ENERGETICS, THERMOREGULATION AND NOCTURNAL HYPOTHERMIA IN AUSTRALIAN SILVEREYES

We investigated whether the Silvereye, Zosterops lateralis (10-12 g) u ses heterothermy to reduce energy expenditure when confronted with adv erse environmental conditions. We also determined the thermal physiolo gy and energetics of this species, Z. lateralis entered nocturnal hypo thermia over a range of ambient temperatures (T-a) from 3-26 degrees C . Below the thermoneutral zone (TNZ), metabolic rate (MR) decreased by up to 50% and this reduction was more pronounced at high T-a than at low T-a. The reduction of MR at night was accompanied by a reduction o f body temperature (T-b) from 40.3 +/- 0.5 to 36.9 +/- 0.1 degrees C, a decrease in the difference between T-b and T-a and a reduction in th ermal conductance. Within the TNZ, basal metabolic rate (BMR) was 2.43 +/- 0.41 mt O-2 g(-1) h(-1), while the corresponding day RMR was sign ificantly elevated at 3.26 +/- 0.42 mt O-2 g(-1) h(-1). The ability of Silvereyes to reduce daily energy expenditure by employing nocturnal hypothermia may be one reason why this species and its relatives are a ble to occupy a wide variety of habitats and climates.