Some like it cold: summer torpor by freetail bats in the Australian arid zone

Abstract

Bats are among the most successful groups of Australian arid-zone mammals and, therefore, must cope with pronounced seasonal fluctuations in ambient temperature (T a), food availability and unpredictable weather patterns. As knowledge about the energy conserving strategies in desert bats is scant, we used temperature-telemetry to quantify the thermal physiology of tree-roosting inland freetail bats (Mormopterus species 3, 8.5 g, n = 8) at Sturt National Park over two summers (2010-2012), when T a was high and insects were relatively abundant. Torpor use and activity were affected by T a. Bats remained normothermic on the warmest days; they employed one "morning" torpor bout on most days and typically exhibited two torpor bouts on the coolest days. Overall, animals employed torpor on 67.9 % of bat-days and torpor bout duration ranged from 0.5 to 39.3 h. At any given T a, torpor bouts were longer in Mormopterus than in bats from temperate and subtropical habitats. Furthermore, unlike bats from other climatic regions that used only partial passive rewarming, Mormopterus aroused from torpor using either almost entirely passive (68.9 % of all arousals) or active rewarming (31.1 %). We provide the first quantitative data on torpor in a free-ranging arid-zone molossid during summer. They demonstrate that this desert bat uses torpor extensively in summer and often rewarms passively from torpor to maximise energy and water conservation.

How to keep cool in a hot desert: Torpor in two species of free-ranging bats in summer

“…The selection of dead and hollow roost trees with the absence of a closed tree canopy probably facilitated this feat. Such entirely passive arousals in bats have only been previously observed in M. petersi studied at the same location, 7 suggesting that entirely passive rewarming from torpor is an important and general strategy of desert bats to minimise energy expenditure and water loss. However, roost selection in this two habitat generalists suggested also the avoidance of temperature extremes with roosts generally being low to the ground and situated in stands of trees along creek lines and water bodies.…”

“…Only adult non-reproductive and/or post-lactating individuals were used for this study. Captured bats were fitted with calibrated temperature-sensitive external radio transmitters (0.3–0.5 g LB-2XT/LB-2NT, Holohil Systems Inc., Carp, Ontario, Canada) and radio-tracked daily to their roosts at sunrise following Bondarenco et al 7,4 For each bat, a receiver/data logger was placed near the roost tree to remotely record T skin data every 10 min 36 that were then downloaded to a portable computer. After locating the roost tree, a coaxial cable attached to a 4-m long fishing rod connected to a receiver (Icom, IC-R10) was used to determine the exact bat location.…”

“…Prior to use, iButtons were calibrated following Bondarenco et al 7 T a recorded at the research site ranged from 12.7 to 48.1°C over the study periods.…”

“…Even during summer, T a can fall as low as 12°C at night in arid areas limiting foraging opportunities 7 . Therefore, the maintenance of constant high body temperature would be energetically counter-productive.…”

“…basking) 12 . Moreover, a recent study on inland freetail bats, ( Mormopterus petersi , ∼9 g; previously known as Mormopterus species 3), showed that this desert specialist can arouse from torpor using entirely passive rewarming without an obvious active component 7 . However, published data on how desert bats deal with energetic challenges and minimise costs of rewarming from torpor are limited to this one bat species, and it is unknown whether it is a common pattern employed by other Australian desert bats especially those that are not desert specialists, but occur over a wider range of habitats.…”