Figure 9.

Summary diagram of energy balance. At homeostasis, energy expenditures equal the incoming calories contained in food, plus short-term mobilization of stored calories. Expenditures include the basal metabolic rate (BMR), together with anabolism (including cell growth of renewing lineages and deposition of glycogen and fat), and “on-demand” processes such as neural function, motor activity, immune defenses and digestion. All these processes, except anabolism, generate heat (indicated by red boxes for heat generation, blue for endothermy). If this does not maintain body temperature at sub-thermoneutral temperatures, (non-shivering) thermogenesis is activated. The rate of heat generation required depends upon the factors indicated, the ambient temperature (including wind speed and humidity), the body temperature set-point, and the rate of heat loss, determined by animal behavior, and the properties of their body coverings, including the skin. The illustration of skin shows the biophysical processes that govern heat loss (Q), taken from Figure 2, a combination of evaporation, convection and radiation.