Complex sentence: The Moties are sequential hermaphrodites, changing sex over and over again over the course of their lives--with one quirk: if a Motie remains female for too long without becoming pregnant, the hormone imbalance will kill her.
Split into simpler sentences: The Moties are sequential hermaphrodites, changing sex over and over again over the course of their lives. However, if a Motie remains female for too long without becoming pregnant, the hormone imbalance will kill her.