How Frogs Cope with Winter

Birds migrate. Mammals have their winter hair, store fat, & some hibernate. Female Homo sapiens stop shaving their legs. But some animals have no hair, no migration option nor fat storage to cope with winter. It seems like a freeze to death situation, but somehow they’re back hopping around in spring. Yes, I’m talking about frogs!

Coming from a megadiverse tropical country with over 150 species of amphibians, moving to Europe was quite a herpetofaunal sacrifice. But the lower herpeto-richness is not the only thing I’ve had to adapt to: also the negative degrees. This freezing winter – that the locals seem to love- poses harsh challenges on me – I mean, on the frogs! You might think that they cope well because they’re cold-blooded (the frogs, not the locals). Although it’s true that they’re ectotherms, going from 40°C in Summer to -20°C in Winter is not such an easy range to adapt to!

Frogsicles

Some frogs hibernate. They burrow themselves below the frost line (the depth at which the soil doesn’t freeze). Some more aquatic species can overwinter at the bottom of lakes & ponds where the water doesn’t freeze.

Some frogs hibernate outside of the water & above the frostline, squeezing on crevices, logs, or other hibernacula. Since most of animal cells’ content is water (yes, yours too!), these frogs risk freezing. Having ice crystals forming inside of them can become problematic, even deadly, since they could puncture cells or even organs.

Therefore, they’ve evolved an anti-freeze system: the liver stores glycogen, which is here used to fabricate lots of glucose (sugar!) that is then mobilized to other cells. Increased plasma glucose prevents freezing by lowering the freezing point of the water in their cells! There’s some other compounds and proteins that come into play to help this happen. They also dehydrate some tissues to reduce the chance of dangerous ice crystals forming. So these frogs “pseudo-freeze”, they stop breathing, their heart stops beating, but they’re not dead! So cool, isn’t it?!

We’ll talk about the implications in a later post, but I’ll leave you this to think about: temperature cues indicate when they should start “thawing”. What will climate change mean for these frog populations?

Recommended:

1. Costanzo, J.P., et al. 2013. Hibernation physiology, freezing adaptation and extreme freeze tolerance in a northern population of the wood frog. Journal of Experimental Biology.