Free-ranging bats alter thermoregulatory behavior in response to reproductive stage, roost type, and weather

Abstract

Heterotherms vary their use of torpor and choice of refugia to deal with energetic stresses such as reproductive activity and extreme weather. We hypothesized that a temperate-region bat would vary its use of heterothermy in response to air temperature but use of torpor would also be influenced by reproductive stage and roost choice. To test this hypothesis, we collected data on skin temperatures of female Indiana bats (Myotis sodalis) carrying temperature-sensitive radiotransmitters during the summers of 2013–2015. We also measured internal temperatures and external characteristics of roosts used by these bats. We analyzed the influence of daytime air temperature, roost canopy closure, roost type, and bat reproductive stage, on daily heterothermy index and torpor characteristics of 17 bats during 103 full roost days (data collected consistently from when a bat entered its roost in the morning to when it emerged at night). Our data showed that Indiana bat heterothermy was influenced by reproductive stage, roost choice, and weather. Although they used torpor, pregnant bats were the least heterothermic (daily heterothermy index = 3.3 ± 0.6°C SE), followed by juvenile bats (5.6 ± 0.5°C), lactating bats (5.7 ± 0.5°C), and one postlactating bat (13.2 ± 1.6°C). Air temperature also influenced heterothermy of pregnant bats less than bats of other reproductive stages. Thermoregulatory strategies varied on a continuum from use of normothermia in warm roosts to use of long and deep bouts of torpor in cool roosts. The thermoregulatory strategy used seemed to be determined by potential reproductive costs of torpor and energetic consequences of weather. Because Indiana bats used different degrees of heterothermy throughout the summer maternity season, managers should offer maternity colonies an array of refugia to facilitate varying behaviors in response to weather and energetic demands.

Evaluating bat boxes: design and placement alter bioenergetic costs and overheating risk

“…This validates the assertion that female bats choose solar-exposed roosts to reduce the energy costs of normothermia ( Callahan et al , 1997 ; Vonhof and Barclay, 1996 ), thus facilitating the avoidance of torpor use and the negative effects of low body temperature on reproduction (e.g. Hoying and Kunz, 1998 ; Wilde et al , 1999 ; but see Bergeson et al , 2021 ). In contrast, forest box placements minimized DEE for heterothermic bats.…”

“…Because Indiana bats’ thermoregulatory behaviour can range from near perfect endothermy to extreme heterothermy ( Bergeson et al , 2021 ), we decided to model energy expenditures of two distinct thermoregulatory scenarios—continuous endothermy and facultative heterothermy. As detailed metabolic data are unavailable for the Indiana bat, we used data for the well-studied, morphologically similar little brown bat ( Myotis lucifugus) .…”

“…Consequently, many reproductively active individuals will use torpor (i.e. become heterothermic) when roosts are cool ( Bergeson et al , 2021 ; Willis et al , 2006 ), even though this behaviour involves reproductive costs of delayed parturition ( Racey and Swift, 1981 ), reduced milk production ( Wilde et al , 1999 ) and slowed juvenile development ( Hoying and Kunz, 1998 ).…”

The small-bat-in-summer paradigm: energetics and adaptive behavioural routines of bats investigated through a stochastic dynamic model

“…The copyright holder for this this version posted September 30, 2022. ; https://doi.org/10.1101/2022.09.28.509895 doi: bioRxiv preprint employ frequent deep torpor bouts (Speakman & Rowland 1999;Lausen & Barclay 2003;Lourenço & Palmeirim 2004). Non-reproductive and post-lactating individuals may also spend considerable periods of time awake during day-and night-time in summer, although the expression of torpor is found to vary greatly with weather and roost type (Lausen & Barclay 2003;Bergeson et al 2021;Fjelldal et al 2021).…”

“…Bats strategically choose warm roosts during summer to facilitate lower thermoregulatory costs of being awake, which is important particularly for pregnant or lactating females as they face added costs to their reproduction if they employ frequent deep torpor bouts (Speakman & Rowland 1999; Lausen & Barclay 2003; Lourenço & Palmeirim 2004). Non-reproductive and post-lactating individuals may also spend considerable periods of time awake during day- and night-time in summer, although the expression of torpor is found to vary greatly with weather and roost type (Lausen & Barclay 2003; Bergeson et al . 2021; Fjelldal et al .…”