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 44ºC. Within the zone of thermal neutrality (30-39ºC) night-time Tb averaged 38.9ºC (range 37.6-41.lºC). Above 30ºC, Tb increased with increasing To, reaching a maximum of 44.1ºC at a Ta of 43.8ºC. 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 (ºC) 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 34ºC (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.