Basal metabolic, equivalent to the idling speed of mammals (measured during rest in non-reproductive, mature, fasted animals) is highly variable in mammals. Based on the hypotheses put forward by the Metabolic Theory of Ecology higher basal metabolic rates are expected to correlate with higher reproductive outputs. Yet in mammals this has consistently been shown to false. I argue that the level and degree of body temperature regulation (which also varies considerably among mammal species) as well as environmental temperatures, and the difference between the two, are key to understanding the lack of relationship between metabolic rates and reproductive outputs. Tropical and subtropical small mammals, for example, routinely experience temperatures above the lower critical limit of the thermoneutral zone and often have highly variable body temperatures. Therefore, unlike temperate species that must consistently generate heat to maintain an elevated body temperature, low latitude species spend more time at thermoneutrality and therefore can spend the energy elsewhere. I present an analyses of basal metabolic rates, body temperatures, thermolability, and life history traits in mammals and propose key areas in need of future research.