The size of mammals has shrunk significantly over the last 125,000 years. A new study finds that the blame is entirely on humans for having caused the reduction in the size of mammals.
Humans changed the size of mammals, and it wasn't just by hunting them to extinction.
Big Mammals Decline
A new study by Felisa Smith of the University of New Mexico reveals ancients humans were the cause of the shrinking size of mammals. Smith's finding was published in the journal Science. They analyzed the fossils going back 65 million years to see what was the cause of the changing size of mammals.
She found that size didn't play a role in the survival of large mammals until humans showed up. Large and small mammals thrived for a time, and then humans changed the odds of survival. Smith found that when humans settled in a region, the chances of survival for large mammals dipped.
As humans made their way out of Africa, the extinction of large animals followed their migrations. This followed when humans settled in the Americas. This was the large place on Earth where big animals were still thriving.
As humans spread, the size of animals declined. The average body mass of mammals in Eurasia dropped by around half over the course of 100,000 years. In Australia, average mammals' body mass dropped 90 percent over the last 125,000 years. In North America, the average mammal dropped from 216 pounds to 17 pounds.
The main reason for humans hunting large mammals to extinction was hunger. Large mammals made easier prey for humans. They are able to get more out of a hunt. Large mammals make for larger meals. It's also possible that humans feared the large animals. Due to their size, ancient humans may have perceived them as an existential threat and hunted them.
Hunting the large mammals would have reduced their numbers significantly. There usually tends to be a smaller number of animals the larger that they become.
Not Just Hunting
Humans had a severe effect on the survival of large mammals. This wasn't only due to the hunting of the mammals by humans but also other activities that the humans were responsible for carrying out.
Humans were responsible for burning forests and grasslands where the large mammals lived and fed. They were also in competition with large mammals for food sources. While hunting humans began using dogs, this gave them an advantage over the large mammals.
These changes also had an adverse effect on the environment. Getting rid of the large mammals caused faster erosion of land. Smith says that when large animals walk up a hill, they walk in a zig-zag. When smaller animals walk up a hill, they walk up straight. Water follows these trails, and a more direct route would make it easier for the vegetation and soil to be washed away.