What would a zoo be without animals? PETA might argue "paradise." But whatever your opinion, it would be a sad—and maybe a little creepy—series of empty cages. All that space should be used for something, right?
Jan and Antonina decide that is Hitler if going to treat people like animals, then these people might as well hide them in the empty zoo habitats. As Ackerman writes, "The great apes (including us) have been staging clever deceits, lying on purpose […] for at least 12 million years" (16.1). By hiding refugees in the zoo, Jan and Antonina are using their animal instincts against the nastiest animal of them all.
To complete the deceit, Jan and Antonina create code names for all their zoo Guests, naming the visiting humans after animals. The Kenigsweins, for example are known as the "sables" (24.12), and Antonina dyes their hair blonde to help them escape. She messes up, of course, turning their hair orange. (Don't quit your day job, Antonina.) From then on, the Kenigsweins are called "Squirrels" (25.4).
This craziness leads the Fox Man to complain that the zookeepers "use animal names for people and people's names for animals!" (24.11). Well, yeah: the code names provide an additional buffer of protection for the Guests. Because "the Nazis were ardent animal lovers and environmentalists" (8.16), they usually look the other way from the zoo. They have no problem exterminating people, but they wouldn't hurt a sable or a squirrel.
By getting in touch with their animal sides, the Guests and the zookeepers are able to do what animals have done best for millions and billions of years: survive.