Wood frogs have a rather unconventional approach to surviving winter: they simply freeze solid. Flooding their bodies with glucose – which protects delicate cell structures from being shattered by ice – the frogs emerge in spring unscathed by a process that would prove fatal for most other creatures. Brent Sinclair, from The University of Western Ontario, Canada, his students, and Ken Storey from Carleton University, Canada, explain that freezing frogs have to burn precious lipid stores to convert glycogen into glucose, but no one knew just how much of these valuable reserves they consumed to pull off the freezing stunt (p. 292).
Freezing and thawing frogs as they measured the amount of carbon dioxide produced by the amphibians to find out how much energy the animals consumed, Sinclair and his colleagues found that the frogs increased CO2 production to 104 ml at 1°C (just before freezing) before drastically increasing it to 565 ml when they froze solid. And when the team recorded the amount of CO2 produced when the frogs thawed, they found that this too incurred a metabolic cost as the animals produced another 564 ml pulse of CO2. The team says, ‘We interpret these increases in metabolic rate to represent the energetic costs of preparation for freezing, the response to freezing and the reestablishment of homeostasis and repair of damage after thawing.’
But how much energy do real wood frogs – buried beneath insulating snow and leaf litter during the harsh Canadian winter – expend to emerge unscathed in spring? Burying temperature data loggers in a wood near Ottawa, the team recorded 23 freeze–thaw cycles and then calculated the amount of energy that frogs in the vicinity would have consumed. Realising that the frogs only spent 6–8% of the winter frozen, the team estimates that a 7.1 g frog consumes 0.28–0.37 g of its lipid stores and 0.05–0.12 g of carbohydrate while weathering the winter. The team adds that the high cost of freezing and thawing probably outweighs the energy saved while frozen, suggesting that the frogs probably evolved their inventive freezing strategy for reasons other than energy conservation. However, they warn that climate change could cause hibernating frogs to run out of fuel by exposing the frogs to more frost if snowfall occurs later in the season.