Internal temperature can be controlled by regulating the amount of radiation, conduction and convection near the physical interface between body and environment (the integument). These can be altered by varying the area, orientation with respect to the sun and wind, posture with respect to the ground, colour, texture, reflectivity and thermal resistance of the exposed surfaces, and the use of insulation (hair, feathers, and layers of superficial tissue). Note that the integument does not necessarily have to be at the same temperature as the core of the body. Varying the integument's temperature not only directly influences the rate of heat transfer between body and environment (Fourier's law of heat flow [13]), but also indirectly affects the amount of convection in the air surrounding the body [6, Chap. 33,](which is also dependent on posture).
The above applies to both the cooling and heating of the body; for cooling only, the evaporation of liquid (eg. water, saliva, sweat, or urine) from the body (skin, mouth, respiratory system) or the adjacent environment is extensively used by animals. `Forced air' cooling is also used by panting, fluttering of the mouth and throat [18], or fanning with wings.
An important strategy in temperature regulation is to control the flow of blood between parts of the body at different temperatures; a typical example is the restriction of bloodflow between cold extremities and the core. Some animals have a specialised arrangement of blood vessels to perform countercurrent heat exchange, allowing parts of the body to be at different temperatures even when there is a considerable bloodflow between them. Examples include maintenance of the core at a higher temperature than the gills, legs, or tail; allowing particular organs to be held at a different temperature to the core (eg. testes), or to have their temperature regulated more precisely than the rest of the body (eg. brain).
The analogies with thermal management in present-day electronics are strong.
The design procedure is typically as follows. Firstly, the designer calculates
the power consumption ( 
heat generation) of the silicon chip, which
is a flat thin slice of silicon of about 
area, stuck to the inside
of a cavity inside a larger plastic or ceramic package (which allows for the
mounting of the device on a circuit-board). For a particular packaging method,
a `thermal resistance from junction to ambient' is specified, where `junction'
refers to the transistors on the silicon. This allows the temperature of the
silicon to be calculated for a given ambient air temperature. If the silicon
would be too hot (the silicon being cold is not normally a problem), then
either a different type of package is used or the heat transfer between the
outside of the package (or case) and the ambient must be improved. The
latter is done by mounting a heatsink onto the case using a paste of
high thermal conductivity. The heatsink is usually a matt-black finned metal
structure of large surface area, giving a very low thermal resistance to the
ambience. Using the sum of the junction-to-case thermal resistance (also
specified for the package) and the heatsink-to-ambient resistance, the silicon
temperature can again be calculated, and a suitable heatsink selected.
Often forced-air cooling is used by mounting a fan on the heatsink or nearby to the package (sometimes there is just one fan blowing air through a large box of electronics), which effectively decreases the thermal resistance to the ambience [19]. A fluid other than air can also be used (water or freons; the latter are electrical insulators and can be allowed direct contact with the chip), and this fluid can be refrigerated rather than at ambient temperature: such methods are currently only used in rather exotic applications. Finally, it is possible to mount a Peltier-effect heat-pump [20] in the form of a wafer between the case and the heatsink. These electrically powered devices are used to pump heat from the case to the heatsink: the heatsink gets hotter with respect to the surrounding fluid, accelerating its heat-loss, and the case becomes cooler. Note that these devices can also operate in reverse to heat the case, and can be electronically controlled to maintain the silicon at a constant temperature: both are currently unusual.
The analogies with nature are obvious. The main difference is that silicon chips are normally just kept below a maximum temperature: we have seen that digital systems become slower with rising temperature. For conventional digital design methodologies, faster operation at low temperatures is not a problem. Another consideration is that the ageing mechanisms leading to device failure accelerate with temperature. Consequently, the amount of cooling is kept fixed at that necessary to guarantee a particular maximum temperature, and is not adaptive. A final difference is that evaporative cooling -- extremely effective in nature -- would be inconvenient (though theoretically possible) in electronic systems.