Gregory S. Paul, freelance scientist and artist, and editor of The Scientific American Book of Dinosaurs, which will be published in November, offers this explanation:

T. rex
TYRANNOSAURUS REX was a colossal carnivore, but meat-eating mammals never reached such proportions.

Gigantism applies to animals that exceed 1 tonne. Today's land giants include elephants (which weigh up to five to 10 tonnes), rhinos, hippos and giraffes. Yet these creatures represent only a small percentage of the terrestrial giants that have existed in the Mesozoic era and Cenozoic era (which we are still in). Indeed, gigantism has been a common feature of land animals since the beginning of the Jurassic period, over 200 million years ago. The first true giants of the land were small-headed, long-necked, sauropod dinosaurs that appeared at the beginning of that time. Soon some of the predatory dinosaurs exceeded 1 tonne. Toward the end of the Jurassic many sauropods reached 10 to 20 tonnes, some weighed as much as 50 tonnes, and a few may have exceeded 100 tonnes and 150 feet in length, rivaling the largest modern whales. A few theropod dinosaurs approached five tonnes, and the spike-tailed stegosaurs, at two to three tonnes, were also giants. In the Cretaceous period stegosaurs disappeared, but sauropods continued to thrive as 10- to-100-tonne monsters. During that time a number of plant-eating ornithischians--club-tailed ankylosaurs, horned ceratopsids, and duck-billed iguanodonts and hadrosaurs--weighed in at two to eight tonnes, as did some theropods, including tyrannosaurs. Dinosaurs went extinct 65 million years ago, with the exception of birds, which many scientists consider to be living dinosaurs.

With regard to gigantism among mammals, before the middle of the Cenozoic some herbivores exceeded a tonne. Millions of years later giant mammoths and rhinoceroses known as indricotheres reached 10-20 tonnes. No true flesh-eating land mammal has weighed over a tonne.

Why do animals become gigantic? Some reasons are simple. The bigger an animal is, the safer it is from predators, and the better it is able to kill prey. Antelope are easy prey for lions, hyenas and hunting dogs, but adult elephants and rhinos are nearly immune--and their young benefit from the protection of their huge parents. For herbivores, being gigantic means being taller and therefore able to access higher foliage. Giraffes and elephants can reach over 18 feet high, and elephants can use their great bulk to push over even taller trees.

Other reasons for being gargantuan are less obvious, although important. The cost of locomotion decreases with increasing size, thus it is much cheaper for a five-tonne elephant to walk a mile than it is for a five-tonne herd of gazelle to move the same distance. Metabolic rate also decreases with increasing size. A shrew must therefore frantically eat more than its own weight each day. The elephant, on the other hand, needs to take in only 5 percent of its own weight. And whereas big herbivores have long digestive systems that allow them to process and digest tougher plants, small herbivores can survive only on higher-quality foods. Also, as size increases, great bulk acts as a form of mass insulation. Large animals are thereby less affected by temperature extremes.

But there are disadvantages to being big. Because big animals eat more, there cannot be as many of them. Before human hunting, the population of elephants and rhinos in Africa was in the low millions. Rodents, in contrast, number in the countless billions. Nor can giants do a lot of things that smaller creatures can do, such as burrow into the ground, climb trees or fly.

On land only dinosaurs and mammals have become gigantic; reptiles have never done so (the biggest tortoises and lizards have only weighed a tonne). One reason may be the rate of growth. Land reptiles cannot grow rapidly. It takes many years for a alligator to reach 100 pounds, whereas an ostrich does so in less than a year. This is probably related to metabolic rate. Only those animals with high metabolic rates can grow rapidly, so only they can grow fast enough to become gigantic within a reasonable amount of time. Elephants reach as much as 10 tonnes in just three decades. It would take well over a century for a reptile to do the same. Yet animals almost always die by around 50 to 60 years old, owing to predators, disease or accident. Dinosaur bones show evidence that they formed rapidly, like those of birds and mammals, rather than slowly, as in reptiles. It is therefore probable that dinosaurs became gigantic because they could grow fast enough to do so. The bones of sauropods indicate that they experienced a sharp slowdown in growth with maturity. But it is possible that the larger examples were still putting on some weight when they died, in the same way that bull elephants often continue to grow throughout life.

Why were sauropods the only land animals to exceed 20 tonnes? Mammals give live birth and nurse their young. An elephant can drop a large calf only every few years, then they must spend years nursing and raising the calf before it can survive on its own. Lots of adult elephants are required to raise the young. With so many adults being supported by a fixed amount of food production, this puts a limit on the size of the adults. Sauropods, on the other hand, laid dozens of eggs each year, and the juveniles could survive on their own. A small population of adults could produce enough eggs to sustain a large population. Adult sauropods could therefore be much larger than can slow breeding mammals.

It is widely believed that it is more logical for giants to dwell in oceans, where they are buoyed by water, than on land, where gravity is a force that must be constantly resisted. Yet sauropods were the most successful giants in the earth's history, having survived for 150 million years--the 100-tonners themselves lasted for 80 million years. Gigantic whales, in contrast, have swum the seas for only 40 million years, and only the blue whale has reached 100 tonnes.