Worried Woolly Bears

 Photo by Tony Fischer. Wooly Bear Caterpiallar to Tiger Moth. CC. https://flic.kr/p/5DzVRB

Photo by Tony Fischer. Wooly Bear Caterpiallar to Tiger Moth. CC. https://flic.kr/p/5DzVRB

While biking to school these days I’ve had to dodge the fuzzy black and orange caterpillars scurrying across the bike path. What are these critters, and why are they running so fast?

Child stars

Apparently woolly bears are the most recognizable caterpillar in North America, which is funny because I had never heard of them until this year. They’re much more famous than their adult form, the Isabella tiger moth, or Pyrrharctic isabella for you Latin fans. That’s probably because the moth is nocturnal and a dull yellow-tan. The fuzzy orange-and-black caterpillars are more noticeable, especially with their habit of crossing roads on sunny fall days.


The real reason for the woolly bear’s fame isn’t its cute black head or its orange and black fuzz. It’s because early North American settlers thought the native caterpillars could predict the winter weather. The wider the orange stripe, the milder the winter would be. They were basically a fall version of Groundhog Day.

This seems silly to us now, but in an era where your survival depended on how much food you stored up the fall, any information these settlers had about the winter would have been comforting.

Scientists aren’t really sure why some caterpillars have wider stripes than others. It could be climate, natural variety in the population, or something that changes as a caterpillar grows. Woolly bears may not predict the weather, but they are awesome for a lot of other reasons.

Autumn angst

Photo by D.Fletcher. Wooly Bear. CC. https://flic.kr/p/sjk7A

Photo by D.Fletcher. Wooly Bear. CC. https://flic.kr/p/sjk7A

People usually see woolly bears in the fall when they’re in a hurry. They’re looking for somewhere to wait out the winter, and if they don’t rush, they’ll freeze. Well, they freeze anyway, but more on that later.

In the spring and summer woolly bears are solitary beasts. Native to southern Canada, they spend their days in meadows and fields hidden in patches of wildflowers. They’ll eat just about anything, from grass to maple leaves to dandelions.

But when winter rolls around they need to find logs, rocks, or tree bark to hide under, none of which are plentiful in wildflower meadows. So in the fall they wander out of their meadows and into more forested areas, which is why we see them crossing roads and bike paths. They’re just looking for somewhere cozy to wait out the winter.

Fuzzy popsicles

Over the winter woolly bears freeze solid. Starting with their heart. Think about how that would feel for a moment. Freezing is usually a supremely bad idea for a living thing. Water expands as it freezes, and prickly ice crystals damage cells and tissues. But woolly bears make a natural antifreeze called cryoprotectant, which keeps their bodies largely undamaged from freeze and thaw cycles.

If it warms up enough during the winter, the caterpillars might thaw and walk around a bit before refreezing again when it gets cold. However, just because they’re full of antifreeze doesn’t make them superheroes. Freezing and thawing multiple times over the winter increases their chances of death and organ damage. When it warms up in the spring, the caterpillars wake up and continue to eat before making a cocoon and transforming into a less-famous adult moth.

Why so hairy?

Woolly Bear fuzz close up. Photo by LadyDragonflyCC->;<. Woolly Bear close up. CC. https://flic.kr/p/7X2AYD/p/oBBdcR

Soft and squishy caterpillars are the ideal snack for birds, rodents, frogs and snakes. The woolly bear has a few ways to keep from becoming their next meal. First, they can run away pretty quickly. They will also curl up into a ball and play dead, keeping all their hairy bristles called setae on the outside. These setae do discourage some predators, but skunks and a few other animals have been known to roll them off before chowing down. It’s also thought that the hairs act as insulation during the caterpillar’s long winter’s nap.

Heal thyself

Besides being surrounded by predators, some woolly bear caterpillars get eaten alive from the inside out. Parasitic flies lay their eggs inside the caterpillar’s body. After they hatch, the larvae munches on the woolly bear’s insides before exploding out of its side. Ick. A 2009 study at the University of Arizona found that woolly bears self-medicate on certain plants which cure the parasite. Caterpillars infected with parasites ate way more alkaloids than their non-infected peers. However, too many alkaloids are poisonous, so woolly bears have to balance poisoning themselves with getting eaten alive by fly babies. Not a fun choice.

So watch out for these speed demons on a road near you this fall, and please try not to squish them.



About Amelia

I am a recent biology graduate and current journalism student exploring career opportunities in science communications.

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October 2015


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