Little brown bats prefer hibernation sites with relative humidity above 90 percent.
High humidity helps these small bats limit evaporative water loss while they remain in torpor. A hibernating bat cannot easily replace lost water because it is not feeding, and excessive dehydration can trigger costly arousals from hibernation.
Their winter shelters, called hibernacula, are usually caves, abandoned mines, or similar underground spaces. Besides humidity above 90 percent, preferred sites generally remain above freezing and maintain relatively stable temperatures through winter.
The figure is a preference, not an absolute rule for every individual or population. Surveys and regional habitat studies sometimes report broader ranges, such as roughly 70–95 percent humidity. That variation does not overturn the standard species account: little brown bats favor the very humid end of the range, unlike big brown bats, which can tolerate drier hibernation sites.