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Endotherms allocate large amounts of energy and water to the regulation of a precise body temperature (Tb), but can potentially reduce thermoregulatory costs by allowing Tb to deviate from normothermic levels. Many data on heterothermy at low air temperatures (Ta) exist for caprimulgids, whereas data on thermoregulation at high Ta are largely absent, despite members of this taxon frequently roosting and nesting in sites exposed to high operative temperatures. We investigated thermoregulation in free-ranging rufous-cheeked nightjars Caprimulgus rufigena and freckled nightjars Caprimulgus tristigma in the southern African arid zone. Individuals of both species showed labile Tb fluctuating around a single modal Tb (Tb-mod). Average Tb-mod was 39.7°C for rufous-cheeked nightjars and 39.0°C for freckled nightjars. In both species, diurnal Tb increased with increasing Ta. At Ta ≥ 38°C, rufous-cheeked nightjar mean Tb increased to 42°C, equivalent to 2.3°C above Tb-mod. Under similar conditions, freckled nightjar Tb was on average only 1.1°C above Tb-mod, with a mean Tb of 40.0°C. Freckled nightjars are one of the most heterothermic caprimulgids investigated to date, but our data suggest that during hot conditions this species maintains Tb within a narrow range above Tb-mod, possibly reflecting an evolutionary tradeoff between decreased thermal sensitivity to lower Tb but increased sensitivity to high Tb. These findings reveal how general thermoregulatory patterns at similar Ta can vary even among closely related species. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]