Humans May Be Main Cause Of World’s Shrinking Mammals

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A new report suggests we’re dealing with a case of shrinking mammals – and humans are largely to blame due to our significant impact on the world around us.

Around 100,000 years ago – long before humans had a significant impact on the world around them – life on Earth was much more impressive from a size perspective. With sloths that were as long as a giraffe to bears that were well over 6 feet tall, the size of mammals was generally much bigger – causing scientists to question why, exactly, we don’t see such impressive specimens in the modern day.

Previous theories suggested that the asteroids may have caused their extinction – similar to the method in which dinosaurs went extinct – or that we saw some sort of climate change or new disease, but it turns out that the main factor that caused shrinking mammals is the impact of humans, at least according to a new study published in Science on Thursday.

According to lead author Felisa Smith, a paleoecologist at the University of New Mexico, “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?’” It turns out that being big was a good thing in most cases, and that for a period of 65 million years we weren’t seeing any sort of case of shrinking mammals. That all changed around 100,000 years ago – a momentary blip in the timeline of our planet but also the time when humans started to have a large impact on the world around them.

As ancient humans started to spread out from Africa, the tool-wielding hunters started to cause the mass extinctions of large animals – leading to shrinking mammals over time as the larger species started to go extinct. Due to the time and energy expended during a hunt, it was important to humans that the largest prey possible be killed in order to make it worth the investment.

“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 largely for this reason that, over thousands of years, we started to see shrinking mammals overall, with these large and impressive species that were dominant before our influence starting to become less and less present, until we ended up with species that were small enough to potentially avoid being threatened.

However, there are other factors that caused shrinking mammals, according to William Ripple, director of the Global Trophic Cascades Program at Oregon State University. Generally, there are fewer of the big animals – making the hunting that much easier to cause an eventual extinction.

“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 humans started to spread across Eurasia, we saw shrinking mammals becoming more commonplace, with the average body mass of a mammal dropping by around 50% over the course of around 100000 years. In fact, in Australia, the average mammal body mass today is only around one-tenth of it was around 125000 years ago. It appeared that, for the longest time, being big was better. But as humans started to tackle the big animals and establish themselves as the apex predator, shrinking mammals became more advantageous, with the overall size of many species much smaller today than it was in the past.

Although this information regarding shrinking mammals is interesting from an academic perspective, it’s important to recognize that the phenomenon is far from over. Humans remain a major threat to a variety of species – whether through direct hunting or through habitat destruction – and we may soon see shrinking mammals to a much greater extent.

In order to extrapolate the process and see what we might see over the next few hundred years, Smith followed the same pattern that Earth experienced previously and assumed all endangered species would become extinct. The results were shocking. Within a few hundred years, there’s a possibility we may see a world where the largest animal around is the domestic cow. With such majestic animals as Blue whales, Elephants, Polar Bears, and more, the list goes on and on. The case of shrinking mammals may continue to claim species that are a valuable part of our ecosystem until we don’t have anything larger than the average dairy cow.

“I think this paper is a significant contribution to what I call the ‘downsizing of nature,’ ” Ripple said of the new study. “So it may be that humans have evolved to [hunt]…But nowadays, we have well over 7 billion humans on planet Earth. And 7 billion humans have a huge impact.”

The results of the study suggest that if we don’t take efforts to rein in our impact, we may soon see a world where the unfortunate process of shrinking mammals leaves us without many beloved species.


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