Early November, giant hammerheads arrive off Bimini, in the Bahamas. But why? Leading shark scientist Tristan Guttridge tags and follows these normally solitary sharks to find out. Many appear to be pregnant females, as an ultrasound of a female hammerhead shows. Guttridge believes they spend winter in Bimini to feed the growing young in their bellies – and then move on to the Florida coast to prey on migrating blacktips – and the evidence is unmistakable. Produced by Icon Films.