Mammal Mass Shrank as Hungry Humans Learned to Hunt

A downward trend in mammal size beginning 125,000 years means the largest land animal may soon be the domesticated cow.

An illustration of a rabbit observing mastodons | Daniel Eskridge/Stocktrek Images via Getty Images
An illustration of a rabbit observing mastodons | Daniel Eskridge/Stocktrek Images via Getty Images

In our defense, we were hungry.

Scientists examining the disappearance of large prehistoric mammals have found evidence that humans and their ancestors drove a sharp reduction in the size of land mammals as hunting skills and weaponry advanced. As a result, the average mammal mass has shrunk more than tenfold over the last 125,000 years, University of New Mexico paleoecologist Felisa Smith told Seeker.

That’s not because species are getting smaller — it’s because the biggest ones went extinct, most likely because they provided the most meat. And that trend has continued into the modern era: Earth’s largest land animal soon may be the domesticated cow, Smith said.

“We used to have animals on the Earth that weighed over 10 tons,” she said. “Now the biggest thing is an elephant that on average is only about three and a half-ish … and if they go extinct, then we’re talking about things no bigger than 900 kilos (2,000 pounds). And that’s maximum size. If you look at mean size, it’s much, much different.”

Smith has studied megafauna extinctions for about 15 years. She and a team from the University of California San Diego, the University of Nebraska, and Stanford University looked back at global fossil records dating back to the start of the Cenozoic Era, after the dinosaurs became extinct. Their findings, published Thursday in the research journal Science lay out a connection between the rise of humans and the shrinking of other mammals over the past 65 million years.

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Megafauna like the mastodon, wooly rhinoceros, or the saber-toothed tiger lived on every continent until the Pleistocene epoch, about 125,000 years back, when the human branch of the evolutionary tree spread from Africa to other continents.

North America was rich in large animals, including giant sloths, a bear species that stood as tall as 12 feet, or the glyptodon, an armadillo-like creature “about the size of a Volkswagen Bug,” Smith said.

One finding Smith called surprising was that those trends didn’t appear to be affected by shifts in climate during that period. Animal species tended to adapt by changing their body size, or moving. Meanwhile, it seems people were killers even before we were technically human.

“The only time we see a spike in extinction rates and this huge size bias, where large-bodied things are disproportionately at risk, was where hominids are involved,” she said. “From that, we conclude that it’s probably related to human exploitation of large-bodied animals.”

Evidence of large mammal extinctions appeared as early as the beginning of the Pleistocene in Africa, where human ancestors were evolving alongside them, Smith said. By 40,000 years ago, when today’s Homo sapiens edged out Neanderthals in Eurasia, mammalian body mass dropped about 50 percent. By the end of the Pleistocene, when the last great ice age ebbed, other human ancestors were gone, humans had settled the Americas and long-range weapons like spears and arrows were common — and the average mass of mammals had fallen from nearly 100 kg (220 pounds) to less than eight.

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While Smith said climate did not appear to play a role in that process in the past, today may be a different story. With biodiversity facing threats from human encroachment and fossil fuel-driven climate change, the evolutionary escape routes that saved earlier mammal species have been largely closed off, she said.

“Now we’re in a situation where a lot of us are a lot more prosperous than we ever were before, and we have the luxury to say, ‘Oh, we don’t have to use everything to live, now we need to transition to being stewards of the land,’ ” she said.

But that shift in attitude still bumps up against the reality of hunger in places where large mammals still live.

“Somehow we’re going to have to cope with that,” Smith said. “Because if we don’t cope with it, we actually are going to end up with an Earth where there is nothing bigger than a cow — and that’s a depressing thought for me personally.”