There’s a saying that, when Rome falls, the whole world will fall.

Which frankly seems a little melodramatic and egotistical on the part of the Romans. Except that they kinda had  a point when you realize the fall of Rome affected  basically everything in Europe up to and including the body size of wild animals.

You don’t exactly see foxes dressing up ike centurions or deer reenacting chariot races, so it doesn’t seem like they should care. But they did. What happened to them and  why is helping us understand how humans shape our environment.

So here’s why even animals  think about the Roman Empire. [intro] Back in the middle of the 19th century, paleontologists began to  notice that ancient animals seemed to get bigger over geological time. For example, horses used to be dog-sized, and now they can be taller than a person. And dinosaurs, pterosaurs, and  marine reptiles in the mesozoic all crept up in size before  the impact did them dirty.

The trend became known as Cope’s Rule, after American paleontologist and  Bone War-haver Edward Drinker Cope, one of the theory’s early champions. But as with many things he did, Cope’s Rule has sparked controversy. Modern studies into thousands of species,  and simulation of evolutionary trends, have supported the idea of a general increase.

But there seem to be plenty of  exceptions to the rule as well. In specific habitats where competition  is high, food might be scarce, keeping animals smaller. This is often the case on small islands, leading to local populations that are basically miniature  versions of normal animals.

But remarkably, over the last few thousand years, another pattern has emerged. In a number of locations around the world, the trend for growth seems to have  gone in the opposite direction. Many wild animals have been  getting smaller over time.

One of the big turning points seems  to be the end of the last ice age, around 12,000 years ago. Around this time, the giants of that frozen era, like woolly mammoths, giant ground  sloths, and dire wolves became extinct, and were replaced by smaller species like foxes, deer, and wild pigs. This came partly down to thermodynamics.

In colder climates, large body sizes do well because they have a smaller  surface area for their volume, so they can retain heat easily. But when it warms up, those  large animals easily overheat, and smaller animals, with relatively  more surface area for their volume, find it easier to cool down. Additionally, as the frozen tundra  retreated and forests grew in its place, habitats also became disrupted and  food sources became more unreliable.

So, it was goodbye ice age megafauna, and hello small furry mammals  for the duration of the Holocene. But warming temperatures weren’t the  only thing to change during this period. As the ice retreated, humans began to  get a real foothold on the landscape.

Specifically, Neolithic communities, who had previously been nomadic hunter-gatherers, settled down and started doing agriculture. That basically meant that they took  wild species of plants and animals and domesticated them, making them amenable to being  kept close and looked after. Which sounds like a pretty cushy deal to me!

Humans in different parts of the world  domesticated animals at different times. The earliest was about 10,000  years ago in southwest Asia, ranging to around 8000 years ago in Europe. And if you look at domesticated  farm animals today, it’s easy to see our influence  on their shape and size.

Thanks to centuries or even  millennia of selective breeding, they’ve become supercharged for  meat, wool, and milk production. They’re way chunkier than their wild  relatives beyond the farm gates. But remarkably, this growth seems to have  come at the expense of the wild animals.

As domestic animals have grown  bigger in recent centuries, wild animals have shrunk, but it's  not clear why this should be the case. So in 2025, a team of French researchers took a fresh look at animal sizes  in the western Mediterranean, charting the changes over the last 8000 years to try and figure out what’s going on. For this, they studied ancient animal remains from archaeological sites  all across southern France, dating from the beginning of  animal domestication in the region.

In total, they looked at more  than a quarter of a million bones from more than 300 sites, and tracked the size changes in domestic sheep, cows, goats, and chickens, as well as wild rabbits, hares, deer, and foxes. Alongside this, they cross-referenced with local  climate records, plant success, and archeological records  of human activity over time, to build a complete picture  of change in the region. With such detailed information, the researchers were actually able to identify four distinct phases of animal size changes, and link them to climatic and human events.

The first phase, between 6000 and 2000 BCE, corresponded to the Neolithic  period of human civilization, and saw all animals decreasing in size. Despite an increase in the numbers  of domestic animals like cows, both the domesticated and wild  species in southern France shrank. This is consistent with records  elsewhere in central and southern Europe, and with the general trend  of post-ice age shrinking.

The researchers think that this  consistent drop is a result of a decrease in rain changing  how much vegetation was available for all animals in the area. The second phase of change  spanned from 2000 BCE to 300 CE, corresponding to the Bronze Age, Iron Age, and the rise of Roman  civilization in western Europe. During this time, domestic  animals began to grow larger, and wild animals stopped shrinking, possibly for the first time  since the end of the ice age.

Wild foxes even started to increase in size. The researchers noted that this was a time  of transformation for human societies. With growing populations, they changed the way they  used land for agriculture, and engaged in a lot more trade with  communities from further afield.

It’s possible that these  long-distance interactions are responsible for the increase  in domestic animal size. New, larger breeds could have  been brought into the area, marking the beginning of selective breeding as humans actively chose which  breeds they wanted to rear. This, in addition to improvements  in pastoral practices, led to bigger farm animals than ever before.

As growing populations went all-in on farming, they started to clear the  ancient forests to make space, creating grasslands and croplands that favoured the survival of wild animals too. Foxes, being a predatory species, fared especially well from this shift, since the increase in size  and number of domestic animals meant there was plenty of food to prey on. So it was a booming time for everyone: humans, their animals, and the  wild animals at the fringes too.

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The success wasn’t to last, because the third phase identified  by the French researchers lasted from 300 CE to 1000 CE, and corresponded to a widespread drop  in wild and domestic animal sizes. Things had been going so well, but a decline in the number of archeological sites from 200 CE onwards hints at the  major societal change of this period: The decline and fall of the Western Roman Empire. The empire ultimately  shattered from the inside out, thanks largely to political and social forces, but the effects of that change  rippled out all across Europe.

Where there had previously been large  farms supplying crops and animals to the vast Roman cities, there were now smaller farms and villages focused on seeing to their own needs. This created less of a surplus  available for domestic and wild animals, ultimately stalling their growth. It’s likely that the climate played a role too.

The landscape experienced what the  researchers call an ‘erosive crisis’, as a result of increased annual rainfall. In the past, extra rain could have been handled by Roman-built land drainage, but following the civilization’s fall, all of this infrastructure was abandoned. The open landscapes were flooded  and eroded by surface waters, sending humans into new territories, which in turn weakened and  shrank wild animals’ habitats.

With less room to roam, and less food to find, it’s little wonder they stopped getting bigger. Intriguingly, it’s also  possible that both the humans’ and the animals’ fall from grace  were caused by the same thing. There’s growing evidence  that the influx of disease may have contributed to the  fall of the Roman empire, especially under a changing climate.

The trade and connectivity that had  been such a boost in the beginning ultimately opened humans and their animals up to new and exotic diseases from far away. And throughout the 700 years  following 300 CE onward, there is archaeological and  historical evidence of mass mortality among domestic animals. While the effects of these diseases  on wild animals is still unknown, it’s possible that natural  selection favored disease resistance over body growth throughout this period.

The fourth and final phase identified by  the researchers spans the last 1000 years, from the Middle Ages to the Modern age. Throughout this time, the growths of domestic   and wild animals diverge and  follow very different paths. Domestic animals grow larger, while wild animals, with the exception of rabbits, continue to shrink.

The increase in domestic animal  size is a result of humans really getting the hang of pastoral  agriculture and selective breeding, but on its own this shouldn’t be  enough to shrink their wild cousins. That doesn’t mean that  humans didn’t find other ways to inadvertently alter the environment, though. In response to continuing climatic changes creating warmer temperatures and lower rainfall, European societies expanded their  agricultural lands into new areas, which further fragmented and  isolated wild animal habitats.

Not only that, but there also seemed to be an  increase in hunting throughout this period. It’s been noticed before  that when animals are hunted, their average size tends to decrease, because hunters are preferentially  removing the bigger, trophy animals from a population. So, as a result of this unnatural selection and the continuing destruction  of natural habitats, wild animals found themselves  shouldered out and diminished.

And these are findings that are supported  by observations elsewhere in the world. A study in 2003 that tracked the size of wild  mammals in Denmark over the last 175 years showed that all but the smallest species have become progressively smaller  along with the fragmentation of their habitats from agricultural expansion. Rabbits were alone in avoiding  this widespread shrinkage, probably because they were on their way to being domesticated through the Middle ages.

The introduction of larger, domesticated breeds to southern France blurred the lines between wild and domestic  rabbits throughout this period. On the whole, for the last 1000 years, the human transformation of the environment has been so significant that  for the first time it was this, rather than any other environmental factor, that drove the changes in animal size. Our favoritism for certain animals has exerted a divergent  pressure on their body sizes, allowing some to grow large,  ultimately at the expense of others.

So, although we may not imagine that us humans are directly affecting wild populations, it’d be silly to assume that our  actions don’t have any knock-on effects. Often, the changes might not be  caused by human activity alone, or by just the environment, but rather a combination of  interactions and feedback between them. Which serves as a reminder  for us to be careful stewards, not just of the animals in our care, but of the Earth as a whole.

We may never know the impact  our actions are having. [ OUTRO MUSIC ]