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

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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 O2 consumption [.ovrhdot.V(O2)] and evaporative water loss (.ovrhdot.mwe) at night at ambient temperatures (Ta) between 17 and 44.degree. C. Within the zone of thermal neutrality (30-39.degree. C) night-time Tb averaged 38.9.degree. C (range 37.6-41.1.degree. C). Above 30.degree. C, Tb increased with increasing Ta, reaching a maximum of 44.1.degree. C at a Ta of 43.8.degree. 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, .ovrhdot.V(O2) (in ml g-1 h-1) varied with Ta (.degree.C) in accordance with the equation .ovrhdot.V(O2) = 8.37-0.218Ta. The slope of the line relating .ovrhdot.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 .ovrhdot.mwe at Ta below 34.degree. C (mean 7.68 mg water g-1 h-1), but exhibit little capacity to increase the rate of evaporation at high Ta. Review of metabolic data for birds reveals that the standard metabolic rate of tropical species averages 67% of the level predicted from body mass. Adaptation to hot-humid climates appears to have involved a reduction in the level of metabolism.