For warm-blooded animals that don’t migrate, one way to survive the winter is to sleep through it. Hibernation is a great strategy that enables animals to conserve energy when food is scarce. During hibernation, body temperature drops, breathing and heart rate slows, and most of the body’s metabolic functions are put on hold in a state of quasi-suspended animation.
It is almost as if the warm-blooded animal becomes cold-blooded, as its body temperature drops considerably. But they are still alive, and they live off their fat reserves. Hibernation for extended periods of time is only accomplished by those animals that can store a great deal of body fat, such as bears, groundhogs, and chipmunks. A black bear loses 15%–30% of its weight while hibernating.
Cold-blooded animals hibernate, too. But they need to store less fat than warm-blooded animals because they require less energy. Turtles and frogs bury themselves in mud under lakes and ponds for up to six months at a time, and for all practical purposes, they appear dead. There are no external signs of life.
When many cold-blooded animals hibernate, something interesting happens at the cellular level. The fluid around the cells, but not in the cells, is frozen solid. As water freezes outside the cell, water from within the cell is drawn out through osmosis. Osmosis is a process in which water moves across a semipermeable membrane—in this case, the cell membrane—from an area of low solute concentration to an area of high solute concentration.
As water freezes outside of the cell, the solute concentration increases, because the quantity of liquid water decreases while the amount of solute stays the same. As a result, water flows out of the cell to equalize the concentrated solution outside of the cell (Fig. 2).