ESCAPE is the six-letter word that is the theme of chapters 10 and 11.
You can see that the word "escape" is the theme in the very first sentence of chapter 10: "Yet there will always be animals that seek to escape from zoos." It doesn't matter how well they are taken care of or how much good food or companions they are given. The animals are still confined.
The animals are wild and aren't meant to be confined!
Pi is sure to mention that even animals born and bred in zoos will STILL have the urge to escape sometimes. If they didn't have this urge, the animal species would not be able to adapt in the wild and would become extinct.
Whatever the reason for wanting to escape, sane or insane, zoo detractors should realize that animals don't escape to somewhere but from something."
The word "escape" appears in the first sentence of chapter 11 as well. In fact, the word "escape" appears 13 times in these 6 pages! The rest of these two chapters are detailed accounts of animal escapes from many different zoos. Even though these chapters seems kind of plucked out of the story, they are actually very important thematically. Escape and confinement are very definite themes, especially when one finds oneself on a lifeboat with a tiger for 277 days.