Different species use different combinations of the temperature-altering mechanisms studied above, and according to different adaptive strategies, to maintain a relatively constant internal temperature. Species are often categorised as hot-blooded (homeotherms), cold-blooded (poikilotherms), endotherms, ectotherms, regulators, compensators, or conformers. These classifications of species are sometimes used inconsistently in the literature, and in ways potentially misleading for this paper, so I have avoided them until now. `Homeotherm' means mammals and birds. The others -- cold-blooded animals or poikilotherms -- differ from homeotherms in lacking the central autonomic thermal controls (the hypothalamus in mammals, and the spinal cord in birds), the continuously high body temperatures, and the emphasis on thermoregulation as a balance between metabolic heat and insulation (in the form of feathers or fur). However, poikilotherms can use many of the mechanisms detailed above to maintain a remarkably constant body temperature in their natural environments [21, 22].
Both homeotherms and poikilotherms have biological stabilisation in the face of changing external temperature by homeostatic feed-back control mechanisms: in homeotherms it is the temperatures of particular parts of the body which are the controlled variables, whereas in poikilotherms the controlled variable might be, for example, metabolic rate [6, Chap. 5,]. This does not necessarily imply a reduced involvement of the nervous system in the thermal regulation of poikilotherms. For example, adaptive responses in the thermal resistance of tissue in several poikilotherms have been found to be regulated by photoperiod (not by heat), indicating involvement of photoreceptors and the neuroendocrine system even in this compensatory adaptation [1].
The hypothalamic thermal control in mammals is reflexly activated by thermoreceptors of skin and mucous membranes, and by temperature change in the hypothalamus itself, or the blood circulating through it. Efferent nerves control muscles (eg. for panting or shivering), and, via other organs, the cutaneous blood vessels, the sweat glands, and the piloerector muscles (for raising hairs); the response via the endocrine system can also be on a much longer timescale, as we have seen. This does not mean that these things are solely under hypothalamic control, for example cutaneous vessels are also directly influenced by temperature.
Hypothalamic control, then, can be viewed as a discontinuous nonlinear negative-feedback control system, with a limited range of operation, aiming to maintain the temperature of various parts of the body at their particular set-points (which can be altered, eg. during exercise or by fever). Note that not all observations are easily explained from this viewpoint. See [23] for details of various control-theoretic models. They model response close to the set-points: further away, less-finely controlled emergency responses (such as massive shivering independent of skin temperature) come into play.
Mammals (derived from ancient poikilothermic reptiles) are often considered the most highly developed animals, breaking shackles of innate habits, and being able to adapt behaviour to changing circumstances. They are aided in both by the constancy of their internal environments [24]. Hoar notes that `... there is a good general correlation between the precision of temperature control and the complexity of behavioural organisation.' [1] There is a graded series from the poikilothermic reptiles through the primitive mammals (monotremes -- platypuses and spiny ant-eaters -- and marsupials) to the Eutheria (placental mammals). `Increasing complexity of organization (especially behavioural organization) makes homeothermy a necessity; or conversely, one may argue that complexity, both physiological and behavioural, becomes progressively more feasible as the internal environmental temperature, especially that of the nervous system, is stabilized.' [1]
These observations are tremendously suggestive for evolutionary electronics. Should we accept that the inclusion of a thermostatic control system is the price to be paid for an efficient complex `electronic nervous system' able to operate in a wide range of ambient temperatures? For thermostatic regulation is indeed costly: in nature we even see animals prepared to give up activity altogether in order to abandon homeothermy when it becomes too costly to maintain (eg. hibernation). The cost of regulation depends on the degree to which conformity with the ambient temperature must be resisted: various trade-offs are found even within mammals [25]. In electronics, there would be the cost of the thermal sensors, the feedback controller, the actuators, and the energy required by these. Such a system is quite simple to build, however, and the components are only expensive in relation to the incredibly low cost of electronics currently achieved through economies of scale.
Thermal sensing might be done by measuring time-delays on the silicon (see §3), measuring the current through a slightly forward-biased electrostatic-discharge (ESD) protection diode (present around the edges of the silicon), or by mounting a thermocouple on the outside of the case. Use of both internal and surface thermosensors can enable more sophisticated control strategies [26]. A simple feedback controller is easy to construct using conventional electronic design principles, and a suitable actuator would be a Peltier-effect heat-pump used as described in §5.1.2 (or even the thermogenic subcircuits proposed in §5.1.3). All of this could be provided without having to try to evolve it, once one was convinced that thermostatic control was appropriate.
The energy consumption of thermostatic regulation could be prohibitive in many electronics applications. It is conceivable that in some situations the circuit could go into dormancy during periods of extreme ambient temperature when the costs of cooling/heating would outweigh the benefits of continued operation. As in animals, some minimal capabilities would have to remain, to `wake up' in an emergency or when conditions are more comfortable, and to maintain functions that are indispensable (the equivalent of cardiac and respiratory rhythms in animals). It seems ludicrous to think of a hibernating circuit; however, some small-bodied animals (which can rapidly change body temperature) abandon homeothermy while asleep at night, and resume it during daytime. It is easy to think of applications where a piece of electronics could be allowed to do the same. Note that this idea can apply to both hot and cold extremes (in nature the summer equivalent of hibernation is estivation, and less profound forms of torpor also exist). Shutting-down of electronic sub-systems when they are not required is already widely used when power consumption is crucial, and indeed dormancy is also used by animals when food or water is scarce.
In an electronic system made of many parts, thermoregulation of the whole could emerge from the individual actions of the parts. There is a biological precedent for this: `Colony activity resulting in some temperature control is common at least among some insects: avoidance and preference, shivering and quieting, fasting and feeding, varying metabolic water production, clustering and dispersal of swarms, nest placement and structure, opening and closing nest entrances, fetching water into the nest and fanning to ventilate and evaporatively cool it. Appropriate ones of these mechanisms are effective in maintaining quite narrow ranges.' [6, Chap. 37,].