If you were to take a walk through North America a mere 10,000 years ago, there would have been an awful lot more animals with you on their menu. From giant sloths to short-faced bears, to perhaps even the 9ft-long (three metre) sabertooth salmon. If we think of big animals now, then our imagination takes us to sub-Saharan Africa with elephants, giraffe and hippo. Yet travel back a mere blink of the eye and monster mammals roamed across almost all of Earth’s continents—how did they get so big?

Firstly, to grow super-sized you need food—a lot of food. For this reason, many of the largest animals were actually herbivores. We know from fossilised dung and teeth shape that the ferocious giant ground sloths of South America ate plants such as grasses or yucca. Megatherium, one of the largest land mammals to ever live, was a ground sloths weighing up to four tons, yet it supported this mass largely through munching on leaves. So in this sense, food is rarely the limitation as long as you can move around enough to find it.

So with enough food, growing large has some major advantages. In particular it reduces the number of other animals that might be able to predate you. Yet size comes with costs too; enormous species are likely to have smaller population sizes making them more vulnerable to freak events like a volcanic eruption or climate change. They often have slower reproductive rates too, with larger animals taking longer to grow and reach maturity, meaning populations recover more slowly from hunting or disease.

Yet the biggest and most inescapable challenge is gravity. As size increases, bones must become stronger and thicker, muscles must become more powerful and the heart must work harder to pump blood around the body. In fact, this is where the most insurmountable biological limits seem to be. Longer muscles for example, can exert no greater force than shorter ones; larger hearts produce proportionately less power and an animal too large would produce so much heat that proteins would quickly become denatured.

To gain an idea of how land animals might be limited by gravity, we need to look no further than our oceans. The largest animal that ever lived is the Blue whale (Balaenoptera musculus), growing up to 30 metres, weighing 173 tons and arteries so wide that you could swim through them. As expected from a giant animal, it survives almost exclusively on super abundant tiny crustaceans called krill, eating up to 40 million (3.6 tons) in a single day! Buoyancy helps balance the pull of gravity, meaning muscles and bones do not have to support the weight of the entire animal. When whales accidentally beach and become stranded they often die from internal damage under their own mass. This is one of the main reasons why marine mammals are able to grow so much larger than land animals.

So could animals get bigger even bigger? On land, certainly. The largest land animals known to have lived were the Titanosaurs weighing 100 tons. Compared to elephants that weigh around five tons—nothing alive today even comes close. The blue whale is the largest animal we know to have existed in the oceans, but it is certainly conceivable for something substantially larger to evolve. The only problem is that we will have to wait a long time to find out.

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Icons of the Wild: The Vegetarian Giants

Their multi-ton frames shake the ground as they roam across the African mesa. Luckily, these massive animals are herbivores and lead mostly solitary, peaceful lives. As these giant non-ruminants graze on shrubs, trees and plants, their nutrient rich droppings fertilise the earth and allow new life to prosper.

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