Powered flight has enormous energy costs. Flight is energetically cheaper than walking or running once the bat is up in the air. However, it takes a considerable amount of calories to get airborne. Flying is very demanding on bat physiology. In some species, the heart rate may rise to approximately 1,000 beats per minute in order to supply oxygen to the tissues during flight. Because of these demands, the heart and lungs are larger in bats than in comparably sized mammals.

Bats do not consume fibrous plant material. Such a diet simply would not supply enough calories. Also, the gut passage time and the gut modifications needed to digest highfiber material would increase the weight of the animal. Bats consume easily digestible, high-calorie items such as insects, fruit, or nectar. Some species also eat small vertebrates like fish, frogs, mice, or even other smaller bats.

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