Also the ability to adapt. Many of the animals that wind up near our homes and scavenge around in our garbage cans, and use our sewer lines for their personal highways are actually the ones who are going to make it. Animals with a set character, and highly specialized diet are the ones who are not going to make it. For example, rats are excellent examples of able-bodied animals. They can feed on anything they can find and can quickly change their lifestyle and habits to suit their environment. There are over 1,000 different species of the Muridae family (rats and mice), living testament to their superb survival skills. On the other hand, the felines (cats and panthers) which have one of the most specialized diets among mammals, are not likely to make it. Proof of that is being seen all over in today's world, where several feline species are becoming very scarce, or extinct in the wild. It is believed some, like cheetahs, are so low in numbers because the sperm count in the males is incredibly low. Cheetahs may be totally extinct within the next 50 years.
Rats will not always be the small, sewar-inhabiting mammals they are today, as history has been an indication, rats will surely change in shape and lifestyle. In this site you will see that some may be as big as modern tigers, some bigger. It all depends on environment and ecological needs.