Cardiorespiratory function and metabolism of heterothermic bats

Abstract

Bats are fascinating in their ability to maintain coordinated cardiorespiratory function at the extremes of metabolism- during flight and torpor. As the only mammals capable of powered flight, bats have developed relatively large and highly efficient hearts and lungs compared to their non-flying counterparts. In addition to this expensive form of locomotion, bats must cope with high heat loss associated with their high surface area to volume ratios because of their small size. To balance these energetic challenges many bats are capable of entering torpor where metabolic rate (MR), cardiac function and body temperature (Tb) are substantially reduced. Torpor use is essential to many bats, can occur almost daily in some species and is often expressed throughout the year. There are two forms of torpor exhibited by heterothermic mammals including bats; daily heterothermy, which consists of short term daily torpor <24h or longer multiday hibernation, which comprises longer multiday torpor bouts punctuated by periodic spontaneous arousals. Previous work on torpor physiology in bats has primarily been undertaken on the thermal energetics of northern hemisphere temperate zone insectivorous species that hibernate in thermally stable environments such as caves or houses. On the other hand, many tree-dwelling bats in Australia enter torpor in thermally labile roosts under tree bark or among exposed foliage and can experience large fluctuations in ambient temperature (Ta). Although the cardiorespiratory system is central to the coordination and maintenance of torpor, as its role in the circulation of blood gases and hormones directly impact on thermoregulation, to date there has been limited investigation of cardiac function in heterothermic bats. Therefore, I aimed to determine the patterns of cardiorespiratory function during torpor in two species of Australian bats that use either hibernation, Gould’s long-eared bat ('Nyctophilus gouldi' Vespertilionidae), or daily torpor, common blossom bat ('Syconycteris australis' Pteropodidae). I also examined the influence of Ta on the relationships between heart rate (HR), ventilation rate (VR), oxygen consumption (V̇O₂) and subcutaneous temperature (Tsub), either at a constant Ta or an increasing Ta profile.