http://www.dallasobserver.com/news/dallas-has-a-rat-problem-and-it-may-be-getting-worse-7321281

"Rats have evolved to depend on us. They are highly intelligent and acrobatic, with noses capable of snuffing out traces of poison down to 1 part per million. They have sex up to 20 times a day (so if you're reading this in or around Dallas, chances are you're not far from rats going at it, making more)."