Energetic Cost of Locomotion of Some "Primitive" Mammals

Abstract

Most mammals possess much higher metabolic rates than reptiles, but three species of insectivores have been found to possess lower reptile-like rates. It has been proposed that the earliest mammals possessed similar low metabolic rates that were retained by these insectivores during the course of mammalian evolution. The purpose of this study was to investigate the energetics of additional insectivores and some other mammals that might also have been expected to retain low reptile-like energetics. We have measured the metabolism of running rather than resting animals to avoid much of the variability inherent in resting measurements. We have found that four species of "primitive" mammals, including armadillos, musk shrews, elephant shrews, and short-tailed opossums, all possess energetics that resemble typical mammals rather than reptiles. We conclude that the low metabolic rate observed in the three species of insectivores was probably a specialization for conserving energy and was not retained by these animals during the course of evolution. It is important, however, to note that the fact that these animals can maintain a constant body temperature in a niche similar to that occupied by the first mammals means that it would have been possible for the first mammals to do this as well.