by The largest animal to ever walk the Earth was probably the dinosaur Argentinosaurus, a massive 77-ton (70 metric 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), which weighs less than 7 tons (6 metric tons). Both look positively elegant next to blue whale (Balaenoptera musculus), which, at an average of 165 tonnes (150 metric tons), may be the heaviest animal to have ever lived.
But could any animal top it? Is there a limit to how big an animal can get?
“We’re looking at blue whales and the question is whether we could get something bigger.” Geerat Vermeij (opens in new tab), a professor of geobiology and paleobiology at the University of California, Davis, told Live Science. “I’m not sure I’d be willing to say no to that question. Size depends on many factors and I have a relativistic view.”
At least in theory, however, there may be a hard limit—imposed by the laws of physics—of about 120 tons (109 metric tons) for land animals, according to Felisa Smith (opens in new tab), professor of paleoecology at the University of New Mexico. “To be bigger than that, on land, your feet would have to be so wide to support your body that you couldn’t walk effectively,” he told Live Science in an email.
Smith is mentioned 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 areas”. In other words, as an animal increases in size, its volume will increase faster than its surface area, so larger animals need much larger limbs to support their weight. If we were to simply scale an elephant by several orders of magnitude, the square-cube law holds 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 to have disproportionately large and thick feet. But even then, at around 120 tons, the limbs necessary to keep the mega-elephant on its feet would be impossibly massive. “The largest animals in the fossil record are 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 is not the only limitation on animal size. If it were, we’d be living in a world full of 100-ton land animals, carefully following the line of Galileo’s square cube. Resource availability is also an important factor — megafauna have to eat. “Animals that live in more productive environments with high quality food are generally able to have larger maximum body sizes,” he said. Jordan Oki (opens in new tab), a quantitative biologist at Arizona State University. “Whales, elephants and other megafauna tend to live in productive, nutrient-rich environments.”
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Nutrient requirements also explain why reptiles, such as titanosaurs, grew much larger than even the largest land mammals, according to Smith. Because warm-blooded mammals have faster metabolisms, they require about 10 times more food to support a given body size than reptiles, Smith explained. Reptiles, on the other hand, have lower body temperatures and slower metabolisms, so they can afford to eat less and can grow on a calorie budget that would starve a mammal.
“Not surprisingly, the largest dinosaurs on land 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 them unique environment explains their success (opens in new tab). Marine megafauna can take advantage of their buoyancy to grow in size without straining their muscles and bones, growing in ways that would shatter the limbs of land animals. And whales have miles of open ocean at their disposal, which they travel to hunt for meals.
“Animals in water are expected to be less constrained by biomechanical constraints,” Okie told Live Science in an email. “Oceans also provide abundant, nutrient-rich resources for those animals that are mobile and resourceful.” In particular, the evolution of baleen plates allowed the whales to consume zooplankton efficiently enough to support their enormous sizes, Okie added.
Apart from various limitations, the planet can clearly support megafauna. For hundreds of millions of years, megafauna were widespread. However, in the last 20,000 years or so (opens in new tab), a mere glimpse into evolutionary time, the megafauna are almost extinct. Large land mammals such as elephants and rhinos are in decline, existing only in certain parts of the world. several groups of marine megafauna, such as whales, are constantly teetering on the brink of extinction. So where did all the giants go?
“People have ruled most of them out,” Vermeij said. “Mammoths, elephants, bison, big carnivores – we’ve wiped out 90% of the big animals, maybe more, and certainly all the bigger ones.”
Humans are also the main obstacle to the revival of these large species.
“You would have to have no humans for the megafauna to come back,” Vermeij said. “We are the dominant species, by far, and no animal is going to grow up under our hegemony. The chances of us ever having something as big as a Cretaceous dinosaur again are unlikely.”
