Phew. There's so much animal imagery in this book, we'd have a hard time pointing it all out, but here's a start:
Nick "prowled around the circumference of the rug" (2.155)
"Nick bared his teeth at her in a silent snarl" (3.206)
Nick "felt like an animal going back into a harness" (9.174)
Nick felt "like a clumsy animal who should not be allowed in here" (11.13)
Nick felt like "a savage dog kept on a chain so he would not fly at throats. He felt like flying at throats. He made a sound that was almost a snarl" (14.56)
Nick felt like "a tiger in a cage. Arthur looked at him with gentle interest, and Nick only just stopped himself from snarling again" (14.62)
See what those all have in common? Yep—Nick. He's always snarling and growling and stalking and prowling, which makes us start to wonder about his savagery and what it might mean. So ultimately, all of this animal imagery really helps us to zero in on the theme of Man and the Natural World, which you probably want to know all about so just hop on over to the "Themes" section.