Fee-fi-fo-fum! It may not make a great deal of grammatical sense, but this phrase is certain to conjure up mental images of magic beans, rampant beanstalks and one very angry giant.
Colossal-sized humans and beasts are the stuff of our oldest legends, our favourite fairy tales and our scariest films. But as is often the case in the natural world, fact is stranger – and far more interesting – than fiction.
While car-sized ants, a skyscraper-scaling ape, and a physics-defying mosasaur from a certain Hollywood dinosaur franchise may be scientific impossibilities, the physiology, anatomy and ecology of supersize wildlife are still fascinating, full of rules that nature ignores at its peril.
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Why is bigger better in the animal kingdom?
Before exploring why it is impossible for an elephant-sized cat – or even a cat-sized ant – to exist, it is worth looking at why a species might find itself on an evolutionary pathway to becoming huge. The main advantage of biological gigantism is likely to be reduced predation. If you find yourself among the ranks of the mega-sized, you’re more likely to be able to defend yourself, or you’ll simply be too large to consume.
A bigger body also means a bigger digestive system, enabling larger animals to exploit a different dietary niche, subsisting on lower-quality food simply by eating more of it. For instance, by developing multi-chambered stomachs and increasing the surface area of the gastrointestinal tract via the addition of specific internal structures – or even just increasing the overall length of the digestive tract – large dinosaurs could eat trees such as the infamously unpalatable monkey puzzle (Araucaria araucana).
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Giant African ungulates, meanwhile, including the ancient rhinoceros relative Paraceratherium, could make a fair meal from the tough bark and nutritionally poor leaves of the acacia.
Finally, being large decreases the risks posed by heat loss, which can be a matter of life and death for many animals. Small mammals such as shrews have a fiercely high metabolism and an almost constant need to eat to keep their bodies warm and functional.
Larger animals, by contrast, retain more heat because they have a smaller surface area relative to volume. In other words, less area for heat to escape compared to the amount of heat produced.
Why aren't all animals giant then?
With gigantism offering so many plus-sized points, you might wonder why the natural world isn’t packed with huge beasts. Yet going supersized is neither a simple nor quick process. It involves complex biomechanics, carefully applied to form and function.
A mouse, for example, cannot simply be ‘stretched out’ to be the size of an elephant. A mouse skeleton comprises a delicate framework of long, slender limb bones, connecting at angles that allow these nimble rodents to leap and bounce. Without the thick, column-like legs and reinforced joints that elephants have evolved to support their 5-tonne bulk, an ele-mouse would, sadly, collapse under its own weight.
An organism increasing (or decreasing) in size is known as scaling, and there are two approaches to this. One, known as isometric growth, is when an animal increases in size while retaining the proportions between its features. Frogs are a good example, but the phenomenon certainly doesn’t apply to all species.
Say, for instance, a hamster was scaled up to be as large as a rhino. It might be cute but a mega-hamster would run into problems pretty quickly, because of something known as the square-cube law. As the hamster (or any organism) doubles in length, its surface area increases fourfold, but its volume increases eightfold.
To unpick the maths, imagine your hamster simplified down to a single 1×1 square. The area of this ‘hamster’ would be 1 (1×1 = 1). The volume (because your hamster isn’t flat) would also be 1 (1×1×1 = 1). If we now attempt to double our subject in size, to 2×2, we get an area of 4 (2×2 = 4). And if we look at this new, larger hamster in 3D, we immediately see the problem of volume, because 2×2×2 = 8.
Our poor huge hamster now has eight times more biological tissue to support through nutrition and respiration, yet the surface area of the features that support those processes – lungs and bones, for example – has only increased by four. This fundamental mathematical principle is why we don’t, sadly, see enormous hamsters roaming around.
The other way an organism grows is through allometric growth, where certain bodily features develop at a different pace to the overall growth-rate of the organism. It’s much more common than isometric growth, and it explains why human babies are born with proportionately big heads and why a fiddler crab has a single large claw.
However we try to scale up an animal, its volume will grow faster than its surface area. It becomes heavier more quickly than it can gain the strength or surface area to manage that weight, and will ultimately reach a threshold where its limbs can no longer support it. Even if those limbs did grow big enough, the bones inside would be so thick and heavy that the animal wouldn’t be able to move adequately.
This imaginary super-giant would also need lots of food. In order to sustain a more demanding metabolism, mammals need roughly 10 times the amount of food compared to a similar-sized reptile or dinosaur. It might not be so coincidental, then, that the largest dinosaurs were around 10 times bigger than the largest land mammals.
As massive land animals go, you can’t really do much better, or bigger, than the titanosaurs, a group of sauropod dinosaurs, which theoretically may have been able to obtain an upper limit of 120 tonnes. But when you take into account the biological considerations, such as food requirements and sufficient mobility to avoid predators, the sizes reached by the Patagotitan and the Argentinosaurus (around 60 and 70 tonnes respectively, according to the Natural History Museum, though these figures are debatable), are a realistic maximum.
What about aquatic giants?
Aquatic gigantism tells a different story. Living in water is not quite cheating but does allow animals to sneakily side-step many of the biomechanical constraints seen on land. Water’s buoyancy removes the stress and load on limbs, while effectively reducing the overall weight of the body.
Long, filter-feeding plates allow baleen whales to continuously cruise nutrient-rich waters to eat enough plankton to sustain their massive bodies and, while there is some debate about a truly huge prehistoric ichthyosaur, the blue whale is still the official record-holder. With a brain weighing nearly 7kg, a heart the size of a small car and arteries so wide an adult human could squeeze down them (I am still to test this theory), this 30m-long ocean wanderer has been known to reach a staggering 190 tonnes.
It is no coincidence that we see larger animals across our ocean ecosystems. It is a trend that has been running for more than half-a-billion years. Research shows that over the past 542 million years, the average size of a marine animal has increased by a factor of 150, allowing them to move faster, exploit new habitats and consume larger prey.
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Are there are any 'rules' in evolution?
In addition to simply making a standard animal bigger and bigger, there is a series of scientific rules and principles by which organisms are bound, in an evolutionary context. Under the ‘island rule’, for instance, introducing a large species to an island typically results in it shrinking over time, whereas a small species will find itself becoming much larger.
‘Allen’s rule’ states that species living at higher latitudes often have shorter, thicker limbs, explaining why a polar bear’s limbs are proportionately stockier than those of a sun bear. And ‘Bergmann’s rule’ refers to the trend that, within a biological group, populations and species of a larger size are found in cooler regions, while species of a smaller size stick to warmer climes.
The classic example to demonstrate this is the bear. The relatively small spectacled bear inhabits equatorial regions, larger black and brown bears live in more temperate regions, and the largest of the group, the polar bears, are found at the highest latitudes, in Arctic habitats.
How would you create the largest living animal of all time?
All things considered, if we wanted to create a supersized land animal, we may need to build upon the idea of taking a population of small dinosaurs and sticking them on a large island, somewhere near the Arctic, with enough food and not too many predators, and see what happens over time.
If we wanted to create the largest aquatic animal, then maybe we are looking in the wrong place with our supersized cetacean celebrities. As a curveball, maybe we could look at colonial organisms such as the giant siphonophore – a vast, tubular community of smaller, identical organisms known as zooids, all working as one and effectively existing as a superorganism.
Reaching a diameter of 15m and stretching nearly 50m in length, some of these biological entities could engulf a blue whale whole. The only real downside for a siphonophore is its structural fragility. Also, the fact it weighs next to nothing, and is technically not a single animal, means it may not fit within the criteria of being a ‘giant’ animal.
Achieving ‘giant’ status is only half the story. In order to sustain a colossal size, animals need to employ a suite of anatomical and physiological features to make daily life achievable.
Pneumatised, ‘air-filled’ bones combine into lightweight but strong skeletons that can also be co-opted into the respiratory system, reducing overall weight. Layers of tight-fitting connective tissue known as fascia, wrapped around the legs, prevent blood from pooling and help increase blood pressure across the circulatory system. Both these adaptations were seen in our largest dinosaurs, and many of our largest land animals today employ these biological ‘pressure stockings’.
It is easy to assume the giants in nature have disappeared. The fossil record shows that in the past there were bigger dinosaurs, reptiles, birds, fish and insects. Yet huge and impressive beasts still roam among us, from the African elephant to the giant squid.
And let’s not forget the largest animal ever to have lived on Earth: the blue whale.
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Top image: African elephant. Credit: Getty
