Whether in dark caves, under bridges, or in inaccessible crevices, bats intrigue us with one unique habit: they sleep hanging upside down. “As they evolved from land mammals to fliers, bats started by gliding, much like flying squirrels,” she told Live Science. Unlike birds, whose hollow bones lighten their skeletons and facilitate lift during flight, bats have solid bones—a characteristic of land mammals. Hanging upside down, bats gain a significant energy advantage thanks to their anatomy. However, while most bats sleep hanging upside down, not all do.