Can woolly bear caterpillars really predict the weather?

As the seasons change and the world gets colder, most caterpillars in the United States and Canada will have already transformed into adults. But not woolly bears: Also called fuzzy wuzzies and woolly worms, these fascinating creatures are some of the few that hunker down and brave the winter while still in their larval stages.

While lots of people probably can’t identify the larvae of many other moth species, woolly bears are relatively well-known. And that’s not just because they are very prominent in the fall, when the insects can be seen crawling over sidewalks and roadways at a rather impressive clip—at least for a caterpillar.

Since America’s colonial period, it’s been said that woolly bears can predict the weather—with their markings suggesting a harsher or milder winter.

There are all sorts of variations on this main theme, but what you really need to know is that they are all bunk.

(These animals can freeze solid in the winter. Here's how they survive.)

"I hate to be a spoilsport but there’s just never been a really clear set of data that says, ‘Yes, this is absolutely the case,’” says Katie Marshall, a comparative physiologist at the University of British Columbia who studies how animals adapt to cold.

However, woolly bears do have a very real winter-weather superpower: freezing.

Why they don't actually predict winter

While there are hundreds of woolly-looking species in Erebidae family, the most common and well-recognized woolly bear caterpillar is Pyrrharctia isabella, or the larval form of the Isabella tiger moth, says Marshall.

According to the legend, the more black setae—long prickly hairs—found on a woolly bear, the harsher the coming winter will be. Likewise, larger bands of red or brown on the caterpillar’s midsection are supposed to forecast a warmer, milder winter.

Some scientists have actually studied all of this and failed to find any relationship between woolly bear prickle patterns and the severity of oncoming weather—although Marshall cautions that most of this work has been more tongue-in-cheek than serious.

(Science also says that groundhogs can't predict the spring.)

But woolly bear colors can tell you other things. For example, these caterpillars tend to shed their exoskeletons six times before transforming into moths—a process known as molting. And each time they do, they become more rusty in color and less black. This means that the youngest caterpillars are usually all black, while more colorful caterpillars tend to be the wizened, elders of a given population.

What’s more, there are some connections between a woolly bear’s appearance and the climate. Marshall says color patterns could indicate how early or late in the year a caterpillar last molted, which could potentially reflect trends in weather and temperature. Similarly, caterpillars with more black coloration would theoretically absorb more sunshine.

“So there are all these sort of plausible, quasi-mechanisms that could maybe show that there is something to this,” she says.

Even though the weather-predicting powers of this caterpillar seem to be overblown, the folklore has caused people to fall in love with these creatures all the same. There are even several places that hold festivals in the woolly bear’s honor.

How they survive the winter

Most insects can’t stay active when winter sets in, so some migrate, like the monarch butterfly, while others dig into the ground or trees to avoid cold temperatures. The latter are known as freeze-avoiding species.

But woolly bears are something different altogether—they’re known as a freeze-tolerant species, says Marshall. They can survive freezing and thawing multiple times over the winter.

“There aren’t a ton of species that do this,” says Marshall. “The liquids within their body will turn to ice.”

Interestingly, the caterpillars do not freeze solid, which would be lethal. Instead, their bodies encourage ice crystals to form in between their cells, while a special, anti-freeze-like substance known as glycerol keeps their cell contents safe.

When the world warms back up, the caterpillars spin a cocoon around themselves and transform into their adult form: Isabella tiger moths, which are pumpkin-orange with black spots lining the top of their abdomens. Once emerged, the Isabella tiger moth will be active during nights throughout the summer.

(How does metamorphosis actually work—and why is it so elaborate?)

The species is thought to be common and not in danger of extinction.

However, because tiger moths are generally attracted to lights at night, you can help these animals by reducing light pollution coming from your home or neighborhood. And who wouldn’t want to help protect everyone’s favorite wiggle-worm?

“We have this idea that they're little, tiny insects, how complicated can they be?” says Marshall. “And the truth is, they can be massively complicated, right? They have all of this stuff going on. They can do things that humans can't do with our bodies.”

“And so I always find it just endlessly fascinating,” she says.