Australia: The Land Where Time Began

A biography of the Australian continent 

Dinosaurs - Gigantism

Dinosaurs evolved from protodinosaurs that were small and many remained small, including birds, but there was a tendency among the group to gigantism. The average mammal size is about that of a dog, the average dinosaur is more like the size of a bear. The largest of the predatory theropods weighed as much as 10 tonnes, as big as elephants, the largest carnivorous mammals are small by comparison, by a factor of 10 or more. The largest land mammal was the indricothere rhinos, long-legged animals that grew to about 15-20 tonnes, were dwarfed by the sauropods by a factor of 5 or more.

The only animals that became gigantic while remaining terrestrial forms, whose energetics are known, were tachyenergetic. Among the reptiles the biggest fully terrestrial forms such as big tortoises and monitors didn't get to more than 1 tonne. Land reptiles grow too slowly to reach very large size in a reasonable time, and there may well be other factors limiting their size. One suggestion is that gravity, 1 g, requires high levels of sustained aerobic power for animals to reach the size of dinosaurs. According to this suggestion bradyenergetic animals, that don't produce enough power to pump their blood much above their heart height, simply don't have the power to grow to gigantic size on land, needing to take to the water to get much bigger.

The extreme heights of sauropod heads when they had their necks raised indicates that their blood needed to be pumped many metres to the head at pressures of 2-3 times higher than the 200 mm Hg required for giraffes to get oxygen to their brains. The author1 suggests that as these animals were so tall and massive it could be catastrophic for them to fall because of even a passing bout of 'light headedness" caused by a faltering blood (oxygen) supply. This indicates that the sauropods must have had hearts that were oversized and required very high oxygen consumption just to power the heart. He also suggests that it may not be possible for a land animal to grow more than 20 m (65 ft) tall, both as the result of the very high pressure produced in the feet by such a tall liquid column and because of the great pressure that would be required to pump the blood up to such heights to the brain. Vascular adaptations would have been needed for the supertall animals to cope with blood pressures that would have varied greatly between the posture needed for drinking and reaching as high as possible to feed on trees when they possibly raised their heads to their maximum vertical reach.  .

Terramegathermy is the hypothesis that tachyenergetic animals are the only ones that can grow to enormous dimensions on land (without the assistance provided  by being buoyed up by water).

Gigantothermy is an alternative hypothesis that proposes that the metabolic systems of giant reptiles converges with the metabolic systems of giant mammals with the result that there is energy efficiency in all giant animals. In this view giant animals rely on their great mass to achieve thermal stability, not on the production of large amounts of heat. The author1 suggests that the idea of gigantothermy is a reflection of a misunderstanding of how the power systems of animals work. The power required to sustain high levels of activity is not supplied by a consistently high body temperature, it merely allows animals that are tachyenergetic, and only animals with high aerobic exercise capacity, to sustain high levels of activity at all times. Based on this, gigantic reptiles with high body temperatures would not be able to be highly athletic for extended periods. Studies have shown that the metabolic rates and aerobic capacity of elephants and whales are as high as expected in their size of mammals, and are much higher than are found in turtles and the biggest of the crocodilians, both have low heat production levels that are typical of reptiles. See Dinosaurs - Sauropod Necks.

The question of how sauropods ate enough food with their small heads, a problem made worse if they were tachyaerobic animals of such great size. As with the small heads of emus or ostriches the small heads of the sauropods were 'basically all mouth'. In herbivorous mammals the heads mostly consist of the dental batteries that are used to chew the food after it has been cropped with the mouth at the front end of the jaws. The heads of the sauropods are only small in relation to the body size, not in actual dimensions, in the biggest of the sauropods the mouth was big enough to fit the entire head of a giraffe. Compared with herbivorous mammals that have been scaled up to the same size and body mass the width of the mouth is the same. If a 50 tonne sauropod ate the same amount of food that would be expected for a mammal of its size it would need to eat more than half a tonne of fresh fodder each day, about 1 % of its body mass. It has been suggested that if the sauropod spent 14 hours per day feeding, taking 1 bite every minute it would need to bite off about 1/2 kg of plant material with each bite. As the sauropod head weighed as much as a human body and had a mouth that was 0.5 m wide (1.5 ft), it would not have been a problem to eat enough to maintain its mass.

Some have suggested that the giant dinosaurs would have overheated in the warm climatic conditions of the Mesozoic if they produced energy at the same levels as mammals or birds. The author1 suggests that big birds and mammals are the largest animals living in the modern tropics, including deserts. Some of the largest elephants are found in the Namib Desert of the Skeleton Coast of southwestern Africa where is it common for them to need to tolerate extreme heat and sun without shade being available.

The author1 disputes the widely held belief that elephants cool off by flapping their ears when the temperature is extremely hot, one trick dinosaurs could not have mastered as they lacked the external pinnae on their ears. He says elephants actually flap their ears when the temperature is lower than that of their bodies. Heat can't flow out when the ambient temperature is the same as their bodies, the ears picking up heat when the air is warmer than the body. Until fairly recently the African elephant was not the main savanna elephant, the previous dominant savanna elephant was Elephas recki, one of the biggest ever land mammals, that was closely related to the Asian elephant, and has been suggested to have had small ears that would have had little use in dissipating body heat regardless of the ambient temperature. He suggests it is actually the small animals that are in greatest danger of heat exhaustion and stroke, as heat is picked from their environment rapidly. In drought when water is in such short supply it cannot be used for evaporative cooling the danger is especially acute. Large animals are protected from the high heat loads that occur on very hot days because of their low surface to volume ratio and they can store heat generated  by their bodies internally. The heat produced by the bodies of large birds and mammals can be stored, both allowing their temperatures to rise a few degrees above normal, dumping the heat into the cooler air at night in preparation of the next part of the cycle the following day.

The mode of reproduction of dinosaurs is suggested by the author1 to possibly be another reason the dinosaurs could grow to such enormous size. Mammals are K-strategists that care for the small number of young they produce, meaning a large population of adults is necessary to raise the next generation. In an elephant herd there are about as many breeding adults as there are juveniles requiring parental care to survive. To avoid overexploitation of their habitat the size of the adults needs to be limited, as there needs to be a lot of adults. If the habitat is overexploited the habitat collapses. 10-20 tonnes appears to be the limit for mammalian herbivores that are slow to reproduce. As they prey on the surplus herbivores there is an even smaller resource base for the carnivores, and the suggested limited of size for the carnivores appears to be between 0.5 and 1 tonne.

One feature in which the gigantic dinosaurs differed from the large mammals was they were r-strategists, producing large numbers of offspring that were independent from the time they hatched. Many young could be produced each year by a small number of adults. The eggs and offspring could survive and even thrive even if all the adults were wiped out in a single year, so long as the disaster occurred after the breeding season. The author suggests that because the giant dinosaurs could survive with smaller adult populations they could grow to such enormous sizes without overexploiting their resource base. The herbivorous dinosaurs were permitted by this scheme to grow to enormous size, up to possibly 100 tonnes. This situation of low populations appears to be supported by the remains of these animals in the fossil record, indicating that they had relatively small populations. It has been suggested that as the bulk of the biomass of adult herbivorous dinosaurs was in oversized giants, this led to the theropods increasing to larger sizes in order to tackle the giants. The author1 suggests it is not logical for theropods to grow to 6-10 tonnes then play it safe by hunting the smaller juveniles. He suggests there may have been a size race between the theropods and the sauropods that pushed each other to grow to even larger sizes.

He suggests that among very tall animals such as sauropods and giraffes the necks evolve in a feedback loop involving 2 factors. The length of the neck is a dominance display that intimidates rivals and enhances reproductive success, in a similar manner to the peacock's tail and among big cervids the possession of big antlers. As a result of the head being raised higher the animal also gets a competitive height advantage over others with shorter necks that provides the power source needed to pump the blood far above the heart to the brain. The heads of sauropods were relatively small as they lacked big brains and dental batteries making them able to evolve necks that were extremely tall that required enormous bodies to anchor them as well as contain the big hearts that were needed to pump the blood to the brain.

Cope's Rule was proposed by Edward Cope in the 1800s that refers to the tendency among animal groups to evolve gigantism. The dinosaurs took this evolutionary pattern to the extreme. At the present such gigantism is seen only in the oceans.


Sources & Further reading

  1. Paul, Gregory S., 2010, The Princeton Field guide to Dinosaurs, Princeton University Press.


Author: M. H. Monroe
Last updated 05/02/2012 



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