they don't have a pleural cavity, elephant lungs are attached directly to the ribs
so when their chest expands the lungs are forced to expand rather than rely on the negative pressure in the pleura like us,

they said it might be due to the pressure an elephant is under when it is submerged and swimming, using its trunk as a cool snorkle
( glued eel /questions/questionsyoudliketoask/post1648081, Tue 30 Jun 2009, 14:21, archived)