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Lab Rat

Exploring the life and times of bacteria

Arctic creepy-crawlies part II: woolly bear caterpillars

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This is the second part of my two-part mini series on Arctic creepy-crawlies. Part I: ice worms can be found here.

Part II: Woolly bear caterpillar

The Arctic woolly bear moth (Gynaephora groenlandica) is found in Greenland and Canada around the Arctic Circle. Unlike the ice worms the caterpillars don't require exclusively freezing conditions to survive, in fact they become dormant during the long winter. However they have adapted to survive to temperatures below −60 °C.

The Woolly Bear Caterpillar. Mike Beauregard from Nunavut, Canada. Uploaded by Tillman. CC 2.0

After hatching from the egg the woolly bear caterpillar, like all caterpillars, must build up enough biomass through eating to turn into a moth. Unlike other caterpillars they only spend about 5% of their lives eating, feeding on the Arctic tundra during the month of June. The rest of their lives are spent frozen and dormant. Because of this the woolly bear caterpillar has the longest life-cycle of any butterfly or moth. It can take up to 14 years to get from the egg to the final moth!

As the caterpillars don't need to be fully functional during their frozen hibernation they can make more drastic internal changes compared to the ice worms. When exposed to near zero or freezing temperatures (which in the wild would be as winter starts to approach) the caterpillars break down all their mitochondria and start to synthesise glycerol. Without mitochondria they are unable to metabolise properly and go dormant. The glycerol acts as an antifreeze to protect the cells from the freezing conditions.

Greenland in summer. Silje Bergum Kinsten - Nordic Co-operation website (norden.org)

Interestingly the dormant caterpillars are not found randomly scattered over Greenland, but in specific micro-habitats. In a field study on Ellesmere Island all the caterpillars were found attached to rocks rather than vegetation. This may be as rocks are a more stable surface to attach to or it may be that snow on rocks thaws quicker in the summer, allowing the caterpillars to emerge sooner to maximise their eating time. The dormant caterpillars were also more likely to be found on the lee-ward side of the rocks giving them protection from the wind.

Although they both live in similar conditions, the woolly bear caterpillars and the ice worms both employ very different strategies to survive the frost. The caterpillars become dormant for long stretches of time, stretching out their lifecycle to take advantage of small windows of warmth and light. In contrast the ice worms altered their internal structure and metabolism to allow them to function at freezing temperatures, rendering them permanently incapable of living anywhere else.

Next month will return to bacteria!

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Kukal O, Duman JG, Serianni AS. Cold-induced mitochondrial degradation and cryoprotectant synthesis in freeze-tolerant arctic caterpillars.J Comp Physiol B. 1989;158(6):661-71.

Bennett VA, Lee RE Jr, Nauman JS, Kukal O. Selection of overwintering microhabitats used by the arctic woollybear caterpillar, Gynaephora groenlandica. Cryo Letters. 2003 May-Jun;24(3):191-200.

The views expressed are those of the author and are not necessarily those of Scientific American.

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