The largest animal that ever walked the earth was probably the dinosaur Argentinosaurus, a massive 77-tonne (70 long tons) titanosaur that lived about 90 million years ago during the Late Cretaceous Period. For comparison: The heaviest animal on land today is the African elephant (Loxodonta) that weighs less than 7 tons (6 tons). And both look positively petite next to it blue whale (Balaenoptera Musculus), which, averaging 165 short tons (150 long tons), is possibly the heaviest animal that has ever lived.
But could an animal ever top that? Is there a limit to how big an animal can get?
“We’re looking at blue whales and the question is if we could get something bigger,” Geerat Vermeij (opens in new tab), professor of geobiology and paleobiology at the University of California, Davis, told Live Science. “I’m not sure I would be willing to say no to that question. The magnitude depends on many factors, and I take a relativistic view.”
However, at least in theory, there may be a hard limit — enforced by the laws of physics — of around 120 metric tons (109 metric tons) for land animals Felissa Schmidt (opens in new tab), Professor of Paleoecology at the University of New Mexico. “To be taller, on land, your legs would have to be so wide to support your body that you couldn’t run efficiently,” she told Live Science in an email.
Smith refers to the square cube law (opens in new tab), a mathematical principle first described by Galileo Galilei as “the ratio of two volumes is greater than the ratio of their surfaces”. In other words, as an animal increases in size, its volume increases faster than its surface area, so larger animals need much larger limbs to support their weight. If we were to increase an elephant by just several orders of magnitude, the square-cube law would say that it would collapse—its mass would increase by a power of three, while its limbs would increase in size by a power of two.
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The only way our imaginary mega-elephant could overcome this limitation would be if it had disproportionately large and thick legs. But even then, at around the 120-ton mark, the limbs necessary to keep the mega-elephant on its feet would become impossibly bulky. “The largest animals in the fossil record weigh just under 100 tons [90 metric tons]which supports this theoretical maximum,” Smith said, adding that “it is not clear that larger ones could not have evolved.”
But physics isn’t the only limitation on animal size. If it were, we would live in a world full of 100-ton land animals carefully following Galileo’s square-cube line. Resource availability is also an important factor – the megafauna need to feed. “Animals that live in more productive environments with high-quality food can generally accommodate larger maximum body sizes,” he said Jordan Okie (opens in new tab), a quantitative biologist at Arizona State University. “Whales, elephants and other megabiota tend to live in productive, nutrient-rich environments.”
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The nutrient requirements also explain why reptiles, like titanosaurs, grew much larger than even the largest land mammals, Smith says. Because warm-blooded mammals have faster metabolisms, they need about 10 times the amount of food to support a given body size than reptiles, Smith explained. Reptiles, on the other hand, have lower body temperatures and slower metabolisms, allowing them to afford to eat less and grow on a calorie budget that would starve a mammal.
“Not surprisingly, the largest dinosaurs in land areas were about 10 times larger than the largest mammals,” Smith said.
Blue whales, which can weigh around 165 tons and are warm-blooded mammals, are glaring exceptions to several of these rules. But her unique environment explains their success (opens in new tab). Marine megafauna can use their buoyancy to increase in size without straining their muscles and bones, and grow in such a way that the limbs of land animals would crumble. And whales have miles of open ocean to travel to forage.
“Animals in the water are expected to be less constrained by biomechanical limitations,” Okie told Live Science in an email. “The oceans also provide plentiful, nutrient-rich resources for the animals, which are mobile and resourceful.” In particular, the evolution of baleen plates allowed whales to consume zooplankton efficiently enough to support their enormous size, Okie added.
Aside from various limitations, the planet can clearly support megafauna. For hundreds of millions of years, megafauna were ubiquitous. But during the last 20,000 years or so (opens in new tab), a mere blink of an eye in evolutionary time, the megafauna has all but disappeared. Large land mammals such as elephants and rhinos are in decline and are only found in certain parts of the world; Several groups of marine megafauna, such as whales, are constantly on the brink of extinction. So where have all the giants gone?
“Humans eliminated most of them,” Vermeij said. “Mammoths, elephants, bison, big carnivores – we eliminated 90% of the big animals, maybe more and certainly all of the biggest ones.”
Humans are also the main obstacle to the revival of these large species.
“You wouldn’t have to have humans before megafauna could make a comeback,” Vermeij said. “We are by far the dominant species, and no animal will thrive under our dominance. The chances of growing back to the size of a Cretaceous dinosaur are unlikely.”