MAXIMUM METABOLISM AND THE AEROBIC FACTORIAL SCOPE OF ENDOTHERMS

Minimum and maximum metabolism in response to cold were measured in 30 species of Australian monotremes, marsupials, eutherians and birds. I n marsupials and the echidna, maximum metabolism was also determined d uring treadmill locomotion. These data were used to determine, for the first time, the relationships between maximum metabolism and body mas s in the four endothermic groups and to compare aerobic factorial scop es (the ratio of maximum to minimum metabolism) elicited by cold and l ocomotion. The effect of body mass on maximum metabolism is the same i n marsupials and eutherians (the therians) but is significantly less i n birds. At the same body mass, there is no difference between the two therian groups for either minimum or maximum metabolism induced by ei ther cold or locomotion. Aerobic scope during cold is significantly hi gher in marsupials (8.3) than in eutherians (5.1), birds (5.4) and mon otremes (5.4). Aerobic scope during locomotion in all groups is almost twice that observed in cold conditions.