Similarity of Mammalian Body Size across the Taxonomic Hierarchy and across Space and Time

Abstract

Although it is commonly assumed that closely related animals are similar in body size, the degree of similarity has not been examined across the taxonomic hierarchy. Moreover, little is known about the variation or consistency of body size patterns across geographic space or evolutionary time. Here, we draw from a data set of terrestrial, nonvolant mammals to quantify and compare patterns across the body size spectrum, the taxonomic hierarchy, continental space, and evolutionary time. We employ a variety of statistical techniques including "sib-sib" regression, phylogenetic autocorrelation, and nested ANOVA. We find an extremely high resemblance (heritability) of size among congeneric species for mammals over approximately 18 g; the result is consistent across the size spectrum. However, there is no significant relationship among the body sizes of congeneric species for mammals under approximately 18 g. We suspect that life-history and ecological parameters are so tightly constrained by allometry at diminutive size that animals can only adapt to novel ecological conditions by modifying body size. The overall distributions of size for each continental fauna and for the most diverse orders are quantitatively similar for North America, South America, and Africa, despite virtually no overlap in species composition. Differences in ordinal composition appear to account for quantitative differences between continents. For most mammalian orders, body size is highly conserved, although there is extensive overlap at all levels of the taxonomic hierarchy. The body size distribution for terrestrial mammals apparently was established early in the Tertiary, and it has remained remarkably constant over the past 50 Ma and across the major continents. Lineages have diversified in size to exploit environmental opportunities but only within limits set by allometric, ecological, and evolutionary constraints.

Niche conservatism above the species level

“…Clearly, there is a role for the number of species in the genus, because those genera that became extinct in North America at the terminal Pleistocene were predominately represented by single species (79%; ref. 29). Similarly, species richness within genera did not significantly influence the size or rank of generic geographic range.…”

“…Body sizes are taken from ref. 29. The list of genera and the number of species per genus is calculated from the North American mammalian database (52).…”

“…We recognize that in some kinds of animals and many kinds of plants, the grouping of ''genus'' and ''family'' may not reflect monophyletic groups; thus, we confine our analysis to mammals, in which these ranks typically do reflect monophyly and aggregate similar evolutionary distances (27)(28)(29). We focus on mammals, because there is little disagreement that most genera are monophyletic and do in fact reflect a real evolutionary grouping, although we of course recognize some may object to even using the term ''genus'' in view of current phylogenetic practice.…”

Can unified theories of biodiversity explain mammalian macroecological patterns?

“…allometric scaling relationships) [2,[77][78][79]. Much of the variation in mammalian life-history traits can be related to body size variation [80][81][82][83].…”

Body size distributions signal a regime shift in a lake ecosystem

“…Many nonexclusive hypotheses attempt to explain why these discontinuous patterns have emerged [4]. These hypotheses range from those that consider trophic and biotic interactions [5] that take place over relatively short timeframes, to energetic [6] and macroevolutionary mechanisms [7] that occur over much longer timescales. The textural discontinuity hypothesis [8] posits that body size aggregations are the product of organisms' interactions with the discontinuous distribution of resources and opportunities within their environment.…”