 Although many of the characters are " Earth @-@ human " – " human " is ambiguous because all species refers to themselves as " human " in their own languages – the stories feature medical staff from a wide range of species . In each story characters belonging to species different from that of the protagonist are treated as neuter and referred to as " it " . Some species are touchy about any reference to sexual roles and behavior , some have more than two sexes and some change sex once or more during their lifetimes . 
