The comparative energetics of neotropical marsupials

Abstract

1) The rates of metabolism and body temperatures of neotropical marsupials were measured and compared to those of Australian species. 2) Neotropical marsupials have somewhat higher basal rates of metabolism and higher thermal conductances than Australian species. The higher basal rates of neotropical species are correlated with food habits that exclude folivore specialists and to some extent with life in a mesic climate. The high conductances of neotropical marsupials relate to high, stable environmental temperatures. 3) Many small marsupials, even in a tropical rain-forest, enter torpor, store fat in their tails, and maintain unusually small temperature differentials, all apparently in response to the high cost of thermoregulation at small masses. 4) Mammals with low basal rates can compensate for a small size by decreasing thermal conductance, thereby maintaining a larger temperature differential with the environment than would otherwise be expected. 5) An appreciable difference in basal rate of metabolism exists between marsupials and placentals in terrestrial carnivores and grazer-browsers, but no such difference is present in arboreal folivores, arboreal frugivore-omnivores, terrestrial omnivore-insectivores, and ant-and termite-eaters. 6) High basal rates of placentals may insure as short a gestation period as is compatible with extended intrauterine development. 7) It is suggested that marsupials have a form of reproduction that may be energetically more expensive than that found in placentals, which may require marsupials to maintain a low basal rate of metabolism.