Life on Earth started with the formation of small organisms and, in general, larger ones evolved from smaller ones. Today, the size range of adult mammals spans eight orders of magnitude, from 1.5 to 150,000,000 g. Although all mammals have a common design principle, they are by no means geometrically similar, either in form or in function. If an elephant had the shape of a blown up shrew, its legs would not be able to carry the weight of its body. If the energy turnover rates of the elephant and shrew were directly proportional to differences in body mass, the elephant's body temperature would be at boiling point. The field of biology dealing with the scaling of the structural and functional properties of organisms is called allometry. Allometric tools are used not only to phenomenologically describe the body size dependence of parameters but also to investigate the underlying scaling laws. One of the most important parameters governing life is energy turnover or metabolic rate, usually measured as oxygen(O2) consumption. For terrestrial mammals, the best fit of the relationship between measured basal metabolic rate (BMR) and body mass (M) is the power function BMR = 3 M0.7 (BMR in watts, M in kg) (1). On a double logarithmic scale, this function is a straight line with a slope of 0.7 (Figure 13.1). In addition to studying the biological basis for scaling laws, it is also of interest to investigate why some species deviate significantly from the allometric mean. When considering extremely small endotherms, the following questions arise: What are the structural and functional properties that constitute the lower end of the mammalian size range, and why do no smaller mammals exist?