It's a fairly valid question to ask how big can an animal get, especially when considering the behemoths that have existed on this planet throughout its lifetime. But with this question comes more questions, such as what family does the animal fall under? Or what are the environmental conditions? Is it even practical? What does one even mean by 'big'?
Let's try and answer them...
Quick disclaimer: The organisms I will mention are all the largest that we know of – just thought I'd clear that up to save me typing that out every time!
What do you mean by 'large'?
So, straight off the bat, the largest organism to ever live in terms of surface area is Armillaria ostoyae. Also known as 'honey fungus', this fungus doesn't appear on the surface as a single being, rather lots of small sprouting mushrooms. However, they are all connected by an underground network, all sprouting from a single organism that can cover around 3.4 square miles!
On a similar vein, we have the pando tree forest, which looks like a 5-mile wide forest, but is connected by a singular root system. Though this is larger than the fungus in terms of length, the fungus does have more surface area.
So this begs the question: what is large? Is it the longest? The heaviest? The one with the largest surface area? Most can't decide, so they categorise them as 'the heaviest' or 'the longest' so as not to become too vague.
Colonial organisms are the 'largest' organisms, since their structures allow for such vast weights to be spread and supported. Animals are a little more compact, however, so can't really reach anywhere near this before being crushed under its own body weight.
What is the largest animal to ever exist?
Well, you're probably aware that the blue whale is the largest (both in terms of surface area and weight) animal alive. Not only that, it is also the largest animal to exist ever. No animal has ever reached this size and for good reason. Not only does it require certain conditions, such as having mass supported by large bodies of water (hence why a whale being beached is a death sentence since it can't physically move itself due to shear weight) but it also comes with a variety of inconveniences.
Blue whales have evolved to filter feed, simply because they require so many calories to live. Hunting takes a lot of energy (especially when you're a big animal) and the animals it would have to hunt simply wouldn't contain enough calories. On top of this, activities such as travelling and mating are extraordinarily difficult tasks that us tiny things take for granted.
Water supporting such masses can really take the load off, though, even for animals that spend much more energy eating for far fewer calories, such as the largest (in both mass and length) predator to ever exist; the Megalodon.
Things get a little more difficult on land, as surface area and mass go up logarithmically. In simple terms, if you increased a 1m tall cube to 10m, its volume would increase to 1000m cubed. The height (therefore the surface area supporting it) has increased but this multiplies the volume (by proxy the mass) by a greater amount. Obviously this maths gets even more complicated when you start taking into account different shapes, but that is the basics of it. So the more surface area you have, the faster your weight multiplies.
Why did animals get so large?
So why evolve that way? Well, there's a variety of reasons here, too. An animal can grow big enough that other predators don't want to mess with it. You can also outmatch the competition in terms of mating, so each generation ends up bigger and bigger. There needs to be two main factors for animal gigantism to take place: food and oxygen. Both of these need to be in abundance for the animal to grow and both of these have fluctuated throughout history.
What doesn't fluctuate (by any relevant amount) is gravity. To combat this, animals need to get clever with their body plans and how their mass is supported. As stated before, whales do this by taking advantage of the buoyancy of water. Another way that was adopted by many non-avian dinosaurs was the use of air sacs.
Did stegosaurus have two brains?
Ever heard the myth that Stegosaurus had two brains? Well, while it was indeed a fictional characteristic, it initially came from the fact that palaeontologists couldn't believe that a walnut-sized braincase could physically control a 4 tonne animal, so when they saw a chamber in its pelvic region that offered no other explanation, they assumed it was the brain that controlled the back half. Turns out, not only did it just have one brain (it really was just...well...stupid), but that empty chamber housed part of a complex avian respiratory system.
You see, birds don't just have a pair of lungs, they also have a total of nine air sacs around their bodies connected to the the lungs. This not only allows for much purer oxygen to flow through their bodies, but more of the body is filled with air, making it lighter. As we know, birds are classified dinosaurs and inherited this system from their non-avian ancestors.
Once upon a time, before scientists discovered this system, it was believed that sauropods were semi-aquatic animals, seeing as this was the only way they could support their massive body mass, plus, it explained the long necks. However, thanks to this air sac system, dinosaurs were much lighter than they appeared, meaning that they didn't have as much mass to support as their surface area would suggest.
Given that it was an ancestral trait, we can thank this system, along with abundant food, abundant oxygen and millions of years of evolving into their niche, for the fact that dinosaurs such as Tyrannosaurus rex and Argentinosaurus huinculensis could reach such massive sizes.
Dinosaurs are very unique, even among other reptiles, in many regards including size adaptations. Not many reptiles outside of dinosauria reached such massive sizes, but there were a few...
Marine reptiles
The first lot were marine reptiles, with the famous mosasaurs growing as large as 17 metres or around 55 feet (forget what you saw in Jurassic World). This is huge for a predator but, again, the ace up its...uh...fin... was that it was fully aquatic.
The same goes for the largest ichthyosaur, Shastasaurus sikanniensis, which potentially grew up to 20 metres (65 feet) long, making it the largest aquatic reptile to have ever lived.
Reptiles
Ok, we get it, aquatic animals can be big. So what about other reptiles? Well if you're squeamish about snakes and lizards, look away now, because reptiles had some pretty hefty contenders on land such as Titanoboa cerrejonensis or Megalania (now reclassified as Varanus priscus), the latter of which likely interacted with the indigenous people of Australia!
Or maybe crocodiles that feasted on large dinosaurs take your fancy? Look no further than the likes of Deinosuchus riograndensis, who was a shining example of gigantism being driven completely by competition.
What was the biggest animal to ever fly?
But I don't feel they really match up in terms of biological engineering to Quetzalcoatlus northropi, a pterosaur from the Hell Creek formation (same environment as T.rex and Triceratops sp.) and just so happens to be the largest flying animal that we know to ever exist. Seriously, take a look at this guy:
Now, the ability to get to this size has already been demonstrated, but for an animal who stood as tall as a giraffe to be able to actually fly? Seems almost impossible, but it managed it. However, it probably had to change things around slightly. Remember, the bigger you are, the more caloric energy and effort it takes for to do certain things, so powered flight gets more and more difficult.
Q. northropi (who belonged to the flying behemoths known as azhdarchids) likely would have done every day things like feed when it was on the ground and used very short bursts of powered flight with the aid of a good enough vantage point and catching the right wind, before simply gliding over long distances. It also could have been helped by thermal energy soaring, but that is a whole different physics explanation. Either way, no animal's weight matters more to its function than a flying one and the weight of this azhdarchid is heavily debated, therefore so is its flying method. It's also worth noting that there is another (currently unnamed) azhdarchid, who has been nicknamed 'Dracula', who is potentially even bigger!
Want to learn more about Quetzalcoatlus northropi? Check out my YouTube video:
Bugs and insects
What about bugs? Creepy, crawly, nasty things aren't they? Now imagine them flying around the size of an eagle, or crawling around as long as a tall man. Well, during the Carboniferous, that site wasn't as rare as one might think.
Beasts like this Arthropleura were not an uncommon site during the Carboniferous. In fact this period is known mostly for its gigantic creepy crawlies! The reason for this, again, comes down to oxygen. Arthropods are the group of animals who are invertebrates with an external skeleton that forms armour and includes all insects, arachnids and crabs (among others). This group doesn't quite have the same breathing tools as us vertebrates, instead having a complex air duct system with tiny openings throughout the body, allowing for the oxygen to be delivered straight to that part of the body.
If oxygen in the atmosphere is particularly high, such as during the Carboniferous, those air ducts don't have to grow quite as big or expend as much energy taking in oxygen, leaving room and energy for the rest of the body to grow. This was the secret of such giant bugs!
Having said that, this was really the limit on how big bugs can ever get, thanks to pesky physics. Firstly, if the body becomes too large, the oxygen would have too far to travel through the body for it to function at a sufficient level. Secondly, we look back to support against gravity. Remember that arthropods have soft insides, with no internal structural support. This means that their bodies would be far more vulnerable to that surface area to volume law we touched on earlier. It's like building a house with the external part being made out of bricks and the interior (walls, multiple floors and all) out of play dough. Unless the house is miniature, it can't support itself internally without something harder and would turn to mush.
Mammals
What about us mammals? After all of the organisms we have discussed so far, mammals sound a little underwhelming don't they?
Well...land mammals do anyway...
So why is that? Well part of that is the fact that mammals actually haven't had much of a chance to evolve in certain niches, since they only started to gain dominance ~55-60 million years ago. In fact, during this time, most mammals have found they thrive more by getting smaller. Many of the largest mammals of any given type are extinct, with examples such as:
Glyptodon, an armadillo the size of a Volkswagen beetle
Arctodus, also known as the North American short-faced bear
Gigantopithecus, the largest ape to ever live and the closest thing we have to a real-life big-foot:
Paraceratherium, the giraffe-cross-elephant... who is actually a hornless rhino... oh and the largest land mammal to ever live.
The main limiting factor on mammalian tetrapods, however, is that they're all a bit too dense. Mammals are actually quite heavy for our size, with body structures that are yet to evolve to support such massive sizes like sauropods on land. Then there is also the fact that mammals haven't really needed to. Bigger isn't always better. In fact, during times of crisis, the biggest die off first due to their high needs.
This post has only scratched the surface for animal gigantism, so you'll just have to keep an eye out for my planned future posts exploring the biggest beasts!
Until next time!