Mass-specific and whole-animal metabolism are not the same concept.

Abstract

Biologists have invested a great deal of effort into studying energy metabolism. Consequently, it is important to analyze energy metabolism data correctly. In his recent technical comment, McNab (1999) discussed the relative merits of analyzing wholeanimal (i.e., total) versus mass-specific energy metabolism. He concludes that (1) “nothing wrong is being perpetuated by the use of mass-specific rates of metabolism” and (2) that “any analysis based on mass-specific rates must also apply to total rates because they are two sides of the same coin” (McNab 1999, p. 644). Unfortunately, these conclusions are incorrect. This comment demonstrates why and makes two main points. First, while there is a necessary relationship between the allometric scaling exponents for whole-animal and mass-specific metabolism, the two variables do not necessarily measure the same biological concept (i.e., they are not necessarily two sides of the same coin). Second, a correlation between whole-animal metabolism and another variable typically places relatively little constraint on the correlation of mass-specific metabolism with that variable. Physiological ecologists should follow the advice given in Packard and Boardman’s (1988) seminal paper and avoid analyzing massspecific variables whenever possible. McNab (1999) asserted that mass-specific and whole-animal metabolism are two sides of the same coin and that any explanation that applies to one should apply to the other. Indeed, although it is not correct, it is easy to see how one might conclude that any explanation that applies to one measure of metabolism should apply to the other. As McNab (1999) recognized, there is a necessary, and easily definable, relationship between the allometric scaling exponents for whole-animal metabolism and mass-specific metabolism. The scaling exponent for mass-specific metabolism equals the scaling exponent for whole-animal metabolism minus one. For example, if whole-