Temperature regulation in the Dusky Munia, Lonchura fuscans (Cassin) (Estrildidae)

Abstract

Dusky munias are small (9.5 g) tropical seed-eating members of the family Estrildidae. Adaptations of munias to hot-humid conditions were examined by measuring body temperatures (Tb), rates of oxygen consumption [V(O2)] and evaporative water loss (Mwe) at night at ambient temperatures (Ta) between 17 and 44�C. Within the zone of thermal neutrality (30-39�C) night-time Tb averaged 38.9�C (range 37.6-41.l�C). Above 30�C, Tb increased with increasing To, reaching a maximum of 44.1�C at a Ta of 43.8�C. For the most part, dusky munias were unable to maintain Tb lower than Ta at high temperatures. The mean standard metabolic rate of munias was 1.87 ml O2 g-1 h-1. This is only 51% of the value predicted for passerine birds of comparable size. Below thermal neutrality, V(O2) (in ml g-l h-1) varied with Ta (�C) in accordance with the equation V(O2) = 8.37-0.218Ta. The slope of the line relating V(O2) to Ta below thermal neutrality is 19% lower than predicted, indicating that munias are better insulated than other birds of comparable size. Dusky munias have relatively high rates of Mwe at Ta's below 34�C (mean 7.68 mg water g-1 h-1), but exhibit little capacity to increase the rate of evaporation at high Ta's. Review of metabolic data for birds reveals that the standard metabolic rate of tropical species averages 67% of the level predicted from body mass. Thus, adaptation to hot-humid climates appears to have involved a reduction in the level of metabolism.