An odd beginning, I suppose, but let's talk about pee.

The smell of urine is common in Boston and Cambridge. It comes at you where you most expect it, like rounding a stairway corner on the way to a Red Line platform, but also where the whiff is more surprising. I have been especially puzzled, and increasingly nauseated, by the reliable presence of acrid pee smells each time I enter the passage in Harvard Square that cuts between City Sports and Crate and Barrel so one can walk from Brattle St. to Mount Auburn St. I've taken to breathing through my mouth as I pass.

I find myself wondering, why does it smell like urine every day, in that same spot? Do tipsy bohos stagger up from the Casablanca in the wee hours to relieve themselves against this icon of modern architecture and crass commercialism? Is it a favorite boundary marker for neighborhood dogs? And yet I never see an actual puddle there, or even a hint of dampness. Does the smell live underground and waft up through the storm drain?

I have a new idea: rats. I just finished reading a mostly fascinating, and self-admittedly disgusting at times, book about the urban natural history of rats: Rats: Observations on the History and Habitat of the City's Most Unwanted Inhabitants.

Author Richard Sullivan makes it clear that every large city is home to rats, and that even if you never see them, there are probably some living nearby. I had always assumed it was people who were responsible for the urinary bouquet, but it now seems much more likely to be rats. They come out every night and follow the same paths to and from their nests, and they prefer to move along next to walls or other objects. The book doesn't specifically mention the toilet habits of rats, but it's easy to imagine they piddle while they work, and rat-sized pee spots might dry up by morning.

Not that I find this idea reassuring, but it at least seems a more likely explanation than humans or dogs.