Energy Metabolism and Nocturnal Hypothermia in Two Tropical Passerine Frugivores, Manacus vitellinus and Pipra mentalis

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Abstract
Oxygen consumption ( ) and body temperature ( ) were measured in Manacus vitellinus (mean mass, 15.5 g) and Pipra mentalis (mean mass, 12.3 g) on Barro Colorado Island, Panama. The two species had the same mean euthermic nocturnal (37.9 C). During activity sometimes reached 43 C. In both species, nocturnal basal metabolic rate (BMR) (41.69 cm³ O₂/h in M. vitellinus and 34.9 cm³ O₂/h in P. mentalis) was significantly less than predicted on the basis of mass. At night fasted birds frequently, and unfasted birds occasionally, became hypothermic, with ranging between 27 and 36 C. Their always remained several degrees above ambient temperature ( ). The lowest recorded was 26.8 C at a of 14.6 C. Thermal conductance was the same in euthermic and hypothermic birds. Manakins save substantial amounts of energy by their nocturnal hypothermia. During a 12-h night a 14-g M. vitellinus maintaining a of 27 C in a of 22 C for 10 h would expend 6.4 kJ less than if it maintained its at the mean euthermic nocturnal level. This represents a savings of 58%. The selective pressures that have favored nocturnal hypothermia in manakins have probably operated on other small tropical frugivorous birds. We predict that the main components of the metabolic pattern of manakins will also be found in other small tropical passerines with similar food habits.