Symbolism, Imagery, Allegory
The line between human and animal gets pretty blurry toward the end of the book, when the narrator decides (as part of her effort to connect with the spirits of her departed parents) to shed all the trappings of modern life and live like an animal.
Even before that, though, we get the sense that the novel wants to take human beings down a peg and remind them that they're not really so different or that much better than animals. Being like an animal is a supremely good thing in this novel's universe. For example, the narrator loves her boyfriend's furriness and more "vestigial" qualities:
I remember the hair on Joe's back, vestigial, like appendices and little toes: soon we'll evolve into total baldness. I like the hair though, and the heavy teeth, thick shoulders, unexpectedly slight hips, hands whose texture I can still feel on my skin, roughened and leathery from the clay. Everything I value about him seems to be physical: the rest is either unknown, disagreeable or ridiculous (6.24).
Joe's physicality might seem even more desirable to the narrator just then because it serves as a stark counterpoint to the empty intellectualism and psychobabble that David spouts (and uses to hide the fact that he's not very nice).
In the novel, animals are often discussed or used to highlight the stakes of violence and evil. When the narrator and her friends come across a murdered heron while they're hiking, it's one of the biggest, most emotional moments for the narrator—bigger, even, than the discovery of her father's body. Even after they've moved on in the hike, the narrator remains preoccupied with the evil act of killing a bird and stringing it up to rot "just because"—that is, not for food but just because you can.
The narrator seems to have inherited her respect and even reverence for animals and the natural world, and a sense of the delicate balance of human-animal relations, from her dad. Like her, he almost seemed to prefer animals: "He didn't dislike people, he merely found them irrational; animals, he said, were more consistent, their behavior at least as predictable. To him that's what Hitler exemplified: not the triumph of evil but the failure of reason" (6.37). Although being "animalistic" is often perceived as being one and the same with being irrational, in Surfacing it's more a sign of being disconnected from a perverted form of "humanity" that is anything but humane.