Energy, water balance and the roost microenvironment in three Australian cave-dwelling bats (Microchiroptera)

The ghost bat, Macroderma gigas, and the orange leaf-nosed bat, Rhinonycter is aurantius, occupy similar ranges across northern Australia and are often found in the same roost caves. Both species are considered rare and vulner able to further population decline. A third small species, the large bent-w ing bat, Miniopterus schreibersii, has a similar body mass to R. aurantius, but has one of the largest ranges of any Australian mammal. In the present study we examine the effect and sensitivity of the animals' roosting micro climates on their energy and water balance. M. schreibersii exhibits a basa l metabolic rate about 40% greater than other bats of similar body mass, wh ereas the other two species are close to predicted levels. R. aurantius sho ws a decrease in body temperatures below thermoneutrality. R. aurantius has levels of pulmocutaneous water loss among the highest seen for a mammal, a nd calculations based on nasal tip temperatures suggest that most of this l oss is across the skin. Calculated ambient temperatures at which metabolic water production is equal to pulmocutaneous water loss in dry air are -14.7 degrees C for R. aurantius, 9.8 degrees C for M. schreibersii and -0.3 deg rees C for M, gigas. Exposing the animals to relative humidities of between 80% and 90% shifted these calculated temperatures to 5.6 degrees C, 25.2 d egrees C, and 2.9 degrees C, respectively. For each species the ratio of me tabolic water production to evaporative water loss has been treated as a jo int function of humidity and ambient temperature. The resulting surface plo t shows that under known roosting conditions in caves R. aurantius and M. s chreibersii remain in positive water balance, whereas M. gigas does not.