Do artificial roost design and placement alter the torpor expression of Indiana bats?

Artificial bat roosts are deployed worldwide for conservation and mitigation for loss of habitat. To date, few studies have assessed bat physiological responses to these structures despite the direct fitness consequences associated with artificial roosts. Variations in artificial roost design and placement impact roost temperature and could affect bats' thermoregulatory responses, with potentially negative implications for energy expenditure and reproduction. To address this knowledge gap, we used temperature-sensitive radio telemetry to quantify the torpor expression of female Indiana bats, an endangered species, using five different artificial roost designs placed in sun-exposed and shaded locations during the summer of 2021 at field sites in Indiana and Kentucky, USA. For bats using the large, thermally stratified artificial roosts we deployed, all variants of a reference design, torpor expression was strongly influenced by minimum daily temperature but not by artificial roost design. Indiana bats exhibited three common thermoregulatory behaviors during this study: continuous endothermy, morning torpor, and all-day torpor use. By switching roosts with seasonal weather shifts, Indiana bats could avoid cooler, shaded artificial roosts, thus avoiding excessive torpor use during pup rearing. Bats shifted from shaded roost placements early in the summer to sun-exposed roost placements after canopy leaf-out occurred. Our results highlight the labile behavioral and thermoregulatory responses of bats to their roosting environments. Offering a variety of artificial roost designs and placements to meet shifting thermoregulatory needs could be critical to enabling bats to maintain a positive energy budget over the course of the maternity season.