When you’re walking through a forest and pass a squirrel scurrying up a tree, or watching a robin in your yard make a nest for her eggs, you feel pretty big, right?

It feels normal to us now, wandering through our suburban lifestyles, where the biggest wild animal is a fox who is a little smaller than our neighbor’s dog – in fact, for most of us our only interaction with a ‘big’ animal is in a zoo or on safari.

However, had we been roaming the Earth at the same time as the dinosaurs, our perspectives would have been completely different.

The largest dinosaurs – known as titanosaurs – measured up to 38 meters in length, eight meters in height, and weighing just under 60 tons according to a guide by the UK’s Natural History Museum. This is like us standing beside a Boeing-737, or like a mouse standing beside an elephant.

From that perspective, we would have had an entirely different world view.

Which begs the question: why is nothing that large alive today?

According to a research team from the University of New Mexico, there is good reason for no terrestrial mammal reaching the great heft of a dinosaur.

In their study, which was published in the academic journal Science, Professor Felisia Smith – along with an international team of paleontologists, evolutionary biologists and macroecologists – built a database of the sizes of land mammals to roam every corner of our planet since the extinction of the dinosaurs.

This groundbreaking database, Smith explained in a statement, is the most comprehensive resource to date, encompassing everything from anteaters and elephants to mastodon and mammoths:

“The database is unique because it’s comprehensive, including mammals from all continents since the extinction of the dinosaurs. We estimated body size from fossil teeth, which are the most commonly preserved parts of mammals.”

And what they found, by exploring worldwide ecosystems – including those of extinct mammals – was that though mammals grew to over 1000 times the size they’d been at the time that dinosaurs walked the Earth, there were constraints that now prevented them from growing any bigger.

These factors, the database revealed, were multiple.

After the extinction of dinosaurs meant more resources for mammals to grow, they did so dramatically, with the researchers noted that mammals reached a maximum of 10kg at the time dinosaurs were alive, and up to 17 tons thereafter. And this was the case in various habitats across the world, with all kinds of mammals responding similarly, as Smith continued:

“The remarkable similarity in the evolution of maximum size on the different continents suggests that there were similar ecological roles to be filled by giant mammals across the globe. This strongly implies that mammals were responding to the same ecological constraints.”

But what were those ecological constraints? Well, the study suggests that there were plenty of limits that prevented mammals from growing any larger.

Factors from climate and habitat were limiting factors, with larger animals conserving heat more efficiently in cooler climates, while those living in busier ecosystems were likely to be smaller to save space and require less food.

And with their new insights into these factors, the research team were able to understand more about how different ecosystems and habitats changed over time, as Smith described:

“The results were striking. Global temperature and terrestrial land area set constraints on the upper limit of mammal body size, with larger mammals evolving when the earth was cooler and the terrestrial land area greater.

Our analysis reflected processes operating consistently across trophic and taxonomic groups, and independent of the physiographic history of each continent.”

Fascinating stuff, but how is this relevant today?

Well, the more we can understand about how our species and others have adapted to different environments and climatic changes, the better. Not only can this information help conservationists, it can also give us a clearer picture of how mammals have historically adapted to climate change, and how this might affect our mammals and their ecosystems going forward:

“Size impacts all aspects of biology, from reproduction to extinction. Understanding the constraints operating on size is crucial to understanding how ecosystems work.”

Of course, it does also act as reassurance that, thanks to a multitude of factors on our planet, we’re not going to be confronted by a plane-sized terrestrial mammal any time soon.

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