Why Aren’t there enormous mammals the size of Godzilla?
This is one of those intriguing seminal questions raised by Galileo—though he didn’t, of course, raise the specter of Godzilla. His argument was crafted on the deceptively simple idea that the weight of an animal increases faster than the ability of its limbs to support it so that if the design, shape, and materials remain unchanged, it will collapse under its own weight as its size increases. This gave an elegant demonstration that there are limits to the sizes of animals, plants, and buildings and provides a template for considerations of limits to growth and sustainability.
The greatest mechanical stresses occur during movement and especially during running, which is an essential feature of survival for many animals.
The largest land mammals that have ever existed are the Paraceratheria, a sort of precursor of the modern rhinoceros, which was almost 10 meters long and weighed up to 20 tons.
The very largest of these is the magnificent blue whale, which is a mammal that can be up to 30 meters long and weigh almost 200 tons, more than twenty times heavier than the infamous Tyrannosaurus rex.
Could there conceivably be even larger mammals that are yet to evolve? Well, there are certainly biomechanical and ecological constraints acting on ocean animals just as there are for land animals.
If Godzilla is anything like the rest of us in the biosphere, he will remain a mythical figure. Even if he didn’t collapse under his own weight, he wouldn’t be able to supply most of his cells with oxygen and so would not be viable.
So here are the “facts” about Godzilla.
In his latest incarnation, Godzilla is 350 feet long, which translates into a weight of about 20,000 tons, about 100 times heavier than the biggest blue whales. To support this gargantuan amount of tissue Godzilla would have to eat about 25 tons of food a day, corresponding to a metabolic rate of about 20 million food calories a day, the food requirements of a small town of 10,000 people. His heart, which would weigh about 100 tons and have a diameter of about 50 feet, would have to pump almost 2 million liters of blood around his body. However, to counterbalance that, it would have to beat only just over a couple of times a minute and sustain a blood pressure similar to ours.
His aorta through which this enormous amount of blood flows would be about 10 feet across, easily big enough for us to walk through quite comfortably. Godzilla might live for up to two thousand years and would need to sleep less than an hour a day. Relatively speaking, he would have a tiny brain representing less than 0.01 percent of his body weight, compared with the approximately 2 percent of ours. This doesn’t mean that he would be stupid, but that’s all he would need to carry out all of his neurological and physiological functions. As to the possibly less savory parts of his life, he would need to pee about 20,000 liters of urine a day, comparable to the size of a small swimming pool, and poop about 3 tons of feces, a good-size truckload.
I shall leave speculations about his sex life to your imagination. 😉
Estimating his walking and running speeds is even more speculative because of the biomechanical inconsistency inherent in such an animal. However, blindly extrapolating from other animals leads to an estimate of a modest 15 miles per hour for his walking speed so the average person would have some difficulty escaping his clutches should he be aggressive. But this brings up the catch in all of this: the diameter of each of his legs would have to be about 60 feet and his thighs probably much bigger, possibly close to 100 feet. In other words, he would have to be almost all leg in order to avoid collapse and be mobile, so the design is no longer feasible.
As stressed earlier, to evolve something this big requires new materials and probably new design principles……..