Energetics, Thermoregulation and Nocturnal Hypothermia in Australian Silvereyes

We investigated whether the Silvereye, Zosterops lateralis (10-12 g) uses heterothermy to reduce energy expenditure when confronted with adverse environmental conditions. We also determined the thermal physiology and energetics of this species. Z. lateralis entered nocturnal hypothermia over a range of ambient temperatures (T a ) from 3-26°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 of body temperature (T b ) from 40.3 ± 0.5 to 36.9 ± 0.1°C, a decrease in the difference between T b and T a and a reduction in thermal conductance. Within the TNZ, basal metabolic rate (BMR) was 2.43 ± 0.41 mL O 2 g -1 h -1 , while the corresponding day RMR was significantly elevated at 3.26 ± 0.42 mL 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 able to occupy a wide variety of habitats and climates.