Over the past 125,000 years, the average size of mammals on the Earth has shrunk. And humans are to blame.
That's the conclusion of a new study of the fossil record by paleo-biologist Felisa Smith of the University of New Mexico.
Smith studied fossils going back 65 million years, when dinosaurs died and mammals came into their own. Many of the early mammals went on to get big. Among the giant creatures: "Llamas and camels and sloths and five species of pronghorn [antelope] actually," she says, "and certainly mammoths. And then lots of really cool predators, like Arctodus, the short faced bear." The short-faced bear stood 11 feet tall, about the shoulder height of some species of ancient camel.
And that was just North America.
Being big was just as successful as being small, and had some advantages when it came to surviving big predators. "Taken as a whole, over 65 million years, being large did not increase mammals' extinction risk. But it did when humans were involved," Smith found.
Looking back over the most recent 125,000 years of the fossil record, Smith found that when humans arrived someplace, the rate of extinction for big mammals rose. She says it basically came down to hunger. "Certainly humans exploit large game," she says, "probably because they are tasty"--and because a bigger animal makes for a bigger meal.
Indeed, the Americas had been the last holdout for really big mammals, since they were the last populated by humans.
We still have lots of furry little mammals on the planet. But the pattern is clear: 11,000 years ago, the average mass of a non-human mammal in North America was about 200 pounds. Now it's about 15 pounds. And the researchers say they're getting even smaller.
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