In stabilising internal temperature, many animals (eg. insects, fish, birds, mammals) use the heat generated by all metabolizing tissues ( endothermy). In many cases, this can be adaptively controlled (perhaps via the neuroendocrine system), not only by shivering or otherwise activating muscles, but also non-shivering thermogenesis. Some mammals even have a specialised thermogenic tissue (brown fat) for producing bursts of heat in response to cold stress: it serves no other function. `The tissue is strategically localized in the neck and thoracic regions in relation to major blood vessels so that its heat is quickly transported to those organs (brain and heart) whose continuous high temperatures are vital...' [1].
The heat generated in the brain is a significant component of non-shivering thermogenesis, and a major part of this originates from neural activity; mostly arising from the metabolic processes needed to run the sodium-pump to restore ion concentrations after firing [7, Chap. 5,]. There is a strong analogy with electronics here. Although there are research projects on reversible computing (using almost zero energy, thus generating almost no heat), current technologies are far less efficient than this. If used to perform digital operations, the CMOS FPGA chips used in evolutionary electronics generate heat at a rate proportional to the speed of logic switching (there is negligible heat generation when there is no activity in the circuit). This heat must be dissipated, as seen in the previous section.
In the type of unconstrained evolutionary electronics which motivates this paper, the circuit instantiated on the FPGA is a continuous-time dynamical analogue system, and is probably not `doing' logic. Nevertheless, it is still true that the rate of heat generation rises with increasing frequency of activity in the circuit. This raises an interesting possibility -- could a circuit evolve such that the heat produced by its activity maintains it at a preferred temperature? The circuit could potentially even adapt its levels of activity in response to a change in ambient temperature.
It turns out that there is not a known biological precedent for this:
in contrast to an increase in tissue respiration of liver, heart, and skeletal
muscle, the 
consumption of brain tissue remains constant during
the development of non-shivering thermogenesis [6, Chap. 5,]. Of
course, the lack of a biological counterpart need not necessarily discount it
in engineering systems. If there existed parts of the circuit (possibly
distributed throughout a large system) which played no other role than to be
highly active and generate heat to maintain vital subcircuits at a preferred
temperature, then these would be analogous to brown fat in mammals. This is
not an idle speculation, but a serious proposal: such thermogenic circuits
could be built-in by hand, could be encouraged to evolve, or could be searched
for in the analysis of a thermally stable evolved circuit.