Birds and mammals ere endothermic, which means they regulate their own body temperature internally. Most other animals are ectothermic, so they depend on the environment to heat or cool them. Some have developed adaptations to avoid freezing; however, most ectotherms rely on behavioural thermoregulation of body temperature.
Thermoregulation can be accomplished by posture, orientation and microclimate selection. When a reptile needs to warm its body, it will spread out on top of a rock that faces the sun. Many desert organisms are diurnal. They are only active early in the morning and in the evening after sunset, avoiding the warmest hours of the day. Tropical fish live in warm water and never venture into cold water.
Freezing point depression
Polar arthropods are ectotherms that avoid freezing by concentrating solutes such as sugars or sugar alcohols to decrease their own freezing point by as much as 10 degrees C (50F). Many oceanic fish use glycoproteins as a kind of anti-freeze, allowing their body temperature to decrease below freezing point. Still other animals use supercooling.
Supercooling occurs when an organism's fluids remain liquid in temperature below freezing. Antarctic icefish keep their bodily fluids liquid at temperatures significantly below freezing. In Labrador, fish that live in the fjords supercool their bodily fluids. Frogs use supercooling and spiders are capable of supercooling, lowering their body temperatures to -11 degrees C (12F).
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