Diet, phylogeny, and basal metabolic rate in phyllostomid bats

Aside from the pervasive effects of body mass, much controversy exists as t o what factors account for interspecific variation in basal metabolic rates (BMR) of mammals; however, both diet and phylogeny have been strongly impl icated. We examined variation in BMR within the New World bat family Phyllo stomidae, which shows the largest diversity of food habits among mammalian families, including frugivorous, nectarivorous, insectivorous, carnivorous and blood-eating species. For 27 species, diet was taken from the literatur e and BMR was either measured on animals captured in Brazil or extracted fr om the Literature. Conventional (nonphylogenetic) analysis of covariance (A NCOVA), with body mass as the covariate, was first used to test the effects of diet on BMR. In this analysis, which assumes that all species evolved s imultaneously from a single ancestor (i.e., a "star" phylogeny), diet exert ed a strong effect on mass-independent BMR: nectarivorous bats showed highe r mass-independent BMR than other bats feeding on fruits, insects or blood. In phylogenetic ANCOVAs via Monte Carlo computer simulation, which assume that species are part of a branching hierarchical phylogeny, no statistical ly significant effect of diet on BMR was observed. Hence, results of the no nphylogenetic analysis were misleading because the critical values for test ing the effect of diet were underestimated. However, in this sample of bats , diet is perfectly confounded with phylogeny, because the four dietary cat egories represent four separate subclades, which greatly reduces statistica l power to detect a diet (= subclade) effect. But even if diet did appear t o exert an influence on BMR in this sample of bats, it would not be logical ly possible to separate this effect from the possibility that the dietary c ategories differ for some other reason (i.e., another synapomorphy of one o r more of the subclades). Examples such as this highlight the importance of considering phylogenetic relationships when designing new comparative stud ies, as well as when analyzing existing data sets. We also discuss some pos sible reasons why BMR may not coadapt with diet.