EVOLUTION OF BROWN FAT - ITS ABSENCE IN MARSUPIALS AND MONOTREMES

Species from all extant families of marsupials and monotremes were exa mined to clarify whether these mammalian subclasses possess brown adip ose tissue. To optimize the chance of finding this tissue. special emp hasis was given to sampling species adapted to colder regions, species with small adult body size, and individuals at a stage of development equivalent to the newborn stage of placentals (late pouch life in the case of marsupials). Evidence based on gross morphology and light, el ectron. and fluorescence microscopy failed to show the presence of bro wn adipose tissue in any marsupial or monotreme. All adipose tissue wa s typical white fat, including special instances where multilocularity of lipid droplets occurred in association with white adipocyte develo pment or with fat mobilization resulting from nutritional or cold stre ss. These results, combined with lack of positive identification of br own adipose tissue in birds or other vertebrates, indicate that brown adipose tissue is unique to eutherian (placental) mammals and probably evolved early in the radiation of this subclass. This uniqueness pres ents the opportunity to suggest a more satisfactory name for the subcl ass: Thermolipia (from the Greek for "warm fat") or, commonly, thermol ipials.