Why are species’ body size distributions usually skewed to the right?

Abstract

Summary1. Species' body size distributions are right-skewed, symmetric or left-skewed, but right-skewness strongly prevails. 2. Skewness changes with taxonomic level, with a tendency to high right-skewness in classes and diverse skewness in orders within a class. Where the number of lower taxa allows for analysis, skewness coefficients have normal distributions, with the majority of taxa being right-skewed. 3. Skewness changes with geographical scale. For a broad range, distributions in a class are usually right-skewed. For a narrower scale, distributions remain right-skewed or become symmetric or even close to uniform. 4. The prevailing right-skewness of species' body size distributions is explained with macroevolutionary models, the fractal character of the environment, or body size optimization. 5. Macroevolutionary models assume either size-biased speciation and extinction, or the existence of a constraint on small size. Macroevolutionary mechanisms seem insufficient to explain the pattern of species' body size distributions, but they may operate together with other mechanisms. 6. Optimization models assume that directional and then stabilizing selection works after speciation events. There are two kinds of optimization approaches to study species' body size distributions. Under the first approach, it is assumed that a single energetic optimum exists for an entire taxon, and that species are distributed around this optimum. Under the second approach, each species has a separate optimum, and the species' body size distribution reflects the distribution of optimal values. 7. Because not only energetic properties but also mortality are important in determining optimal sizes, only the second approach, that is, seeking the distribution of optimal values, seems appropriate in the context of life-history evolution. This approach predicts diverse shapes of body size distributions, with right-skewness prevailing.

Body size inequality of carabids along an urbanisation gradient

“…Skewness is the only method used to evaluate inequality and/or asymmetry in size distribution of animal populations or assemblages (Gomez & Espadaler, 2000;Gregory, 2000;Knouft, 2004;Kozlowski & Gawelczyk, 2002;Novotny & Kindlmann, 1996;Poulin & Morand, 1997). In the present paper, we extended the range of methods applied to analyse the inequality of animal body size distribution using two other parameters (the Gini coefficient and the Lorenz asymmetry coefficient) and showed that the Lorenz asymmetry coefficient, S, is a powerful method for studies describing and interpreting variations in body size.…”

Scaling of metabolism inHelix aspersasnails: changes through ontogeny and response to selection for increased size

“…We stress that future studies should reconcile the cellular and physiological (1-3) mechanisms associated with growth rate evolution, and should investigate their role in the origin of metabolic scaling variability on different levels of biological organization. Successful integration of these phenomena promises evolutionary explanations of different large-scale phenomena such as Bergmann's rule in ectotherms or patterns in interspecific body size distributions and life histories (Kozlowski and Weiner, 1997;Kindlemann et al, 1999;Kozlowski and Gawelczyk, 2002;Angilletta and Dunham, 2003;Kozlowski et al, 2003;Kozlowski et al, 2004).…”

“…Metabolic scaling is expected to affect optimal resource allocation to growth and reproduction, so it should substantially influence the life history of organisms (Kozlowski and Teriokhin, 1999;Czarnolę ski et al, 2003). Emerging evidence suggests that adaptive allocation responses to shifts in metabolic scaling can explain different ecological and evolutionary phenomena, such as the so-called 'temperature-size rule' in ectotherms (slower growth and larger final body size in colder environments) (Angilletta and Dunham, 2003;Kozlowski et al, 2004), or the interspecific patterns of body size distributions and life history allometries (Kozlowski and Weiner, 1997;Kindlemann et al, 1999;Kozlowski and Gawelczyk, 2002). In this work we examine the link between size-scaling of metabolism and growth rate in Helix aspersa snails.…”

Evidence for soft bounds in Ubuntu package sizes and mammalian body masses

“…Consequently, different initial conditions can produce markedly different distributions. If the initial mass is sufficiently large, left-skewed distributions can be obtained; such a shape is less common but is nonetheless found in some taxa (32). Note, however, that the fitting distribution for mammals is very nearly stationary (SI Appendix, Fig.…”