As the article says, bats are known for their way to fly in the dark. They have the abilities to avoid obstacles while navigating even if they can't see where they are going. The bats send high frequency sounds which helps them to evaluate the obstacles. One day, Stefan Greif was studying the way bats use to go from place to another. He realized that bats had tendancy to collid with vertical surfaces. He decided to go further and do an experience. The results confirmed that bats have difficulty to recognize vertical surfaces.