Contents Updated: Thursday, August 05, 1999
Nicholas Hotton III of the Smithsonian Institute claims that 80 per cent of mammals are smaller than the smallest dinosaur, claimed to weigh about 20 pounds. Only two per cent of modern mammals are heavier than two tons but 50 per cent of dinosaurs were. Bulk must have been advantageous to the dinosaurs. The reason claimed is that they had no internal mechanism for maintaining a steady temperature and had to retain their heat by being bulky.
A large body loses heat more slowly than a small one because it has less surface area per unit volume. It is the volume that holds the heat and the surface area that loses it. The greater the volume compared with the surface area, the greater the retention of heat. In technical terms, they were mass homeotherms (homoiotherms). By growing to huge size they kept a more or less constant temperature giving them many of the characteristics of warm-bloodedness. Hotton concluded that the dinosaurs had reduced their dependence on the temperature of the environment at a much lower cost than mammals.
The activity of dinosaurs was more sedate than that of mammals. The basic strategy of dinosaurs in general was 'slow and steady', and what it lacked in mammalian elan, it made up in economy.
Yet this solution still leaves us with puzzles. Some dinosaurs grew no bigger than rabbits or crows: archaeopteryx was no bigger than a crow and compsognathus was no bigger than a chicken. Alan Charig of London's Natural History Museum states that the smallest dinosaur was no bigger than a mistle thrush, weighing only a few grams. Pterosaurs were even smaller—some were tiny. How could a dinosaur the size of a thrush, or even a chicken, maintain an even temperature and how could hatchlings survive? Both would have a large surface area to volume ratio and would radiate heat rapidly.
Baby dinosaurs and pterosaurs could hardly have been other than genuinely warm-blooded, (mass homeotherms without the mass?) Why otherwise did some pterosaurs, if not all, have fur?
Fur is an insulator. It makes no sense for an animal to have fur unless it wants to keep heat in, implying internal heat generation and warm-bloodedness. If a small mammal had no insulation it would need to generate so much heat from its own activity to replace heat lost from its skin that it could never rest. If it did it would cool down, become comatose, be unable to forage and would starve to death. Small mammals need fur to live. Most pterosaurs were not bulky enough to be mass homeotherms. Yet, since they had evolved fur, they must have been warm-blooded. Their large eyes and brains also imply an active lifestyle, a characteristic of warm-blooded animals.
Archaeopteryx, the evolutionary link between dinosaurs and birds, was discovered because it was feathered. Feathers, like fur, act as an insulator suggesting that archaeopteryx had some reason to keep heat in—it was warm-blooded. Yet we noted above it was only as big as a crow, apparently counting out mass homeothermy. Its descendants, the birds, are hotter blooded than mammals. Yet archaeopteryx was living 140 million years ago, right in the middle of the dinosaurs' reign. Is it reasonable to imagine that archaeopteryx had evolved warm-blood when the other dinosaurs were still tinkering with mass homeothermy or, according to many, were cold-blooded reptiles.
Moreover archaeopteryx was not the only feathered dinosaur. A fossil dinosaur discovered in the Gobi Desert named avimimus (bird mimic) was described in 1980 by a Dr Kurzanov. Looking somewhat like compsognathus, a sort of miniature allosaur, the main feature about this dinosaur was that it showed signs of being feathered like archaeopteryx yet is much later, coming from the Late Cretaceous about 75 million years ago. The upper arm bone had low projections like those of birds for the attachment of flight muscles while the lower forelimb had a bony ridge similar to bony protrusions on the forearm of birds. The feathering was thought by Kurzanov to indicate warm-bloodedness especially as they were small. Interestingly the skeleton had no tail making it look particularly bird like.
Unless feathers evolved twice this could be evidence that all small therapods were feathered for insulation. Certainly deinonychus was so similar to archaeopteryx that Bakker presumes it too must have been feathered. Possibly many others also were, though the feathers would not have been flight feathers but rather a sort of down. If small dinosaurs were warm-blooded they could not have evolved without insulation. (Did Dinosaurs Have Feathers? Article and link to Dinosaurs On-Line.)
As in many disputes, not least scientific ones, the answer might not be at either of the extremes. One could argue that not all the dinosaur genera were fully warm-blooded. The huge, noble sauropods might have been stately homeotherms, needing less internally generated heat, because their bulk retained it. Natural selection in animals like these might have pushed them to vast bulks. Smaller dinosaurs and perhaps predators were warm-blooded.
But this idea is not supported by the absence of mass homeothermy today. If it offers such an evolutionary advantage over warm bloodedness, except for tiny animals, why are there no large mass homeotherms in the tropical regions where conditions are stable and ideally suited to them. The large animals are elephants, rhinoceroses and hippopotamuses—fully hot-blooded mammals.
Mass homeotherms must necessarily have been displaced by creatures that were actively warm-blooded irrespective of size. A ten degree Celsius drop in temperature approximately halves the rate of a chemical reaction including those that make the metabolism work. Consequently the activity of a cold-blooded animal roughly halves with every ten degree Celsius drop in temperature. That is why cold-blooded lizards get less active in colder temperatures. In cold climates only the animal with a body temperature regulated at the optimum level for its metabolism can keep active. Closer to the poles or up mountains a temperature must be reached where the mass homeotherms would lose body heat becoming less active whereas a fully warm blooded animal with its built in thermostat would remain active. In some such environment the warm-bloods would have had the advantage over the mass homeotherms. Over millennia, the advantage of always being active would translate into total dominance.
As Robert Bakker puts it: In direct confrontation, high metabolism always conquers low metabolism. A Bakker cartoon illustrates this idea wonderfully gruesomely in his book, The Dinosaur Heresies. The cartoon depicts a large sauropod, the mass homeotherm, comatose and unresponsive in the freezing rain, failing to notice tiny rat-like mammals, active despite the cold, eating him alive from the tip of his tail. It never really occurred, of course. Dinosaurs were quite capable of surviving in freezing climates near both Poles.
In 1987 a group of scientists led by Elisabeth Brouwers of the US Geological Survey reported in the journal "Science" that they had found dinosaur remains inside the Arctic Circle. A mixture of young, old, large and small dinosaurs were found near the Colville River. How could cold-blooded dinosaurs or even mass homeotherms survive here in the cold and darkness of winter—they had to be warm-blooded Migration after the fashion of the caribou is just a possibility but again implies a highly active animal. Dinosaur fossils have also been found in Australia which in the Triassic period was much closer to the South Pole having just broken away from Antarctica. Mass homeotherms seem unlikely to have lived in such places.
Furthermore, if dinosaurs were anything other than warm-blooded they could never have displaced the mammals, or rather the mammal-like reptiles, 200 million years ago and could only have survived by taking the role of our mainly small, present day reptiles. That they conquered and then suppressed the mammals, growing eventually to huge size shows that they were fully warm-blooded and not mass homeotherms.
A species evolves to fit different niches in its environment by the process of adaptive radiation. The speed of adaptive radiation depends upon the metabolism of the animal. Cold-blooded animals evolve more slowly than warm-blooded ones. Yet the dinosaurs on several occasions evolved explosively. In one small locality in the US, seven genera of hadrosaurs appeared in only ten million years, all plainly evolved from a known ancestor. Such rates of adaptive radiation have been typical of mammals since the extinction of the dinosaurs.
Evolution is now believed to occur in spurts separated by periods of stability when a species does not change much. This idea, proposed by Eldredge and Gould, was anticipated by an English paleontologist, Hugh Falconer who died in 1866, only seven years after the publication of Darwin's theory. But by the time the arguments of the scientists against the religious bigots had been won the soft voice of Falconer had been forgotten and his message was not heard again until a century after his death.
The periods of equilibrium of species vary according to whether the animal is warm- or cold-blooded. Cold-blooded animals remain stable longer and evolve into new species much less often than warm-blooded ones. The reason is that the higher metabolism of warm-blooded creatures accentuates the competitiveness between them. The evolution of a new species changes the environment of the others which induces them to evolve in response. A positive feedback encouraging rapid evolution occurs and only warm-blooded animals can adapt so quickly.
Dinosaurs speciated at a rate comparable with the mammals and birds, not their supposed role models, the reptiles. An analogy might be a boxer who finds himself champion at a time when there are few contenders and they are of poor quality. He could remain champion for a long time. If there were more and better aspirants for the title, his reign would be shorter. The champion cold-blooded boxer remains champion for longer than does the champion warm-blooded boxer because there is a steady stream of new warm-blooded contenders but few cold-blooded ones. Where the warm and cold-blooded animals are in direct competition the cold-blooded one stands little chance. If a human boxer were fearful that one day he might leave the ring dead, he might be very glad to opt out of boxing and chose instead the life of a grocer or a publican. In a sense, that is what the cold-blooded animals that have survived till today have done. They have opted out of direct competition with the warm-bloods.
If the broader classifications of genera and families are considered instead of species, the differences in rates of adaptation are enhanced. Taxonomic families of genera are even more likely to be long lived. Indeed families of cold-blooded species often seem to go on indefinitely. The family of modern crocodiles has survived for a hundred million years and, according to Bakker, the average for cold-blooded reptiles is 55 million years. If dinosaurs were cold-blooded their taxonomic families should survive for a similar length of time. But the dinosaurs evolved quickly, changed repeatedly, and turned out wave after wave of new species with new adaptations all through their reign, according to Bakker. The average life of a family of dinosaurs was only 25 million years, virtually the same as that of families of mammals.
Desmond wrote:
Nobody before [Bakker and Ostrom] had demonstrated the inextricable relationship between high metabolism, stable temperature and erect posture, yet once explicitly stated this linking seemed obvious and natural. It resolved the long-standing contradictions inherent in the ludicrous sun-basking brontosaur model by scrapping the model altogether and substituting an endothermic dinosaur... The dinosaur's suspected high metabolism and fast energy production places it not with cold-blooded lizards but warmer-blooded mammals and birds.