The main clue we have that pterosaurs flew, rather than glided, is that their bones had enlarged attachment sites for the muscles that, in birds, control flight. Scientist interpret this as meaning that the muscles were large and powerful in pterosaurs, a characteristic unlikely to have evolved unless they were capable of active flight. Another characteristic that pterosaurs shared with birds is hollow bones with internal struts. This adaptation increases bone strength while reducing weight, and is regarded as critical for the evolution of flight. Light, strong bones would have been especially important in pterosaurs, given the enormous sizes attained by some species.