What is a trade rat?: Information Roundup
A large wood rat found in the western United States, Canada and Mexico is known as the trade rat or pack rat from its habit of carrying all sorts of things from one place to another and sometimes leaving something else in exchange. This species, of which there are eight known subspecies differing chiefly in coloration, has prominent [page 37] whiskers and a long, bushy, squirrel-like tail. These bushy-tailed mountain and wood rats do not confine their thievery to things they can eat or use in their nests. Stolen articles may be replaced with anything from mere rubbish to valuable gems. In fact they seem to exchange articles out of sheer mischievousness. There have been many amusing reports about the pranks of these industrious rats. They are attracted to bright-colored trinkets and have been known to carry away table utensils, bits of cloth, buttons and many other articles. On one occasion trade rats removed some rice from a bowl and refilled the bowl with collar buttons. In 1943 a woman in Arizona placed small packages of poison in her garden to kill trade rats that had been eating young plants. The packages of poison disappeared the first night. On the supposition that the rats had been killed by the poison, the woman planted some more seeds. The next day she discovered that the trade rats had carried away the seeds and left in their place the tiny packages of poison they had carried away two nights before. Trade rat is sometimes used figuratively in the sense of "idle acquisitiveness" or "the accumulation of useless property."