Mammals were a lot smaller before the dinosaurs went extinct—but got really, really, really big once dinos were gone, scientists say. New research published in the journal Science shows that, over the course of 25 million years, the largest mammals increased in size 1,000-fold from the time of the dinosaurs, from a maximum of 22 pounds to a whopping 17 tons, the Telegraph reports. That may seem like a long time, but it's "actually a pretty short time frame, geologically speaking," says a researcher. "That's really rapid evolution."
The reason: With dinosaurs gone, mammals faced less competition when it came to eating plants, "and it's more efficient to be an herbivore when you're big," the biologist says.