By definition, homeothermic species maintain their core temperature (Tc) regulated within narrow limits. In most of these species, the hypothalamic thermostat is set somewhere between 36 and 38 C, and Tc is tightly kept there, in spite of highly variable ambient temperatures. Several mechanisms to maintain body temperature have been selected during evolution. Vasoconstriction and piloerection are mechanisms to save heat, while vasodilatation, sweating and perspiration are mechanisms to rapidly dissipate heat. To maintain temperature in environments usually colder than Tc, homeothermic species need to produce more heat than poikilothermic species. Energy transformations generate heat simply by virtue of the laws of thermodynamics and so the energy transformations inherent to life generate heat. Such heat, generated as byproduct of cell vital functions, is called obligatory thermogenesis (OT) and is expectedly higher in homeothermic species, as a consequence of which metabolic rate is substantially higher in these species than in poikilothermic species (Else and Hulbert, 1981). In addition to having a higher OT, homeothermic species can also activate specific mechanisms to produce extra heat in cold environments, which is called facultative thermogenesis (Gordon, 1993). Muscle shivering is the most immediate form of facultative thermogenesis, but it is energy consuming and disruptive for activity, and is rapidly replaced by metabolic heat production or non-shivering facultative thermogenesis (for simplicity we will call the former ‘‘shivering’’and the latter ‘‘facultative thermogenesis’’, FT). There is an ambient temperature range, called thermoneutral zone, where OT is sufficient per se to maintain body temperature, without the need of FT or heat saving or heat dissipating mechanisms (Gordon, 1993). Such ambient temperature zone is quite narrow, but empirically it is possible to define a lower and an upper boundary (Gordon, 1993). In this discussion, I will talk rather of thermoneutrality temperature, TN, which corresponds to the low end of the thermoneutral zone and is defined as the minimal ambient temperature where OT can alone maintain body temperature.