Energy scaling in marsupials and eutherians.

Marsupials have been shown to have basal metabolic rates below those of eutherian mammals. Now metabolic rates below thermoneutrality are found to be equivalent in both taxa. Two models are proposed to explain the observed metabolic patterns: in one, marsupials differ only in having reduced basal metabolic rates; in the other, the reduced marsupial basal metabolic rates combined with a reduced body temperature and elevated conductance. The metabolic costs of existence below thermoneutrality appear to be similar for both taxa. The difference in basal metabolic rate may be of energetic significance or merely a phylogenetic coincidence.