Why did supersized coyotes suddenly shrink 10,000 years ago?

Just 11,500 years ago, coyotes were about 1.5 times as big as they are today. By 10,000 years ago, they had shrunk to their current proportions. What could have made these predators get so much smaller, so quickly?

According to researchers at the National Evolutionary Synthesis Center in North Carolina, the coyotes of the Pleistocene weighed about 15-25 kilograms and their size would have resembled that of wolves far more than modern coyotes, which are a relatively paltry 10-18 kilograms. But when the shrinking started, it happened fast, with the coyotes shedding the species's excess mass in just over a thousand years.

There are a few possible explanations for this sudden shrinkage. Their loss of size broadly lines up with the aftermath of the last Ice Age, when global temperatures recovered and rose six degrees Celsius. The researchers point out that such warming tends to favor smaller animals, but they couldn't find any correlation between warming patterns and the size of coyotes, and there's no evidence that wolves underwent a similar shrinking in response to the changing climate, so this is likely only a minor factor, if at all.

It's also possible that humans played some role in this, as they likely arrived in the Americas around the same time the coyotes were shrinking. Humans might have selectively hunted the larger coyotes or reduced their food supply to the extent that only smaller coyotes could survive long-term. However, there's so little archaeological evidence that it's hard to judge this one either way. It's possible, but it's also almost impossible to prove.

The most likely solution goes back to a mammal extinction event that preceded the coyote shrinking. This extinction wiped out much of the megafauna that had once roamed the Americas, leaving precious little of the large prey that these supersized coyotes had once relied on. The necessity to cut back on food or pick on smaller prey meant that less massive coyotes suddenly enjoyed a major evolutionary advantage, and in less than fifteen centuries the coyotes had shrunk. It also probably helped that the extinction event claimed the massive dire wolves, meaning coyotes no longer need excess size to defend themselves against their adversaries.

It's an intriguing finding because it points to how extinction events don't simply wipe out species — it can also rewrite the evolutionary path of the species left behind, sometimes with hugely dramatic effects. That's of particular interest now, considering we're likely staring down the barrel of another large extinction event.

Original paper at PNAS. Image by Todd Ryburn on Flickr.