Birds and mammals have a unique characteristic in the animal world: They are the only organisms able to maintain a constant internal body temperature regardless of external temperatures, which is why they are called homoeothermic or endothermic organisms. The internal body is taken to include those areas of the torso and head containing all the vital internal organs (intestines, central nervous system, and brain) that also generate heat when the organism is at rest. The body core temperature is generally more or less constant while the temperature of the body shell, including the skin and extremities, fluctuates more strongly.
The body core temperature of humans is 37 degrees Celsius. Hedgehogs have a core temperature of 35 degrees Celsius and swallows of 44 degrees Celsius. The body core temperature of carnivores, horses and humans fluctuates by one to two degrees Celsius throughout the day dependent on activity. An increase of more than six degrees Celsius is life-threatening to most homoeothermic organisms and generally results in death by hyperthermia. In contrast, death by hypothermia occurs when an organism’s body temperature decreases and its body core temperature falls below a species-specific level. Humans for example are in a critical condition if the temperature of their blood falls below 27 degrees Celsius.
Fish, amphibians or reptiles are not greatly impacted by a slight drop in body temperature. They are among the poikilothermic organisms or ectotherms. These terms are used for all organisms whose body temperature is fully dependent on the temperature of their environment and is generally not influenced by the animals’ metabolism. Poikilothermic animals have developed characteristic behaviours which allow them to regulate their own body temperature. Salamanders for example bask in the morning sunshine in order to reach “operational temperature” while many flying insects warm up by means of rapid contractions of their wing muscles.