Nocturnal Heterothermy and Torpor in the Malachite Sunbird (Nectarinia Famosa)

Abstract

Abstract Heterothermy in birds occurs in species that are generally small and whose diet fluctuates. This study of thermoregulation of Malachite Sunbirds (Nectarina famosa) showed that they have circadian fluctuations in Tb and VO2, as in most birds. Of special importance was the high degree of rest-phase hypothermy and exhibition of torpor at ambient temperatures of 10°C and lower. These patterns are significant because they have not been described in detail for a passerine species. Surgically implanted minimitters were used to measure Tb continuously and without disturbing the birds. Minimum VO2 during rest phase was 1.70 (mL O2 g–1 h–1) at 25°C. As ambient temperature decreased, VO2 minimum during the rest phase did not increase to maintain Tb. No birds remained normothermic during scotophase. At 5°C, that resulted in torpor in Malachite Sunbirds with a decrease of 15°C in Tb. Birds increased Tb to active-phase levels with the onset of light. It was difficult to define limits and ranges of physiological parameters associated with observed heterothermy. Individuals showed similar responses; however, those differed with ambient temperature. Malachite Sunbirds conserved energy nocturnally by reducing metabolic rate and, concomitantly, Tb. This plasticity in Tb shows that daily variations in Tb of homeotherms are biologically important. Furthermore, this heterothermy (particularly nocturnal hypothermia and torpor) in a small avian species would be important in an unpredictable environment where food resources fluctuate to prevent an energy deficit.