Humans Killed Off Largest Mammals And Shrunk Their Body Size

Posted: Apr 20 2018, 11:46am CDT | by , in News | Latest Science News

Humans Killed off Largest Mammals and Shrunk their Body Size
Credit: John Poulsen, Duke University

If the downsizing trend continues, the largest terrestrial mammal in 200 years will be the domestic cow

Humans are responsible for the extinction of most of the biggest mammals on Earth and also for the reduction in their body size. As Homo sapiens, Neanderthals and other hominin species migrated out of Africa, large mammals started to die at an exceptional rate, leaving relatively smaller mammals behind. Massive mammals like wooly mammoths, elephant-sized ground sloths and various saber-toothed cats were roaming Earth between 2.6 million and 12,000 years ago. But they disappeared faster than small animals, which is a phenomenon known as size-biased extinction.

Based on fossil records, researchers have estimated that this size-biased extinction started at least 125,000 years ago and the timeframe coincide with known human migration patterns. At the point, the average body size of mammals on Africa was already 50 percent smaller than on other continents. This is surprising, especially given the fact that Africa was a larger continent and continents with larger masses can typically support larger mammals. The extent of this size-biased extinction in Africa was higher than any other recorded during the last 66 million years.

"It wasn't until human impacts started becoming a factor that large body sizes made mammals more vulnerable to extinction," said one of the study authors Kate Lyons from University of Nebraska-Lincoln's. "The anthropological record indicates that Homo sapiens are identified as a species around 200,000 years ago, so this occurred not very long after the birth of us as a species. It just seems to be something that we do.

"From a life-history standpoint, it makes some sense. If you kill a rabbit, you're going to feed your family for a night. If you can kill a large mammal, you're going to feed your village."

Over time, as humans spread around the globe, extinctions of the largest mammals followed and their average body size also reduced. Human activities are still influencing mammal diversity and body size. If this trend continues, researchers say, the largest land mammal in the future will be the domestic cow and the average body mass would drop to less than six pounds - roughly the size of a Yorkshire terrier.

"If this trend continues, and all the currently threatened (mammals) are lost, then energy flow and taxonomic composition will be entirely restructured," said lead researcher Dr. Felisa Smith from The University of New Mexico. "In fact, mammalian body size around the globe will revert to what the world looked like 40 million years ago."

New study also contradicts the idea that climate change drove the extinction of many animals during the last 66 million years. The shifts in temperature throughout that span appear equally harmful for large and small mammals.

"If climate were causing this, we would expect to see these extinction events either sometimes (diverging from) human migration across the globe or always lining up with clear climate events in the record," said Lyons. "And they don't do either of those things.”

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<a href="/latest_stories/all/all/47" rel="author">Hira Bashir</a>
The latest discoveries in science are the passion of Hira Bashir (). With years of experience, she is able to spot the most interesting new achievements of scientists around the world and cover them in easy to understand reporting.




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