Use and pitfalls of allometry: a valuable tool in comparisons and extrapolations between species and in ethical considerations concerning the use of one species to model another.

Where research on one species is justified on the grounds that it will provide benefits to another, the strength of the ethical case depends critically on whether findings can be extrapolated meaningfully. Valid extrapolation depends on the species being sufficiently similar in respects critical to the research and on knowledge of the bases and effects of salient differences. Many biological parameters vary with body weight (W) between species. When species of small size are used to model larger ones, the influence of size on the rates of physiological, immunological and other processes must be taken into account. Between species, the rates of physiological processes tend to increase with W(0.75), and the durations of physiological events tend to increase with W(0.25). Providing potential pitfalls are understood, allometric scaling enables valid comparisons and extrapolations between species. Knowledge of these principles is crucial also in making predictions about many aspects of animals' biology and has application also in making sound ethical judgments about the justifiability of extrapolations between species concerning the wide range of processes linked to rates of metabolism.