An ongoing study by Dr. Dougie Clarke, head of biological sciences at the University of Huddersfield’s School of Applied Sciences, UK, has shown that widespread use of rodenticides has led to rats becoming resistant to previously effective, commonly used rat poisons. Over time, in areas treated with these poisons, the rat population will evolve exclusively into the Super Rat type that pass the resistance gene to their offspring.
And, it gets worse. Australian Museum naturalist Martin Robinson reported, “A lot of suburban rats have become immune to the poison baits. And that when they become immune to the bait, they can actually become addicted to it.”... http://ecowatch.org/2012/world-of-the-super-rat/