Frogcicles, anyone?

Yesterday (07 February 2016) the temperature in Mt. Pleasant reached a high of 46 degrees Fahrenheit.  A blast of Arctic air is going to hit Mid-Michigan over the next few days, dropping temperatures down to near zero degrees by the end of the week.  This drop in temperature will make survival more difficult for many small animals.  The layer of snow that has been insulating the ground has all but melted away.  Animals such as voles, mice, and shrews are exposed to the elements as they forage.  These species all have high metabolic rates, they need to consume enormous numbers of calories for their size just to counteract the effects of cold.  Exposure for many of these species can quickly lead to hypothermia if they are unable to find enough food to maintain their metabolic rate.

Other small animals are less bothered by the change in temperature.  Some of these species spend the winter months securely hibernating below ground.  Mammals that hibernate reduce their metabolic rate and slowly burn through fat that they accumulated during the summer and fall months.  Even though their body temperature drops it remains above freezing.  For reptiles and amphibians this is not an option.  These species are considered ectothermic - meaning that they rely on the external temperature to regulate their internal temperature.  If the temperature of their surrounding is high so is their internal body temperature; if their surrounding are cold so is their internal temperature.

So how to species such as this survive the lows of winter?  If the temperature of their surroundings drops below freezing so does their body temperature.  This exposes their cells and organs to the dangers of freezing solid.  When a cell freezes, ice crystals form in the intercellular fluid - this can cause the walls of the cell to break, destroying the cell.  If enough cells are destroyed the animal dies.  (The same cellular freezing causes plants to die.)

At least one species has developed a countermeasure that prevents its cells from freezing solid.  The Wood Frog (Lithobates sylvaticus formerly Rana sylvatica) survives winter temperatures that should result in complete cellular destruction.  Unlike most frog species that hibernate at the bottom of ponds, where their temperature drops but not to freezing, the Wood Frog hibernates in the leaf litter of North American forests almost as far north as the Arctic.  It is regularly subject to temperature lows that should freeze its cells solid, but in the spring the Wood Frog thaws and goes about its business with no apparent damage.

How?

When a Wood Frog enters hibernation in the fall, it's liver begins to produce large amounts of the simple sugar glucose.  This sugar is directed into the frogs cells, essentially filling them with a syrup solution.  This high sugar content of the intracellular fluid prevents the cells from freezing; at the same time the fluid between the frog's cells (intercellular fluid) does freeze solid.  So the frog is both frozen solid, but not frozen at the same time.  When springtime temperatures thaw the intercellular fluids, the frog is able to hop away with no apparent damage.