Sociality influences thermoregulation and roost switching in a forest bat using ephemeral roosts

Abstract

In summer, many temperate bat species use daytime torpor, but breeding females do so less to avoid interferences with reproduction. In forest-roosting bats, deep tree cavities buffer roost microclimate from abrupt temperature oscillations and facilitate thermoregulation. Forest bats also switch roosts frequently, so thermally suitable cavities may be limiting. We tested how barbastelle bats (Barbastella barbastellus), often roosting beneath flaking bark in snags, may thermoregulate successfully despite the unstable microclimate of their preferred cavities. We assessed thermoregulation patterns of bats roosting in trees in a beech forest of central Italy. Although all bats used torpor, females were more often normothermic. Cavities were poorly insulated, but social thermoregulation probably overcomes this problem. A model incorporating the presence of roost mates and group size explained thermoregulation patterns better than others based, respectively, on the location and structural characteristics of tree roosts and cavities, weather, or sex, reproductive or body condition. Homeothermy was recorded for all subjects, including nonreproductive females: This probably ensures availability of a warm roosting environment for nonvolant juveniles. Homeothermy may also represent a lifesaver for bats roosting beneath loose bark, very exposed to predators, because homeothermic bats may react quickly in case of emergency. We also found that barbastelle bats maintain group cohesion when switching roosts: This may accelerate roost occupation at the end of a night, quickly securing a stable microclimate in the newly occupied cavity. Overall, both thermoregulation and roost-switching patterns were satisfactorily explained as adaptations to a structurally and thermally labile roosting environment.

Keywords: Chiroptera; body temperature; snag; torpor; tree; vespertilionids.

Figure 1

Barbastelle bat Barbastella barbastellus, a small‐sized vespertilionid found in Europe, Asia, and N Africa

Figure 2

Mean hourly ambient (solid line) and internal roost (dashed line) temperatures of Barbastella barbastellus tree roosts. Temperatures were measured while bats were roosting. Error bars represent 1 standard deviation. Differences between roost and ambient temperatures are not significant (paired Student's t test, n.s.)

Figure 3

Simultaneous daily thermal patterns of two female Barbastella barbastellus roosting together in the same social group. Solid line in the upper part of the figure (a) shows skin temperature of a nonreproductive female, dashed line that of a lactating female; point line: torpor onset threshold. Solid line in the lower part of the figure (b) shows ambient temperature. Error bars show ±1 standard deviation. Sunrise and sunset times on sampling day were, respectively, 05.45 and 20.36

Figure 4

Patterns of skin temperature (solid lines with error bars) of the same female Barbastella barbastellus over two consecutive days, that is, when roosting in a group of six bats (a) and alone (b); ambient temperature (solid lines with no error bars in the lower part of the figure) and torpor onset threshold (dotted line) are also represented. Skin temperatures were not measured at night (between 21.00 and 04.00), when bats were active, but only in daytime, when they were roosting. Error bars show ±1 standard deviation

Figure 5

Simultaneous daily patterns of skin temperature of a male and female Barbastella barbastellus roosting in two different roosts. The upper part of the graph shows skin temperatures of male (dashed line) and female (solid line with error bars) and the torpor onset threshold (dotted line). The solid line with no error bars represents ambient temperature (Ta). Error bars show ±1 standard deviation. Times of sunrise and sunset on sampling day were, respectively, 05.48 and 20.34

Figure 6

A male Barbastella barbastellus (left) roosting solitarily beneath the exfoliating bark of a beech tree (right). The bat is also visible in the thermographic image of the cavity (upper left box)

Figure 7

Snag (left) and flaking bark cavity (right) used by a group of female Barbastella barbastellus and their pups; at least three pups are visible