Fundamental laws, such as gravity and thermodynamics, are in common use in the physical sciences but are less applicable to biology. However, although there are fewer biological laws relative to other disciplines, the ones that exist are of great importance, e.g. natural selection. Another example may be a new law of biological network design which explains observations on how metabolic rate changes as organisms change size. It has been known for some time that as animal size increases its metabolic rate falls ( Rubner 1883; Kleiber 1932; Adolph 1949; Schmidt-Nielsen 1984; Calabrese 1991). This has developed from a physiological curiosity to a tool that is used extensively to predict drug effects across species ( Lave et al. 1999 ). The function of this article is to review the development of this concept, and to examine its application in veterinary anaesthesia.
