend to be large, propelling them over long distances at high speeds, resulting in mammals and birds having a broad pelvis to support a large set of thigh muscles. A short pelvis anchoring narrow thigh muscles was common to protodinosaurs, the first theropods and the prosauropods, though they have long, erect legs. no modern animals have such a combination. According to the author¹ these factors suggest the small-hipped dinosaurs probably had an extinct type of metabolic system that he suggests was probably intermediate between that of reptiles and mammals. Large hips were present on all other known dinosaurs that could support the large thigh muscles that are typical of more aerobically capable animals. The therizinosaurs, stegosaurs and the armoured ankylosaurs, that were among the big-hipped dinosaurs, that were relatively sluggish, are believed to have had lower energy budgets than the faster moving dinosaurs.

The author¹ suggests that the height the brains of many dinosaurs could be held, well above their hearts, suggests they had a high level of power production that is present in birds and mammals that are tall. See Dinosaurs - Sauropod Necks.

He suggests the protodinosaurs, early theropods and prosauropods are believed to have had unsophisticated lungs and an intermediate metabolism is compatible with such lungs. When considering the ornithischians not enough is known of their respiration to allow it to be associated with a particular metabolic level, though as they are believed to possibly have had a diaphragm that was mammals-like suggests they possibly had a level of oxygen intake similar to that of mammals. It is widely believed that the bird-like respiratory complex, that is ventilated by air sacs and is highly efficient, was present in avepod sauropods is widely seen as evidence for the evolution of elevated oxygen consumption in these dinosaurs. The sauropods probably needed an efficient respiratory complex as is present in birds to oxygenate a high metabolic rate through their long necks. This also applied to some marine reptiles such as plesiosaurs, though they didn't need air sacs to pull large volumes of air into their lungs as they had low metabolic rates.

Large nasal passages containing respiratory turbinals are present in many birds and mammals, the turbinals aiding in the retention of heat and water from the exhaled air by processing the air, that would otherwise be lost during the high respiration levels associated with a high metabolic rate. Reptiles don't need respiratory turbinals as they breath more slowly. Some have pointed out that there are no preserved nasal turbinals in the nasal passages of dinosaurs, as well as the small dimensions of some dinosaur nasal passages, suggesting it is evidence that dinosaurs had the slow respiration of reptiles that are bradyenergetic. The lack of well-developed respiratory turbinals is also found in a number of birds and mammals, and they are completely cartilaginous with no bony traces in some birds. There are also some birds that don't breathe primarily through their nasal passages. In California condors there are tiny nostrils. There is also a problem in that in some dinosaurs the space available for turbinals has been underestimated, while other dinosaurs had nasal passages that were very large there would have been capable of accommodating examples of these structures that were unossified. The author¹ suggests the evidence doesn't appear to be definitive.

Some strong evidence of elevated metabolic rates is the presence of a blanket of hollow fibres in a growing array of small dinosaurs. The absorption of heat from the environment that is necessary to maintain body heat in ectotherms would have been prevented by insulation, hence it is not found in animals that are bradyenergetic. The evolution of high metabolic rates at an early point in their evolution is indicated by the presence of insulation at these early stages, either at the beginning of the group or in their ancestral forms. As with the very large mammals naked skin is compatible with high metabolic rates in large dinosaurs, many suids, human children, and even a small naked bat. As most dinosaurs lived in tropical climates so the need for insulation was reduced, the bulk of the very large dinosaurs eliminating the need for it.

Land reptiles lack the ability to gather enough food to allow them to grow rapidly probably because of their low exercise capacity. That tachyenergetic animals can eat the large quantities needed to produce the power to gather the additional large quantities of food they need to grow rapidly is said by the author¹ to be an expression of principle that it takes money to make money. The food of tachyenergetic juveniles is either gathered by themselves or provided by their parents. Dinosaurs of all sizes grew faster than land reptiles of similar size indicating that dinosaurs had aerobic capacity and energy budgets that were markedly higher than land reptiles. The small ornithopods apparently grew especially rapidly, and the fast growth of the gigantic dinosaurs suggest they had strongly elevated metabolic levels.

The metabolism of dinosaurs has been assessed with the aid of bone isotopes that have been used to examine the fluctuations of temperature that the bone experienced in life. The dinosaur is believed to be heterothermic, on either a daily or seasonal basis, if the bones display evidence of strong differences of temperature. Such cases could indicate either a bradyenergetic ectotherm or a tachyenergetic endotherm that spends the winter in hibernation. The dinosaurs of all sizes have been shown by this technique to be homeothermic, therefore being more tachyenergetic and endothermic than crocodilians from the same formation. There was evidence that an ankylosaur was heterothermic, but as the animal lived at high latitudes is has been suggested that it hibernated in the long winters, possibly sleeping in dense brush where its heavy armour may have protected it from the cold weather as well as from predators.

It has been found that in the polar regions there was a diverse array of dinosaurs, from the small to the gigantic, in areas that would have experienced freezing conditions in winter, it has been suggested that dinosaurs needed to be able to generate more internal heat than reptiles, which are either very rare or were completely excluded from such habitats. It has also been suggested that it was not practical for the dinosaurs to have migrated far enough towards the equator to escape the freezing conditions of winter at the poles. As well as costing too much time and energy, in some places their path would have been blocked by oceans. The author¹ suggests that the presence of giant sauropods in the habitats that were very cold in winter refutes the proposal that they maintained their body heat by their sheer bulk as only a small amount of heat would be generated by a reptilian type of metabolism. He suggests it would have been necessary for these giants to have had elevated metabolic level to keep their core temperature within the necessary range and to stop the skin from freezing. It has been found by the study of the growth rings of the bones of polar dinosaurs that at least some of the polar dinosaurs didn't hibernate in the very cold winters, something ectotherms would not have been able to do. Since burrows have been found in parts of Australia that were in polar regions at the time it may be that some small ornithopods may have hibernated though winter as bears of the present do.

The ratites, the largest and most primitive of living birds, have been found to have metabolic rates similar to those of marsupials, it has been suggested that the energy budget of dinosaurs may have been such that they didn't exceed that of marsupials. Some bone isotope data suggest that this may indeed be the case with dinosaurs having moderately high food consumption levels, that of most placentals of the same size being somewhat higher. The tall sauropods with their high circulatory pressures and polar dinosaurs that remained active during winter, needed to produce a lot of heat. Early dinosaurs, slower growing armoured dinosaurs, and the therizinosaurs at the opposite extreme, are believed to have probably had energy budgets similar to those of the less energetic mammals. It is believed likely that dinosaurs tended to control their body temperature less precisely than mammals, asdo birds. This accords with the presence of bone rings. It has been suggested that most dinosaurs probably had high temperatures of about 38^o C or more to allow them to best avoid overheating, as they lived at a time when most of the earth was largely hot. High-latitude dinosaurs appear to be a possible exception, which have been proposed to have possibly adapted to their habitat by having a lower temperature, which would have saved some energy if they were active during winter.

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