Food restricted southern blossom bats (Syconycteris australis) reduce energy use and body temperature

The southern blossom bat (Syconycteris australis) is a small (∼18 g), solitary, tree roosting pteropodid, which forages on nectar, pollen, and small fruits. In Australia, the distribution of S. australis extends along the eastern coast from the tropics to subtropical northern New South Wales (NSW). In NSW, Banksia spp. are an important food source for the species. S. australis can lower metabolic rate (MR) and body temperature (T_b) in a controlled manner to save energy and combat short-term food shortage. While it is known that in S. australis torpor use may vary by season, specifically how ambient temperature (T_a) and food availability influence torpor induction is not understood. Our study therefore aimed to provide the first quantitative examination of the relationships between food availability/restriction, T_a, T_b, and energy expenditure in captive male S. australis. To this end, the relationship between T_a (17-37 °C) and diet ('fed'/'unfed') to subcutaneous temperature (T_sub) and MR was examined using flow-through respirometry (in S. australis T_sub is normally within 0.6 °C of T_b). At T_a < 25 °C, mean resting MR and T_sub were consistently lower in 'unfed' compared with 'fed' S. australis. Below the thermoneutral zone (i.e. below T_a ∼30 °C), the effect of T_a on T_sub depended on food availability, with T_sub decreasing at 0.25 × T_a in 'unfed' bats but at only 0.05 × T_a in 'fed' bats. In contrast, above the thermoneutral zone, T_sub rapidly increased with T_a regardless of feeding treatment (at 0.9 × T_a in 'fed' and 1.0 × T_a in 'unfed' bats). The results demonstrate that under a mild thermal challenge, food availability, rather than T_a, is fundamental in determining whether S. australis reduces MR and T_b. The ability for regulated T_b reductions in response to food restriction is likely crucial for survival of S. australis.