The ability for Indiana bats to find suitable hibernating conditions is critical for their survival. A hibernaculum that remained too warm during one winter caused a 45% mortality rate in hibernating bats. They generally hibernate in warmer portions of the hibernacula in fall, then move to cooler areas as winter progresses. During October and November, temperatures at roosting sites within major hibernacula in six states averaged 43.5 to 53.2 °F (6.4–11.8 °C). Roost temperatures at the same hibernacula ranged from 34.5 to 48.6 °F (1.4–9.2 °C) from December to February. Temperatures in March and April were slightly lower than in autumn at 39.6 to 51.3 °F (4.2–10.7 °C). Indiana bat populations increased over time in hibernacula that had stable midwinter temperatures averaging 37.4 to 45.0 °F (3.0–7.2 °C), and declined in hibernacula with temperatures outside this range.