Based purely on the physical strength of bone and muscle, it has been calculated that land animals of at least 100 tonnes and possibly as much as 1,000 tonnes ought to be able to support their own weight and move around. That’s much bigger than even the largest dinosaur (Argentinosaurus probably weighed 80 tonnes at most), but that’s because other limits cut in first. The largest animal to have ever lived is the blue whale. At 180 tonnes, it already has to eat 1.5 million calories a day. Blue whales eat krill, which is one of the most abundant food sources in the ocean. Even so, about half the global population of krill is eaten every year by whales, seals and fish. A single freak blue whale that was double the normal size could still probably find enough food to sustain itself. But if all blue whales grew this big, the population would need to be smaller and they would reproduce even more slowly than they do now, making them more vulnerable to extinction.