Doing wildlife control work and being a farmer I can tell you for every rat you see there are 10 more. Rats have always been with man and can live and thrive wherever people are, and unless there populations get large enough you will probably never know they were there. I cant tell you how many homes Ive been called to over rats and the people are upset because they believe you have to be dirty to have rats, but as folks know in the big city this just isnt so. That is why the rodent control industry is a billion dollar industry. Ive found that a good poison program is realy the most efficient way to deal with these critters, folks around farms, and public places like the Universities are always going to have larger than usal rat populations, the food and cover these areas provide are a welcome mat.
Dont know Ive told you anything you probably didnt already know through your stdies but this is my 2 cents
josh

Rat Terriers work better then cats here!

As a kid growing up on a farm, we learned to live with rats.

All concepts/strategies used for mice (storage containers, dogs, poison, traps), were just doubled or tripled in strength. The cats usually left these things alone. We would nail down the traps (so we didn't have to retrieve them). I'd try to shoot the ones I could see, but they were pretty sneaky. We never got rid of the rats.

Just my story.

I've not seen a rat here but growing up in Maine I did see them in the barn on more than one occasion. They are nasty but I guess they do serve a purpose. As Josh and Bill7 have stated, for every ONE you see there are many more that you don't. For all the precautions you take for mice you do need to triple it for rats as they are actually very smart and find ways to get around traps and the like. They are clever and their size only adds to the problem. My cats bring in adult rabbits but have never hauled in a rat, whether we don't have any or they choose to not mess with them, I don't know. Some of my cats are very mean hunters. We have an old barn filled with debris from previous owners...old straw/hay and tons of other garbage (not literally garbage but boxes of old magazines, car parts, etc.) so I can't imagine that we don't have them, I've just never seen one here. Lori