In The Grapes of Wrath, what is the significance of animal imagery?

In Steinbeck's wonderful novel The Grapes of Wrath, the author frequently uses animals as symbols for the state of well-being of the family members; there are many examples of this throughout the novel, and the animals more often than not symbolize struggle.
One example can be found in the cat that lives at the Joad farm. When Tom returns home from a stint in prison, he finds that the farm is almost entirely abandoned apart from the cat. The cat refuses to go near Tom, for it has become entirely independent in its means of survival. It can no longer rely on others—for it had surely stopped receiving food once the people left—and thus it's forced back to pursue its primal nature as it turns "directly to one of the little piles of rabbit entrails on the ground."
Another example of animals acting as symbolism for the Joad family's struggles is the dog that gets run over by the truck. After a gruesome and painful death, Pa admits that there was no true way to be able to feed the dog. The dog's death is cruel, unexpected, and wholly grotesque, yet the death ends the struggles that the dog would have faced. In a sense, the dog shows the desperation of the family while also symbolizing the cruel, dangerous nature of the outside world.