Bigger is no longer better, at least when it comes to the extinction threat facing the animals who call our planet's oceans home. In a study published in Science on Wednesday, Stanford researchers reveal what a press release calls an "unprecedented pattern of extinction": large-bodied marine animals, not smaller, are the ones now more vulnerable to extinction, and we're likely to blame for throwing the system into reverse. "For every factor of 10 increase in body mass, the odds of being threatened by extinction go up by a factor of 13 or so," says co-author Jonathan Payne. The researchers looked at the interplay between extinction rates and four traits for 2,497 mollusks and marine vertebrates genera: its body size, sea floor vs. open water living, predator vs. not, and swimmer vs. stationary.
The Los Angeles Times explains those relationships were examined over five mass extinction events over the past 445 million years; in essentially all of that time, size either wasn't a factor or smaller animals were a bit more likely to go extinct. But in the last 500 years comes the unprecedented shift: size is the now the leading factor, and significantly so. (Gizmodo reports that previously, the biggest factor was habitat, with those in open waters facing the greater threat.) As for what's behind the reversal, the researchers have a hunch: "After looking at the fossil record and then looking at fisheries studies, we can't think of anything but human hunting that might be the cause," says coauthor Noel Heim. The effects could be profound: "A larger clam will mix more sediments than a smaller one," says Heim, who recommends humans intervene to reverse the trend.