On Being the Right Size: Different Definitions of 'Right'

Abstract

Animal species exhibit a wide range of body sizes both within and across taxa. This range, and the distribution of body sizes within the range limits, have received considerable attention as indicators of pattern in the evolution and ecology of animal assemblages (Hemmingsen 1934, Hutchinson and MacArthur 1959, Lindsey 1966, May 1978, 1986, Calder 1984, Dial and Marzluff 1988, Brown and Nicoletto 1991, Gaston et al. 1993a, Brown et al. 1993, Fenchel 1993, Blackburn and Gaston 1994a, b, c, Brown 1995). Given that all species in a taxon tend not to be the same size, it has frequently been asked what, given the range of sizes encompassed, is the 'optimum' or 'best' body size for a species to be (Damuth 1981, 1993, Peters 1983, Brown and Maurer 1986, 1987, Griffiths 1992, Brown et al. 1993, Brown 1995, Marquet et al. 1995)? In this paper, we examine the principal answers that have been suggested, the assumptions that they make, and the extent to which they can be reconciled. In accordance with the bulk of the relevant published work, much of our discussion will be framed in the context of mammal assemblages. Our aim is to address whether the idea of an optimum body size for the species in a taxon such as mammals is a sensible concept. To achieve this, we must first consider the different proposals for what that optimum might be. We will not, for the most part, be concerned with the closely analogous question of what constitutes an optimum size for individuals within a species (for entries into this literature see e.g. Roff 1986).