Zoos and blockbuster films trade on people's love of large animals. However, for thousands of years, humanity's activities have been ensuring there are fewer and fewer really big beasts. Indeed, a new paper warns that it may not be long before zoos will be the only place you can find any animal larger than the domestic cow.

Half a million years ago, the world was full of large beasts. The variety of species larger than us has declined rapidly since then. Whether this decline was our fault or a consequence of climatic changes has been among the most fiercely debated questions in paleontology.

A paper in Science tracks these changes over the last 125,000 years and puts the blame firmly on humans.

“All habitable continents once harbored giant mammals, the few remaining species are largely confined to Africa," the paper notes. "This decline is coincident with the global expansion of hominins over the late Quaternary."

The current rate of extinction is the highest it has been since the end of the Cretaceous, but even in the early part of the period studied, the loss of large mammals can be seen in the fossil record. 

It is very much the big beasts that have suffered over this period. On every continent, and irrespective of their status as predators or herbivores, large animals were much more likely to become extinct than small ones. Indeed, until humans got there, Eurasia had more big animals than Africa did.

The paper notes that humans hunt large animals that would likely be safe from other predators. The removal of the major herbivores in turn affects the survival chances of big predators that feed on their young. Until 15,000 years ago, the average mass of North America's native mammals was 98 kilograms (216 pounds). Today it is 7.7 kilograms (17 pounds). Projecting forwards, the authors expect it to decline to 4.9 kilograms (11 pounds), and all animals larger than a cow may be gone within two centuries. 

This size effect has decreased since the Ice Age ended because we're now ending small animals as well. Still, it remains real.

To confirm the responsibility of humans, first author Professor Felisa Smith of the University of New Mexico and colleagues compared extinction rates of small to large species in other eras. No period during the last 65 million years showed an equally strong bias against animals of a particular size. Interestingly, the closest comparison was around 29 million years ago, but then it was the smaller animals that were more likely to die out, something the authors speculate might be associated with the rise of grasslands at the time. Even further back, during mass extinctions, size was not a strong predictor of extinction risk.