In the beginning of chapter one, Steinbeck takes care to describe the Salinas River. It is an idyllic scene, and peaceful. Then, he describes the animals as part of the setting.
On the sandy bank under the trees the leaves lie deep and so crisp that a lizard makes a great skittering if he runs among them.
In this case, the lizard is used to further describe the landscape and the setting. Notice that the lizard is somewhat personified, since it is described as “he” instead of “it” for the pronoun. The animals own the scene.
Rabbits come out of the brush to sit on the sand in the evening, and the damp flats are covered with the night tracks of ‘coons, and with the spreadpads of dogs from the ranches, and with the split-wedge tracks of deer that come to drink in the dark. (ch 1)
When the men to do appear, it is as if they are encroaching on the territory of the animals. They interrupt the peace.
And then from the direction of the state highway came the sound of footsteps on crisp sycamore leaves. The rabbits hurried noiselessly for cover. (ch 1)
When the men are described, it is with animal imagery. George is described as “small and quick, dark of face, with restless eyes and sharp, strong features” like a rabbit. Lennie is described as “dragging his feet a little, the way a bear drags his paws” (ch 1). The animal imagery is continued when Lennie thrusts his head into the water, like a dog.