Earlier this year, the Freakonomics podcast conducted a fascinating interview with Kathleen Corradi, New York City’s “rat czar” (official title: director of rodent mitigation). The city, with a staggering 3 million rats, even hosted a national “rat summit” last year. The Top 10 Rattiest Cities in America (thanks Orkin and Terminix) probably won’t surprise you — New York, Los Angeles, San Francisco, Chicago and Washington, D.C., lead the pack — but some “ratical” solutions to end the rat race might. Baltimore, for instance, has tested non-poison “rat contraceptives” in place of traditional bait. Denver has closed entire public parks. Boston last year adopted a rodent action plan, while New York City created a 99-page mitigation report. In Philadelphia County (Pennsylvania) 30% of households reported rat infestations. These cities have much in common, including dense urban housing and aging infrastructure that create ideal rat breeding grounds. As Corradi said: “What we’re effectively doing is making their lives more stressful and cutting off their superpower to breed.”