Just like us, many frog species have to deal with less-than-desireable winter weather. Rana kukunoris overwinter in ice-covered water, but how they handle these cold temperatures and how they maintain their water/ion balance is unclear. Men et al. (2025) ran an experiment to investigate physiological differences in R. kukunoris hibernating in near-freezing water (4°C) or in water that was 12°C. The researchers found that after eight weeks of simulated ice-water hibernation, the frogs had dramatically elevated levels of glycerol in their blood plasma, urea was nearly three times higher, and glucose was modestly elevated as well. On top of that, the frogs increased production of freeze-responsive proteins (Fr10 and Li16), first discovered in wood frogs, which appear to help manage the dangerous process of ice formation in body tissues. And to mitigate potential issues with too much water getting into the body while underwater, frogs hibernating in near-freezing water showed changes in gene expression related to water transport proteins and the kidney started producing extremely dilute urine, nearly half as concentrated as that of non-hibernating frogs. The researchers also found that the frogs that underwent simulated ice-water hibernation survived freezing in air much better than the frogs that were maintained in water that was 12°C. These results provide clues as to how R. kukunoris survives winter "under the ice" in near-freezing temperatures, but it remains a risky time of year, since no R. kukunoris individuals survive if the water completely freezes.

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