The evolution of energetics in eutherian insectivorans: an alternate approach

An analysis of standard energetics in 57 species of insectivorans, small eutherians that preferentially feed on soil invertebrates, indicated that a combination of climate, the use of torpor, substrate, food habits, and log(10) body mass accounted for 92.5% of the variation in log(10) basal rate of metabolism in insectivorans, whereas log(10) body mass alone accounted for 76.7% of the variation. With the addition of subfamily affiliation, this analysis accounted for 95.5% of the variation in log(10) basal rate, the most distinctive subfamilies being Soricinae and the Talpinae, which have equally high basal rates. Sorex species have basal rates that average 2.5 times those of tropical crocidurines, reflecting an approach by Sorex to life in cold climates that does not include the use of torpor, a stratagem widely used by crocidurines in warm-temperate and tropical climates. The absence of torpor in Sorex may facilitate a high reproductive rate through a high basal rate of metabolism, a combination that may be incompatible with a small mass, insectivorous food habits, and life in the lowland tropics, but required in cold-temperate environments. Insectivorans other than shrews, moles, and cold-temperature hedgehogs have low basal rates principally in association with tropical distributions and the use of torpor. Basal rate of metabolism in insectivorans also correlated with ordinal, familial, subfamilial, and tribal affiliations. The suggestion that phylogeny is an important determinant of performance characters like rate of metabolism ignores the requirement that performance must be compatible with conditions in the environment and with a species' other characteristics. The principal reason why performance characters are correlated with taxonomic affiliation is that many clades consist of species that share a common approach to the environment. Thus, clades not only represent evolutionary histories, they also are crude measures of physiological and behavioral performances.