Bobo the baboon apparently died of pneumonia.
After Mr. Pignati comes home from the hospital to find his house trashed because of a party that John and Lorraine had thrown there, the two teenagers feel terrible and want to make amends. They call Mr. Pignati and offer to come and clean up, and when he demurs, they invite him to the zoo, knowing that it is his favorite place to go. They hope that by taking him to see Bobo, his "best friend", they will be able to lift his spirits a little, and make up in a small way for their misdeeds.
When they get to the gorilla cages, Bobo is not there. An attendant tells Mr. Pignati and John and Lorraine that "Bobo died last week...did an autopsy on him, and it looked like pneumonia". Mr. Pignati, who is already very frail, stares in shock at the empty cage for "what seem(s) like an eternity". The blood vessels on the side of his neck pulse as he raises his right hand to his face. His hands begin to shake, and he grabs hold of the railing, letting out "a tiny cry almost like a puppy that had been stepped on by mistake"; he then drops senseless to the floor. The shock of finding out that his beloved Bobo has died on top of all that has happened to him during the past weeks is too much for him. Mr. Pignati is dead (Chapter 14).