To record the heart rate of male common noctule bats during flight, the scientists attached heart rate transmitters weighing less than one gram to the animals, which they then accompanied in an airplane while the bats flew, sometimes for more than an hour, in search of food. The researchers found that the heart rate of bats reaches around 900 beats per minute within only a few minutes during flight. Using the unique recordings of heart beats, the scientists discovered fascinating strategies that bats use to budget their energy consumption in different seasons. They found that male common noctule bats consume up to 42% more energy in summer compared to spring. This is mainly due to the fact that the bats in spring go into a kind of short daytime hibernation known as “torpor”—an energy-saving state in which heart rate can be reduced to six beats per minute. Male bats did not use torpor in summer at all. In summer, food is plentiful and so males stay awake during the day to invest energy in sperm production.