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Earth’s mammals have shrunk dramatically, and humans are to blame

Within a few hundred years, a new study says, the domestic cow might be Earth’s biggest animal. (Justin Sullivan/Getty Images)

Life on Earth used to look a lot more impressive. Just a little more than 100,000 years ago, there were sloths as long as a giraffe is tall, monstrous bears whose shoulders were six feet off the ground, and Bunyanesque beavers that weighed as much as an NFL linebacker. But over time, all of these creatures disappeared in a manner so rapid and so mysterious that scientists still can’t fully explain what went down.

Did an asteroid discharge the mega-beasts, similar to the one thought to have snuffed out the dinosaurs? Or was it widespread climatic change or a plague of new diseases? Did our penchant for hunting play a role?

It’s likely that a combination of factors led to a planet-wide demise in sizable mammals as the Ice Age came to a close. But a study published Thursday in the journal Science provides evidence that the major drivers were humans and other hominids.

“We looked at the entire fossil record for 65 million years, in million-year increments, and we asked the question, ‘Is it ever bad to be big?’ ” said lead author Felisa Smith, a paleoecologist at the University of New Mexico. For most of evolutionary history, the answer was no — larger body mass did not make an animal more likely to go extinct, she said. “For 65 million years, it didn’t matter what size you were.”

That is, until a new kind of predator arrived on the scene: Homo erectus. Around 1.8 million years ago, hominids that had long been dependent on plants became hominids that were “heavily and increasingly dependent on meat as a food source,” Smith said.

As these tool-wielding team hunters spread out from Africa, large-mammal extinctions followed. If you’re going to spend time and energy on a hunt, these early humans and their ancestors probably believed, it’s go big or go home.

“You hunt a rabbit, you have food for a small family for a day,” Smith said. “You hunt a mammoth, you feed the village.”

It’s also possible that hominids actively targeted the mightiest creatures for other reasons — out of fear, perhaps, or perceived competition for prey. In modern times, human conflict with large animals is often about their taste for our livestock, as with wolves and lions, or their destruction or consumption of our crops, as with elephants and orangutans.

But something about substantial animals makes them more vulnerable to population collapse, said William Ripple, director of the Global Trophic Cascades Program at Oregon State University. For starters, there are usually fewer of the big animals, at least compared with the little guys.

“Their life history traits, such as reproduction rates and maturity rates, are much slower,” Ripple said. “Big animals don’t reproduce as fast as small ones.”

As hominids dispersed, the average body mass of mammals in Eurasia dropped by about half over the course of 100,000 years, Smith and her colleagues found. In Australia, the average mammal body mass today is just one-tenth what it was before 125,000 years ago.

North America was late to the game, as far as extinctions went, with most of its massive mammals surviving up to the very end of the Pleistocene. But when they did go, they went fast, a phenomenon Smith says might have to do with the invention of more-effective, long-range hunting weapons by Homo sapiens and the disappearance of all rival hominids. All told, after the dust of extinction had settled, the size of North America’s average mammal dropped from 216 pounds to about 17 pounds — the size of a bobcat.

To see what might happen if this shrinking trend continues, Smith presumed that all animals now listed as endangered or threatened would eventually go extinct and then removed them from the data.

Blue whales? Gone. Elephants? Poached out of existence. Polar bears? Glug, glug, glug.

Go down the line, and within a few hundred years you wind up with a planet where the most substantial mammal is none other than the domestic cow.

Ripple, the ecologist, is unsurprised. He has published numerous papers finding that large mammals are at a disproportionate risk for extinction.

“I think this paper is a significant contribution to what I call the ‘downsizing of nature,’ ” he said of the new study.

Of course, several animals make a living by preying on larger creatures, Ripple said. Gray wolves can take down an elk; killer whales have been seen dispatching gray whales.

“So it may be that humans have evolved to do that,” Ripple said. “But nowadays, we have well over 7 billion humans on planet Earth. And 7 billion humans have a huge impact.”

While the new paper focuses on mammals, Ripple said the same size-selective pressures are bearing down on the world’s grandest fish, reptiles, amphibians and birds. And we’re only now starting to understand what consequences this might have for the ecosystems all around us.

Scientists view many of the largest animals as ecosystem engineers. Elephants have a habit of tearing down trees in their quest for greens, helping maintain open environments such as the savanna. Mammoths probably interacted with their environment in much the same way, which is why the prairie-like habitat that used to stretch from Spain to China is called the “mammoth steppe.”

Smaller animals will hoof it straight up a hill, creating a vertical game trail, Smith said. Bulkier animals must curve their way up an incline, making switchbacks and long, meandering depressions. The difference in these paths can affect erosion, water dispersal and the distribution of vegetation.

“So even something as simple as how they walk through the environment can change everything,” Smith said of big animals. Cows’ ecosystem services, she notes, are no match for those of elephants.

Evidence that hominids have been dooming large species for nearly 2 million years could be read as an excuse for modern humans and all the animals we’ve pushed onto the Endangered Species List. But Smith said there’s a difference between then and now.

“Now we’re at a point where we can be aware of it,” she said.

And we have a choice: Are we going to keep killing them off for food, clothing and talismans, or are we going to break with hominid tradition and find ways to coexist with the behemoths that remain?

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