Amphibians and reptiles are ectothermic, which means that they get their heat from the environment. During the winter, low temperatures make it very difficult to be active and food shortages further contribute to this problem. The best strategy to handle the problem is to hibernate and live off of stored fat. The rate of metabolism is very reduced so not a lot of energy is required.
When temperatures approach freezing, most species of frogs seek protection below the freeze line. Some species jump into the water and prepare to hibernate at the bottom of ponds or under the banks of streams. Because water is a good insulator and retains heat better than other environments, it is a good place to wait out the winter.
An active frog breathes oxygen from the air. At least its nostrils are above water (above photo) and the throat area can be seen moving as it pumps air into the lungs. To enter hibernation, several changes in position and behavior can be seen.
The frog on the left is warm and active, therefore it has a high level of metabolic activity. Notice its upright stance, with back legs tucked close to the body ready to jump if startled. In contrast, the same frog spreads the legs into a "four-point stance" after its body temperature has dropped to near freezing. At the bottom of a pond, this stance provides stability against any activity that might dislodge it from its winter refuge.
An active frog keeps its eyes open (top photo) to be aware of events in its environment. However, during hibernation, debris and other organisms may scrape across the eyes. To protect the eyes from damage, the frog raises a special "eyelid" called a nictitating membrane (see middle photo of a toad eye). At the same time, the bulging eye is pulled closer into the head (bottom photo).
Perhaps the most significant change is in respiration. As the frog cools, it requires less oxygen. A cooler frog can stay completely submerged for longer periods of time, only surfacing occasionally for a needed gulp of air. With additional cooling, even less oxygen is needed--to the point that all of the oxygen that it needs can be obtained by diffusion through the moist skin. There is little need for blood to go to the lungs, so now much of the blood is sent to the skin. This process, called cutaneous respiration, allows the frog to obtain all the oxygen it requires at low temperatures, and simultaneously remain submerged for extended periods of time. This phenomenon can be observed by looking at the light skin on the belly, which may become pink to reddish with the increase of blood flow. The frog above is in hibernation. The skin is reddish due to increased blood flow. Notice also that the arms are outstretched in the "four-point stance."
This same frog is coming out of hibernation and has begun to breathe again. Less blood is in the skin, so it is whiter. Also, the frog is more alert and is trying to turn over--notice the front legs are being stretched as the frog warms. A hibernating frog may look like it is dead. It can be placed on its back and show no inclination to turn over. However, after its body temperature increases, the frog will begin to respond to stimuli and eventually will right itself and jump away.