Why were prehistoric dinosaurs so gigantic? What was the advantage of their large size?

At the onset of Jurassic Period about 200 million years ago many animals living on land were gigantic in size. Notable among them were herbivore dinosaurs having long necks and small heads. Most of them weighed about 20 tons on an average but there were some species which weighed about 50 tons. Many scholars have concluded from the study of fossil skeleton of a dinosaur of Supersaurus species that it must have been minimum 41 meters in length and 98 tons in weight – and that it was a full grown adult at the time of death. From the picture given below one can easily surmise how tiny the man would have been in comparison with a full grown Supersaurus.

Dinasaurs would not have attained giant size unless there were specific advantages of large size. Some advantages are obvious. Larger the animal lesser the threat of predators. A small deer like blackbuck become prey to carnivores like hyena, tiger or wild dogs but larger ones like elephant and rhinoceros are left alone by hunter animals. A giant size herbivore is very tall and can take mouthfuls from high foliage.
Some advantages of giant size are not so obvious. A jumbo-sized animal spends less energy (weight to weight) when foraging in forest as compared with a small animal. For example, one 6,500 kilogram elephant spends less ‘fuel’ in going about foraging than 76 blackbucks weighing 85 kilograms each do for covering the same distance. In other words, it can be said that larger animals get better ‘average’ from the energy in terms of calories spent. According to biological principle metabolic process of digestion and conversion of food into nourishment and energy is lower in case of larger animals. So they do not have to eat much, comparatively speaking. A mouse needs as much food as its weight but an elephant’s diet per day is about only 5% of its weight. Further, larger size protects body from variations in temperature. About two thousand two hundred years ago Carthaginian general Hannibal invaded Rome. There was a squadron of giant African war-elephants in his army. His elephants which were born and raised in extremely hot climate of North Africa had crossed over snow-clad Alps Mountains into Italy.
On the other extreme there is hummingbird of Cuba weighing a mere 28 grams. Daily it faces certain death due to exposure in cold morning unless immediately in the morning it starts sipping glucose rich nectar of flowers. This tiny hummingbird has so delicate constitution that even a few hours delay in getting nourishment becomes fatal.
Having taken into account positive aspects of giant size it will be well to consider negative aspects also. A gargantuan like dinosaur needs equally gargantuan diet compared with other animals. Hence, its numbers were always restricted. If Asiatic and African elephants (or rhinos) number in thousands, then mice number in billions. This means that in the event of a catastrophic natural disaster entire species of gigantic animals may get wiped out due to their limited numbers. But smaller animals can save themselves by taking shelter on the trees, behind rocks or burrow under the ground. When catastrophe in the form of an asteroid or minor planet struck the Earth at the end of Jurassic Period, all the dinosaurs were killed in the resulting turmoil. Among the survivors were some warm blooded prehistoric mammals. They ruled the Earth for millions of years till evolution of human beings at its height staged.
In conclusion it is essential to note that some dinosaurs were about the size of poultry. So it can not be said with certainty that the giant size alone was responsible for extinction of dinosaurs.

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