Mammals Have Been Shrinking for Thousands of Years and It's All Humans' Fault

Humans can be challenging neighbors: We build cities, we turn forests into fields and we enjoy eating a host of other species. Scientists know these actions take a toll on the animals that live around us.

A new paper published in the journal Science has gone to drastic new lengths to quantify exactly what that toll is. It finds a stark correlation between the arrival of humans or our lost relatives like Neanderthals on a new continent and the subsequent extinction of larger mammals that leaves behind smaller survivors. In short, mammals on average have been shrinking for more than 100,000 years, and it's all humans' fault.

Gone are the mammoth that soared to 14 feet and the giant ground sloth that could weigh four tons. Elephants are now the largest land mammals left wandering Earth, and scientists worry they, too, may not last.

Scientists have suspected a correlation before, but never been able to put together such a broad look at the situation. The new study is based on data that covers species over the entire 65 million years the group of animals has been alive. "It is a Herculean effort to put together these databases," lead author Felisa Smith, a biologist at the University of New Mexico, told Newsweek. "But without them you really can't ask these big picture questions."

Read more: New Fossil Whale Discoveries Thrill Scientists Trying to Crack Mysteries of Ancient Species

The project involves pulling together measurements from existing databases and published papers alike, all based on the remains of these animals. Even without a complete skeleton, scientists can calculate a mammal's body size from the size of its molar tooth. And of course, sometimes more bones are available.

"You haven't lived until you've put your arms around a mammoth femur trying to measure it," Smith said.

Then the team combined that mammal data with information about when humans and related species like Neanderthals arrived on a given continent. That was always when the trouble started for the planet's largest ghosts.

"For most of Earth's history, being big has not been a bad thing," Smith said. But after hominins arrive, a big body becomes a big liability.

Other factors, like natural climate change, didn't show any impact on extinction. "How clear the patterns were was really amazing to me," she added.

But the fact that there's a pattern at all is more or less what scientists would have expected. "Frankly no, I wasn't surprised," said Alison Boyer, a biologist at the University of Tennessee, Knoxville. She has studied birds on isolated islands, where they tend to act like mammals in ecosystems and where she says exactly the same trend occurs. "It seems to me a pretty generalizable pattern," she said.

There are a few reasons that could individually or together explain the pattern, says Manuela Gonzalez-Suarez, a biologist at the University of Reading in the U.K. who wasn't involved in the new research. Human hunger is, of course, a clear contender. "We like to hunt, it's more effective to hunt something big because you get more meat for one capture," she told Newsweek.

But other factors could also be playing a role—larger species tend to need more space and more food, which hominins can interfere with. And larger animals tend to have fewer babies that mature more slowly.

"Producing one elephant takes longer than producing one mouse," Gonzalez-Suarez said.

Some species have evaded the curse of large animals, Boyer says. She points to species like certain species of kangaroos, which have survived the arrival of humans, but gradually shrunk. Koalas may have done the same. But that relies on the luck of evolution.

Mammoths and many other large mammals have disappeared after the local arrival of hominins. Andrej Isakovic/AFP/Getty Images

The new paper doesn't just measure the ghosts hiding in Earth's history—Smith and her colleagues also look into the future. They work under the assumption that all species currently considered threatened will die out over the next 200 years. If that happens, they say, the largest mammals left will be cows tended by humans.

"It's not a pretty picture," Smith said. "I certainly hope it's not going to come to pass."

Plenty of people are working to stop those extinctions, and Boyer emphasized that some of the larger species left will probably survive. But that hope also points to a grimmer truth.

"I'm afraid we've already taken care of all the big stuff and there just aren't that many big things left to drive extinct," Boyer said. "We really have already done most of the damage."

Smith says her team wanted to focus on what the fossil record showed about body size over time without getting into political issues. She said the past can shed light on what may happen next, and that's where humans would be able to break the pattern identified in all this data.

"Obviously, we can't change what happened," Gonzalez-Suarez said. "We can only look into the future."

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