Food intake balances energy intake, which is influenced by numerous parameters such as basal metabolic rate (Blundell et al., 2012), physical activity (Sallis and Hinckson, 2014), body weight, life duration (Speakman, 2005), and thermogenesis (Rosen and Spiegelman, 2006). Food intake also varies across individuals of a single species in an adaptive manner according to current needs and environmental conditions. The most important factor determining energy intake is body mass. Individuals that are fed ad libitum eat proportionally more when they weigh less and less when they weigh more (Prentice and Yamamoto, 1974). Energy requirements increase with body mass due to the greater surface area that must be maintained at a constant temperature for larger animals (McKay and Duman, 2002). Basal metabolic rate increases as well; thus, the amount of energy needed per day increases with body mass. Life expectancy influences energy intake because it determines how long an individual can go without eating before dying of starvation. Thermogenesis is the production of heat by any process other than food consumption or radiation exposure. In small mammals, especially rodents, thermogenesis accounts for nearly all of the energy consumed. Thermogenesis may be used as a defense mechanism by organisms that cannot flee from predators-such as birds-or by those that cannot fast for long periods of time-such as humans-to avoid falling into a weakened state.