Torpor and activity in a free-ranging tropical bat: implications for the distribution and conservation of mammals?

Abstract

Bats are most diverse in the tropics, but there are no quantitative data on torpor use for energy conservation by any tropical bat in the wild. We examined the thermal biology, activity patterns and torpor use of two tree-roosting long-eared bats (Nyctophilus geoffroyi, 7.8 g) in tropical northern Australia in winter using temperature telemetry. Bats commenced activity about 20 min after sunset, ended activity about 2.5 h before sunrise and entered torpor everyday in the early morning even when minimum ambient temperatures (T (a)) were as high as 23°C. On average, bats remained torpid for almost 5 h, mean minimum skin temperature (T (skin)) measured was 22.8 ± 0.1°C and daily T (skin) minima were correlated with T(a). Our study shows that even in the tropics, torpor is frequently employed by bats, suggesting that worldwide most bat species are heterothermic and use torpor for energy conservation. We propose that the ability of employing torpor and the resulting highly plastic energy requirements may partially explain why these small insectivorous bats can inhabit almost the entire Australian continent despite vastly different climatic and likely trophic conditions. Reduced energy requirements also may permit survival in degraded or modified habitats, reduce the need for foraging and reduce exposure to predators. Thus, the ability to employ torpor may be one important reason for why most Australian bats and other heterothermic mammals have not gone extinct whereas many obligatory homeothermic mammals that cannot employ torpor and have high energy and foraging requirements have suffered high rates of extinctions.

Will Temperature Effects or Phenotypic Plasticity Determine the Thermal Response of a Heterothermic Tropical Bat to Climate Change?

“…While this was a rare occurrence, torpor was frequently used (95% of all study days) even though weather conditions were mild. Our extensive field study of N. bifax and a recent brief field study on two individual N. geoffroyi during winter in a tropical habitat [11] confirm earlier findings from laboratory work [25]–[26] that torpor is indeed widely used by tropical bats for energy conservation in the wild. Further, data on tropical bats and on other mammals such as lemurs and tenrecs from Madagascar [31]–[33] show that, contrary to the widely held view, torpor use is prevalent in tropical regions.…”

“…Data from N. geoffroyi from a temperate region in summer [22] predict that torpor bout duration at the mean minimum T a of 19.2°C in tropical Northern Territory is only 1.7 h and will decline to 1.2 h with a 2°C rise of T a . Measured torpor bout duration in tropical N. geoffroyi at a mean minimum T a of 19.2°C is in fact 4.9 h [11], 2.8-times that predicted from temperate bats. Winter data [23] predict that torpor bout duration of temperate N. geoffroyi at a minimum T a of 19.2°C and a mean T skin of 26.2°C is only 1.0 h, only 21% of that measured in the tropics.…”

“…However, in the tropics essentially all information on the use of mammalian torpor in nature is currently limited to dwarf lemurs and tenrecs [31]–[34], despite the enormous diversity of tropical bats. Although bats comprise >20% of all mammals and the vast majority of these live in the tropics [27], only two individuals of a single species have been examined with regard to torpor in the wild [11].…”

The importance of temporal heterothermy in bats

“…These data show that saving energy by employing torpor is important not only to bats in cold climates. Even in tropical areas, frequent use of short bouts of torpor in winter has now also been recorded in some free‐ranging insectivorous bats and torpor bouts of longer than a day have also been observed (Geiser et al ., ; Stawski & Geiser, ).…”

Body temperature patterns of big brown bats during winter in a building hibernaculum

“…However, many species in the United States hibernate outside of caves in trees, buildings, and rocks and are likely to use different strategies than cavernicolous bats to survive winter (Boyles and Robbins 2006;Brigham 1987;Mormann and Robbins 2007). Essentially all field studies on the physiology of noncavernicolous bats during winter are from the Southern Hemisphere (Cory Toussaint et al 2010;Geiser et al 2011;Stawski et al 2009;Turbill and Geiser 2008), and no data are available for populations from high northern latitudes. These data are essential to enhance our understandw w w .…”