Claustrophobic boat imagery
On the boat, everything is claustrophobic. An ape sits meditating, to calm its nerves about being in a cage. When the ape becomes free, suddenly the humans feel trapped. They know that somewhere, there is a horrifying animal on the loose, but they are also trapped on the boat. They feel "haunted" by the animal, which is thematic, because that animal haunting becomes a fully-articulated story of its own. Even though the story continues later, the claustrophobic feeling remains through much of the book.
Pictures of the sensible life
In London, the young couple has a perfectly ordinary life where the husband goes to work and the wife stays behind to tend to the house and keep herself entertained somehow. She immediately becomes a nervous wreck and starts abusing alcohol. These pictures of normal life are abysmal and horrifying to Madelene. When she is obeying the illusion of order that society created, she finds herself unconvinced that life is meaningful, and she suffers existential dread. If the boat is Erasmus's cage, this life is Madelene's cage.
Pictures of humans as animals.
Every major character in this novel is directly concerned with animals. Madelene's relationship to the ape obviously speaks for itself, but even the other characters are involved with animals. Adam is a zoologist, for instance. The captain of the boat is a poacher and smuggler. These are all depictions of humans behaving as if "the animal kingdom" is somehow external to them. They don't understand the irony of their lives as animal-workers, because they forget that they are of the same category of being as the animals. Only Madelene learns this.
The picture of natural paradise
Not surprisingly, the novel finds its imagery completed in a natural paradise where Madelene learns that life is much more satisfying as an ape than as a human. That doesn't mean she changed existentially, it just means that she has decided to prefer the animal part of herself, her natural instinct, and her life became heaven. Although nature is horrifying, Madelene's experience with the ape is safe and warm, because as a pure animal, he is competent to survive. The not-so-subtle argument here is that we make a lot of assumptions about the value of society and civilization.
