On the Utility of Uniformity in the Definition of Basal Rate of Metabolism

the intensity of similar or dissimilar components of the energy Energy expenditure is often used as a basis of ecological theory budget of different species or of the same species in different (e.g., Gadgil and Bossert 1970; Charnov 1976; Sibly and Calow environments. From this view, field measurements of energy 1986), and its study has been a means of examining the adjustexpenditure, as interesting as they are theoretically, are the ments made by vertebrates to the environments in which they most subject to misinterpretation because they are the least live (see, e.g., McNab 1994). The view that energetics is a clearly defined. central component of the natural history of vertebrates, and One of the most convenient (i.e., easiest) rates to measure especially of endotherms, given their high rates of metabolism, is the ‘‘standard’’ rate of metabolism, which in endotherms is is usually justified by the conclusion that energy and nutrient the ‘‘basal’’ rate. Basal rate was defined for humans and codified resources have a limited availability in most environments (but by DuBois (1924, 1930); later it was applied to domestic mamsee King and Murphy [1985]). Vertebrates, most notably endomals by Kleiber (1932, 1961) and Benedict (1938). It is the therms, are therefore often required to adjust expenditures to rate (1) in the zone of thermoneutrality when the individuals reflect resource availability. All endotherms do not make the are (2) inactive, (3) postabsorptive, (4) adult (thereby eliminatsame adjustments to accommodate a seasonally or a chronically ing the cost of growth), (5) nonreproductive (eliminating the limited resource base: some remain active and modify energy cost of pregnancy, lactation, egg formation, or incubation), expenditure, some migrate, and others enter torpor. One pracand (6) regulating body temperature, and it is (7) measured tical difficulty with the use of energy as a basis for ecological during the inactive period (also see Aschoff and Pohl 1970). theory, as well as with its use in the narrower objective of These criteria are most specifically discussed by Benedict analyzing the adaptation of individuals and species to the envi(1938). With the extension of the study of energetics to endoronment, is the parameter, or parameters, of energy expenditherms other than humans, some criteria demand special attenture that should be used. tion as the physiological diversity of species being studied inThe rate of energy expenditure, often measured in terms of creases. For example, fermentation in ruminants delays or oxygen consumption, is highly variable because it is influenced prohibits entrance into a postabsorptive state, which raises the by many factors. Each rate must be clearly defined by the question of whether they can be characterized by a basal rate. conditions under which it is measured; this is what separates Furthermore, most biologists use an estimate of basal rate that one rate from another. Rates that characterize vertebrates incorresponds to the ‘‘normally’’ regulated body temperature of clude the cost of body maintenance; the cost of various behavthe species being studied, which is not the same in all mammals iors, such as feeding, territorial defense, courting, and reproand birds; a correction factor in rate of metabolism for the duction; and the minimal and maximal rates of metabolism. differences in body temperature is properly not made, except Each of these rates measures the intensity of a component of that rates associated with torpid states are excluded. Consethe energy budget of vertebrates and therefore is a rate of quently, a mammal or bird can be said to have a basal rate justifiable concern. only if it regulates body temperature by the internal generation The sensitivity of energy expenditure to internal and external of heat, that is, if it is an endothermic homeotherm. Nearly factors is both its strength and its weakness: its strength because the complete array of endotherms is now potentially available it is interactive with most aspects of the natural history of for study, which increasingly means that biologists will examine