In 1947, to keep a close eye on his charges, Calhoun constructed a quarter-acre “rat city” behind his house, and filled it with breeding pairs. He expected to be able to house 5,000 rats there, but over the two years he observed the city, the population never exceeded 150. At that point, the rats became too stressed to reproduce. They started acting weirdly, rolling dirt into balls rather than digging normal tunnels. They hissed and fought.