Oecologia (2004) 138: 13–27 DOI 10.1007/s00442-003-1376-3 ECOPHYSIOLOGY Barry G. Lovegrove . Linda Haines The evolution of placental mammal body sizes: evolutionary history, form, and function Received: 12 May 2003 / Accepted: 13 August 2003 / Published online: 28 October 2003 # Springer-Verlag 2003 Abstract The unimodal, right-skewed distribution, most frequently identified in contemporary descriptions of placental mammal body size distributions, masks an underlying multidistribution structure; a long-term evolu- tionary process that has generated a concatenation of two or three frequency distributions specific to locomotory modes (plantigrade, digitigrade and unguligrade). The Afrotropical assemblages are bimodal, with a tendency towards trimodality, whereas the Nearctic assemblage is unimodal. However, mixtures of two and three normal distributions fitted the Nearctic data well, suggesting a multidistribution structure masked by disproportionate species numbers within locomotory modes. Differences in proportional species numbers within modes between assemblages may reflect the evolutionary history of form and function. However, common interassemblage predic- tions of such proportions in contemporary distributions may be disguised by the relative severity of the Pleistocene megafaunal extinction (patterns supported by the fossil record), geographical scale, and taxonomic composition. A species gap occurs at body sizes around 1 kg at the interface between the largest plantigrade mammals and the smallest digitigrade mammals, coin- cident with the minimum interspecific variance of basal metabolic rate. In terms of the evolution of the optimal body size in the trade-off between mortality and produc- tion, there may be good historical and evolutionary reasons why we should not expect optimization to produce the same results in different zoogeographical assemblages. Moreover, the evolution of diverse mammalian forms and functions, especially with respect to predator-prey inter- actions and diet, render a single body size optimum untenable in the search for an energetic definition of fitness. Keywords Mammals . Body size distribution . Predation . Running speed . Basal metabolic rate Introduction Understanding the evolution of body size is arguably the most essential prerequisite for understanding how mam- mals achieve fitness. The interaction of size dependence of mortality and production give rise to an optimal body size, and the frequency distribution is an emergent property of this process (Stearns 1992; Kozlowski and Weiner 1997). The shape of the frequency distribution of species numbers plotted as a function of log body mass usually shows a right-skewed, unimodal distribution (van Valen 1973; Brown and Maurer 1986; Blackburn and Gaston 1994; Loder et al. 1997; Kozlowski and Gawelczyk 2002) with a modal value of approximately 100 g (Brown et al. 1993). Although biologists have been intensely interested in identifying and quantifying the evolutionary forces involved for more than a century, definitive explanations for the distribution remain elusive (Kozlowski and Gawelczyk 2002). The distribution is the product of at least 100 million years of evolutionary change (Eizirik et al. 2001). Moreover, body sizes have increased gradually with time in a trend well known as Cope’ s Rule (Stanley 1973; Brown and Maurer 1986; Alroy 1998) and, presumably, the change is still taking place. Apart from descriptions of the shape of the distribution (Stanley 1973; Brown and Nicoletto 1991; Blackburn and Gaston 1994, 1998; Loder et al. 1997), much attention has focused on whether discontinuities or gaps exist in the distribution (Holling 1992; Manly 1996; Siemann and Brown 1999), and on the association between body size and energy conversion to offspring (Calow 1977; Brown et al. 1993; Blackburn and Gaston 1994; Marquet et al. 1995; Kozlowski 1996a, 1996b; Kozlowski and Gawelc- B. G. Lovegrove (*) School of Botany and Zoology, University of Natal, P/Bag X01, 3209 Scottsville, South Africa e-mail: lovegrove@nu.ac.za Tel.: +27-33-2605113 Fax: +27-33-2605105 L. Haines School of Mathematics, Statistics and Information Technology, University of Natal, P/Bag X01, 3209 Scottsville, South Africa