The Milieu Souterrain Superficiel as hibernation habitat for bats: implications for white-nose syndrome

Abstract

Recent studies have revealed that western populations of little brown bats (Myotis lucifugus) in North America exhibit different hibernation behavior than their eastern counterparts. Understanding these differences is essential for assessing the risk white-nose syndrome (WNS) poses to western bat populations. We used acoustic monitoring and radiotelemetry to study the overwintering behavior of little brown bats near Juneau, Alaska during 2011-2014. Our objectives were to identify the structures they use for hibernation, measure the microclimates within those structures, and determine the timing of immergence and emergence and the length of the hibernation season. We radiotracked 10 little brown bats to underground hibernacula dispersed along two ridge systems. All hibernacula were ≤ 24.2 km from where the bats were captured. Eight bats hibernated in the "Milieu Souterrain Superficiel" (MSS), a network of air-filled underground voids between the rock fragments found in scree (talus) deposits. Two bats hibernated in holes in the soil beneath the root system of a tree or stump (rootball). At least two hibernacula in the MSS were reused in subsequent years. Average MSS and rootball temperatures were warmer and more stable than ambient temperature and were well below the optimal growth range of the fungus that causes WNS. Temperatures in the MSS dropped below freezing, but MSS temperatures increased with depth, indicating bats could avoid subfreezing temperatures by moving deeper into the MSS. Relative humidity (RH) approached 100% in the MSS and under rootballs and was more stable than ambient RH, which also was high, but dropped substantially during periods of extreme cold. Acoustic monitoring revealed that bats hibernated by late October and began emerging by the second week of April; estimates of minimum length of the hibernation season ranged from 156 to 190 days. The cold temperatures, dispersed nature of the hibernacula, and close proximity of hibernacula to summering areas may slow the spread and reduce the impacts of WNS on local populations of little brown bats.

Keywords: Mesovoid Shallow Stratum; Milieu Souterrain Superficiel; Myotis lucifugus; hibernacula; little brown bat; microclimate; radiotelemetry; rock scree; white-nose syndrome.

Fig. 1.

Map of Juneau, Alaska study area showing sites at which little brown bats were captured and radiotagged during autumn 2011–2014 and locations of 10 hibernacula identified by radiotelemetry. Stars indicate sites where bats were captured, circles indicate hibernacula in the Milieu Souterrain Superficiel (MSS), and squares indicate hibernacula in holes under the rootball of a tree or stump (Rootball). A cross inside the hibernacula symbol indicates the bat was captured at Fish Creek and an × inside indicates the bat was captured at Campground. Karst areas of Southeast Alaska are indicated by dark shading on inset map.

Fig. 2.

Radiotracking outcomes for little brown bats in Juneau, Alaska relative to seasonal acoustic activity at the base of a ridge used for hibernation (Fish Creek) and all other Juneau area detector sites combined (pooled sites), by season and year. Squares indicate dates on which bats hibernated, triangles indicate the last date a signal was detected when the fate of the bat was unknown, and circles indicate dates when we lost radio signals while actively tracking bats near known hibernation areas. The shaded areas represent the span of time during which radiotags were active.

Fig. 3.

Bat acoustic activity during the 2014 active season (April–October) at two sites in Juneau, Alaska. The black bars indicate activity at a hibernaculum in the Milieu Souterrain Superficiel (MSS) used by a radiotagged little brown bat the previous winter; crosshatching denotes that the detector was not operational. The black line depicts activity at a long-term acoustic monitoring site (Fish Creek) located at a pond at the base of the ridge where the bat hibernated.

Fig. 4.

Boxplot of temperature and relative humidity for ambient conditions and little brown bat hibernacula in the Milieu Souterrain Superficiel (MSS) or in holes under the rootball of a tree or stump (Rootball) near Juneau, Alaska during the winters (December–February) 2013–2016. Bold bars represent the median, diamonds are the mean, upper and lower box limits are the 75th and 25th quartiles, whiskers indicate the minimum and maximum values, and circles are possible outliers. Sample sizes were 23, 23, and 9 for temperature and 13, 12, and 2 for relative humidity for ambient, MSS, and rootball, respectively.

Fig. 5.

Proportion of time temperatures were above or below 0°C for ambient conditions and in little brown bat hibernacula in the Milieu Souterrain Superficiel (MSS) near Juneau, Alaska. Data collection in the MSS did not begin until 24 October during the winter of 2013–2014.

Fig. 6.

Daily-average temperatures for ambient conditions (gray line) and in little brown bat hibernacula in the Milieu Souterrain Superficiel (MSS; black line) with 95% CI (gray shading) near Juneau, Alaska, by winter. Light dashed line indicates 0°C. Sample sizes are for the MSS.