We Live in a World of Small Critters And Here is Why

Picture taken completely out of context. 

Extinction is a bummer. It's also somewhat inevitable on a planet that is constantly changing. Seeing as the environment keeps changing, life must change with it or perish. 
But you, the person reading this, might be wondering what that has to do with our planet being a world of smaller beasts.

First off, you might be thinking that's hogwash. There are plenty of large animals around today: elephants, rhinos, hippos, bison, moose, cattle. Hell, the largest known animal to exist in earth's history lives now in the form of the blue whale: nearly 200 tons and 30 metres of pure Holocene giant. 

It's so big, the World Wildlife Fund couldn't fit the entire thing into this picture. 

But a look into the recent geological past says otherwise. Millions of years ago, there were animals that weighed almost 100 tons and could peek into a fifth-story window. Less than a million years ago the world had many more kinds of megafauna all over the world and often much more 'mega' than what we have now: mammoths, wooly rhinos, sabre-toothed cats, giant warthogs, moas, elephant birds and many more besides. It could be safe to say that the majority of the land-based animal giants that have evolved in the last 1 million years are gone now. 

So why are they gone? As tends to be the case with mass extinctions, scientists are happily arguing about it and there are numerous explanations put forward. Generally, the accepted explanation is a combination of climate change at the end of the last Ice Age (about 10,000 years ago) combined with human beings spreading all over the planet and eating all the giant delicious things into oblivion. And here is where we can get into why the the world doesn't have as many elephants as it potentially could...

1. Large Animals are More Vulnerable to Extinction Events

When it comes to evolution, being big is a definite short-term advantage: less things can eat you, you're faster, stronger, able to take advantage of resources that smaller beasts cannot and so on. However, being big has some major drawbacks that render large animals more vulnerable to extinction events:

- Large animals generally breed more slowly, so they evolve slower and can't recover easily from mass die-off.
- They need more food, water and space, so they are more vulnerable to their environment being disrupted. 
- Larger animals also tend to be more specialized in their lifestyle, making them less able to adapt to their preferred food and environment disappearing. 

Examples of this are numerous. An animal as large as a Columbian Mammoth would take years to reach sexual maturity and would require thousands of acres of grassland to roam on to stay properly fed and watered. Sabre-toothed cats like Smilodon that were specialized for hunting larger, slower game were too bulky to consistently hunt the smaller, faster creatures that handled the Quaternary extinction better.  

American Pronghorn antelope top speed: 98 km per hour.
Smilodon fatalis top speed: way, way slower than that. 

You might be wondering then how whales managed to survive all this: they're huge, and many of them are specialized in eating krill (and only krill). The answer to that is that marine environments are slower to change than land-based ones, allowing large animals more time to adjust. The fact that whales are very intelligent, can communicate over huge distances, can travel thousands of miles with little effort and can go for months without food helps as well. Typing that out made me realize again just how awesome whales are.

2. Non-African Megafauna Was Not Ready For Modern Humans

Africa is a bit unusual, biologically speaking, because most of its Ice Age megafauna are still around. Sure, some have died out: you won't find sabre-toothed cats or giant warthogs on the continent anymore. But the elephant is still there (barely), the rhino is still there (almost), the hippo, buffalo, giraffe and many other assorted big things are still there (but watch this space, the poachers are on it or so I hear). 

The reason for this is that African animals evolved right next to humans, had time to adjust to us and get properly afraid of fire, pointy sticks and sharp rocks. This is also the reason why most attempts to domesticate African wildlife throughout history have failed, and why most domesticated animals descend from wild creatures originating outside of Africa, with the notable exception of domestic cats (descended from African wildcats, and arguably still not fully domesticated). 

Fear me. 

Megafauna outside Africa had never encountered modern humans before, were less paranoid and didn't have time to adjust to our superior murder-skills. Many megafauna species outside Africa also were slow, stupid and delicious to human hunters (New Zealand moas, American ground sloths and Australian giant marsupials to name a few). The defence strategies that worked well against their native predators didn't work against humans. A fully grown four-ton ground sloth with thick skin and huge claws would've been too much of a fight for an American lion or sabre-tooth, but wouldn't have been fast enough to outpace human spears or bows. 

They were probably hairless too, by the way: http://markwitton-com.blogspot.com/2019/10/megafuzz-under-microscope-how-credible.html

The end result of this climate-slash-human-winnowing process is a world that's largely made up of animals that are either smaller, more opportunistic or a combination of the two. Modern sloths survived because they are small, can survive on leaves that taste like cardboard and are camouflaged by the moss that literally grows on them. Meanwhile, their bigger ground-dwelling cousins were easy to spot, needed more food and were slow and delicious enough for humans to have an easy time hunting them. 

As for opportunists, most modern large mammals fit somewhat into that category. Modern gray wolves can learn to hunt just about everything from rats to fully-grown bison, and most modern bear species have a very broad omnivorous diet. Elephants are also generalists, with their horribly inefficient digestive systems being the price they pay for being able to eat a wide range of plant matter. Some notable specialists still survive, although it bears (pun intended) mentioning that the ongoing Holocene mass extinction (yes, we are probably in one right now) is putting huge pressure on them. The specialist sea-ice hunting polar bear is the poster beast for the disastrous results of climate change, whereas the ridiculously over-specialized giant panda is the official mascot of the World Wildlife Fund for very good reason. 

That reason being that it's one of the most badly thought-out animals in the world.
Never eat raw bamboo, people. 

The same extinction trend of larger creatures dying out and being replaced by smaller, more opportunistic creatures has happened many, many times before, and is still ongoing. Remember the gray wolves? They were in a perfect ecological position at the end of the Quaternary extinction event, able to take over a top pack-hunting predator niche that had been left vacant by the extinction of many Pleistocene apex predators. But thanks to centuries of habitat loss and persecution by humans, it looks like their time is starting to run out too. 

Which is a problem for some good folks in North America, because in areas where wolves, mountain lions and bears have been wiped out there has been noticeable growth in the coyote population. Coyotes are smaller than wolves, but they are bolder, stealthier and more cunning than their bigger cousins. Being less afraid of people they're more likely to move into your urban neighbourhood, raid your garbage cans, kill your pets and generally be a bother. 

Keep an eye on him, he'll be raiding your fridge next...

So if you've ever wondered why there seem to be a lack of true giants in the world, the short answer is because there have been two mass extinctions within the last 10,000 years, both of them caused by our species. This is why we can't have big things, people.