WOOLLIES IN WINTER
Congratulations to those of you who correctly identified this week’s Caterpillar of the Week, the Woolly Bear (Pyrrharctia isabella)! A couple of our fans even mentioned seeing Woolly Bears crossing the road in fall—a common occurrence in many parts of the continental US, especially in the East.
Where do those Woollies go in fall? And what do they do when they get there?
Woolly Bears spend the entire winter as caterpillars. In fall, when temperatures begin to cool, they seek out a good spot to overwinter. Piles of wood or leaves are favored wintering grounds. When spring arrives, Woollies often wander again, sometimes crossing roads just like they often do in fall. Scientists aren’t sure what all the wandering is about, because Woolly Bears aren’t picky eaters. In fact, they can eat a wide variety of common plants. Regardless, after their spring restlessness, Woollies often resume eating for a little while.
When they’ve had their fill of food, Woolly Bears spin cocoons, which are loosely constructed with silk and the caterpillars’ setae. (Don’t know what setae are? See our post from yesterday, January 25!) In approximately two weeks, the handsome adult Isabella Tiger Moth emerges.
I can barely survive the winter cold in my house. How do tiny Woolly Bears survive outdoors?
Woolly Bears have some very cool (pun definitely intended!) overwintering strategies. We learned about them through articles written by researcher Jack R. Layne Jr. and his students / colleagues at Slippery Rock University. If you’d like to know more than we share here, please check out their fascinating work!
One thing you’ve probably picked up on that may help Woollies get through winter is their sheltered overwintering sites, in leaf litter or similar habitats. But here’s a fun fact about their winter survival—it only works at the right time of year. In September 1999, most Woolly Bears in Layne’s study didn’t survive being frozen for three days at 23*F. However, come November, ALL of his Woollies survived the same temperature for more than twice as long!
In Layne’s 2002 study, he investigated what happens in a Woolly Bear’s body that helps it acclimate to sub-freezing weather at some times but not others. Woolly Bears’ physical winter preparation appears to begin in response to dehydration as well as cold temperature cues. When Woolies experience these cues, their bodies begin to produce additional chemicals, including sugars, in their hemolymph (which is basically insect blood, but doesn’t carry oxygen like our blood). These chemicals reduce the freezing point of the caterpillars’ body fluids, so they can experience temperatures below 30*F without damage to their bodies.
Here’s the catch, though, with Woolly Bears being able to survive freezing temperatures of 23*F and below: it depends on how “survival” is defined. In 2006, Layne studied long-term effects of freezing Woollies at 24*F. Some individuals were frozen for a week, and others for six weeks. The entire weeklong freeze group survived. So did nearly all of the six-week freeze group. But the results didn’t stop there. Woollies that survived the long freeze pupated only about half as often as their short freeze counterparts. Apparently, the longer-term freeze did enough internal damage that half the individuals in this group weren’t capable of pupation. So, does that count as survival? Or, as Layne put it, did these individuals actually recover from their six-week freeze?
Who cares??? What are you, COLD-hearted?
Seriously though, this research is meaningful, beyond providing some neat facts you can share with your vehicle passengers next time you swerve to avoid a Woolly Bear in the road. Understanding how creatures survive temperature extremes can help scientists predict how well the same creatures will be able to cope with and adapt to climate change. And that’s just what we’ll address in our next Caterpillar of the Week installment—Woolly Bears and climate change. Will they go extinct? Or will their sweet, sweet winter hemolymph enable Woollies to take over the world? Stay tuned…
-Liz Kautz, TCL Education Director