Let us remember that to Mr. Pignati, Bobo was almost a replacement for his wife. In his state of solitude, his regular visits to Bobo and the delight that he takes in Bobo clearly illustrates how important this chimp actually is to this lonely old man. It is therefore no suprise that in Chapter 14 when Mr. Pignati discovers that Bobo has died, that he reacts in a way that demonstrates just how important Bobo was to him:
He went to grab hold of the railing, but let out a tiny cry almost like a puppy that had been stepped on by mistake. I can still remember the sound of it, and sometimes I wake up from a nightmare with it in my ears. It was like a high-pitched scream, but it came from deep inside of him, and before John or I knew what had happened, the Pigman dropped to the floor.
The news of Bobo's death is what results in the death of Mr. Pignati, as he realises just how alone he actually is in this world. For Mr. Pignati, who throughout the novel is shown again and again as being an intensely social individual for whom relationships are incredibly important, life just simply wasn't worth living without Bobo, and thus he dies. Let us not also forget the way in which Mr. Pignati had been effectively betrayed by John and Lorraine when they threw the party in his house. The death of Bobo thus also could be said to be symbolic of the death of the trust that Mr. Pignati had with John and Lorraine.