In the novel The Grapes of Wrath what is a big cat?

A caterpillar tractor. The main characters, the Joads, and their neighbors are sharecroppers who have been farming the same plots of land in Oklahoma for over seventy years. Each family has a forty acre plot which they farm by hand with a mule or horse drawn plow. under the pressures of the Great Dpression the company which owns the land is forcing the tenants off of it and switching to modernised farming methods using tractors. One hired hand with a tractor can farm far more land than a family with a mule at far less cost. The men with the tractors, the cats, are plowing the fields and nocking down the sharecroppers' houses to force them off the land.