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Home / Science / The mammals of the earth have shrunk dramatically and people are to blame

The mammals of the earth have shrunk dramatically and people are to blame

Within a few hundred years, according to a new study, the domestic cow might be the largest animal in the world. (Justin Sullivan / Getty Images)

Life on earth used to look much more impressive. Just over 100,000 years ago, there were sloths as long as a giraffe was tall, monstrous bears whose shoulders were six feet off the ground, and Bunyanesque beaver, which weighed as much as an NFL linebacker. But over time, all of these creatures disappeared in such a fast and so mysterious way that scientists still could not explain what was going down.

Has an asteroid unloaded the mega-beasts, much like the one that went out is the dinosaurs? Or was it a widespread climate change or a plague of new diseases? Has our preference for hunting played a role?

It is likely that a combination of factors led to global mammalian death when the Ice Age came to an end. However, a study published in the journal Science on Thursday provides evidence that the main driving forces were humans and other hominids.

"We have looked at the entire fossil record in millions of years for 65 million years. Question:" Is it ever bad to be big? "Said lead author Felisa Smith, paleo-ecologist at the University of New Mexico for most of the evolutionary History was the answer no – greater body mass did not make an animal die out, she said. "For 65 million years, it did not matter what size you were."

That is, until a new breed of predator appeared: Homo erectus. About 1.8 million years ago, hominids that had long been dependent on plants had become hominids that "were heavily and increasingly dependent on meat as a source of food," according to Smith.

When these tool-wielding team hunters spread out of Africa If you are spending time and energy on a hunt, these early humans and their ancestors probably believed that it was great is going or going home.

"They hunt a hare, they eat for a small family for a day," said Smith. "You hunt a mammoth, you feed the village."

It is also possible that hominids actively attack the most powerful creatures for other reasons – for fear, perhaps, or perceived competition for prey. In modern times, human conflicts with large animals are often about their taste for our cattle, such as wolves and lions, or the destruction or consumption of our crop, such as elephants and orangutans.

But something substantial makes them more vulnerable to population decline, 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 to the little guys.

"Their life-historical traits, such as reproduction rates and maturity rates, are much slower," said Ripple. "Large animals do not multiply as fast as small ones."

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

North America was too late to extinction, as most of its massive mammals survive until the end of the Pleistocene. But as they left, they went fast, a phenomenon Smith says could have had to do with the invention of more effective long-range hunting by Homo sapiens and the disappearance of all rival hominids. All in all, after the black dust settled on extinction, the size of North America's average mammal dropped from 216 pounds to about 17 pounds – the size of a bobcat.

Brett Crawford, above, and Matt Fair deconstruct the whirls of a Woolly Mammoth Skeleton at the Smithsonian Museum of Natural History in Washington in 2014. (19659015) To see what could happen if this shrinking trend continues, Smith guessed that all Animals that are now classified as endangered or threatened will be extinguished and then removed from the data.

blue whales? Path. Elephant? Poached from existence. Polar bears? Glug, glug, glug.

Go down the line and within a few hundred years you will end up with a planet where the most important mammal is nothing but the domestic cow.

Ripple, the ecologist, is not surprised. He has published numerous articles stating that large mammals are disproportionately threatened with extinction.

"I think that this work makes a significant contribution to what I call the 'downsizing of nature," he said of the new study

Of course, several animals make their living by they use larger creatures, said Ripple. Gray wolves can kill an elk; Killer whales were watched as they drove gray whales away.

"So it could be that humans evolved," said Ripple. "But today we have well over 7 billion people on planet Earth and 7 billion people have a big impact."

While the new paper focuses on mammals, Ripple said that the same size selection pressure applies to the largest fish the world, reptiles, amphibians and birds, acts. And we are only now beginning to understand the consequences that could have for the ecosystems around us.

An elephant grazes in the Mara Triangle in southern Kenya. African elephants are today the largest land mammal in the world. (Yasuyoshi Chiba / AFP / Getty Images)

Scientists consider many of the largest animals to be ecosystem engineers. Elephants have a habit of tearing down trees in search of greens for open environments like the savannah. Probably the mammoths interacted with their environment in a similar way, which is why the prairie-like habitat that once stretched from Spain to China is called the mammoth steppe.

Smaller animals hauled him directly onto a hill and created him a vertical game path, Smith said. Stronger animals must climb up a slope, forming serpentines and long, meandering valleys. The difference in these paths can affect everything from erosion and water dispersal to the distribution of vegetation.

"So even something as simple as going through the environment can change everything," Smith said about big animals. Cows' ecosystem services, she notes, are no match for elephants.

Evidence that hominids have been spoiling large species for almost 2 million years may be read as an excuse for modern humans and all the animals we pushed for the list of endangered species. But Smith said that there is a difference between then and now.

"Now we are at a point where we can become aware of it," she said.

And we have the choice: will we continue to kill her? for food, clothing and talismans, or will we break with the hominid tradition and find ways to coexist with the remaining giants?

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