Thermally unstable roosts influence winter torpor patterns in a threatened bat species

Abstract

Many hibernating bats in thermally stable, subterranean roosts have experienced precipitous declines from white-nose syndrome (WNS). However, some WNS-affected species also use thermally unstable roosts during winter that may impact their torpor patterns and WNS susceptibility. From November to March 2017-19, we used temperature-sensitive transmitters to document winter torpor patterns of tricolored bats (Perimyotis subflavus) using thermally unstable roosts in the upper Coastal Plain of South Carolina. Daily mean roost temperature was 12.9 ± 4.9°C SD in bridges and 11.0 ± 4.6°C in accessible cavities with daily fluctuations of 4.8 ± 2°C in bridges and 4.0 ± 1.9°C in accessible cavities and maximum fluctuations of 13.8 and 10.5°C, respectively. Mean torpor bout duration was 2.7 ± 2.8 days and was negatively related to ambient temperature and positively related to precipitation. Bats maintained non-random arousal patterns focused near dusk and were active on 33.6% of tracked days. Fifty-one percent of arousals contained passive rewarming. Normothermic bout duration, general activity and activity away from the roost were positively related to ambient temperature, and activity away from the roost was negatively related to barometric pressure. Our results suggest ambient weather conditions influence winter torpor patterns of tricolored bats using thermally unstable roosts. Short torpor bout durations and potential nighttime foraging during winter by tricolored bats in thermally unstable roosts contrasts with behaviors of tricolored bats in thermally stable roosts. Therefore, tricolored bat using thermally unstable roosts may be less susceptible to WNS. More broadly, these results highlight the importance of understanding the effect of roost thermal stability on winter torpor patterns and the physiological flexibility of broadly distributed hibernating species.

Keywords: Perimyotis subflavus; roost microclimate; short-term torpor; thermally unstable roosts; white-nose syndrome.

Figure 1

(a) Summary of torpid skin temperatures recorded every 15 min from tricolored bats using thermally unstable roosts in the Upper Coastal Plain of South Carolina, 2017–19. Skin temperatures favorable to Pd growth (i.e. <19.5°C) occur above the grey shaded regions, while potential optimal growth conditions (i.e. 12.5–15.8°C) are represented by solid black shading. (b) Skin temperature trace (dotted purple) containing longest observed torpor bout duration of 15.5 days from a tricolored bat (SR0257) in 2019 using a tree cavity with roost temperature (solid orange) and ambient temperature (solid gray) for reference.

Figure 2

Observed (a) Pd bout duration, (b) torpor bout duration and (c) normothermic bout duration as a function of mean ambient temperature for tricolored bats using thermally unstable roosts in the Upper Coastal Plain of South Carolina, 2017–19.

Figure 3

Probability of arousal by time of day (24-hour notation) for tricolored bats using thermally unstable roosts in the Upper Coastal Plain of South Carolina, 2017–19.

Figure 4

(a) Probability of general activity as a function of daily mean ambient temperature and the (b) probability of night-time activity away as a function of nightly mean ambient temperature and (c) mean nightly barometric pressure for tricolored bats using thermally unstable roosts in the Upper Coastal Plain of South Carolina, 2017–19.