Cats & rats go together like, well, soup and salad (rats playing the role of both courses :-).  Cats have been prized as rat-catchers on farms for many centuries.  And, it is widely believed that the persecution of cats that began in the Dark Ages allowed the rat population in Europe to grow uncontrolled and led to the Bubonic Plague, which killed 25,000,000 Europeans in just three years.

So, how effective are cats at reducing rat populations in urban envrionments?  Scientists from  Johns Hopkins University and the University of Florida recently studied this question, coming to some surprising conclusions.

The scientists performed the study in a natural environment: alleys in Baltimore.  The study required that the scientists introduce some am,using new measurement units: “rats/alley”  and “cats/alley”  (I’m thinking these are not likely to become internations standards any time soon). The cat population averaged around 3 cats/alley during the period of their observations of 20 alleys.  The rats were measured at upwards of 50 cats/alley during this time.

 The study (which ran for three years) came to two surprising conclusions:

1. The rat population was not affected by the cats predation.  Conversely, the cat population is not affected by changes in the rat population (done via artificial means).
2. The cats overwhelmingly chose smaller (typically younger and healthier) rats, as opposed to larger (typically older and and less healthy) rats.

The first point seems counterintuitive, but I suspect it holds true in crowded environments where the species’ natural reproduction rates are very high, but the actual reproduction rate is limited by space.  In other words, the rat population will expand rapidly to fill available living space.  Once filled, reproduction slows down to the point where it is just frequent enough to replace dead rats.

So, no matter how fast the cats can eat the rats, the rats will reproduce fast enough to fill the available space.  Note, that even with no cats munching on them, the rat population will not expand greatly.

Beware: I am an amateur and could be completely off base here.

Point two I find very confusing.  General biological wisdom holds that predators weed out the weakest, sickest prey, primarily because they are easier to catch.  The opposite appears to be in play here.  There is obviously come aspect of natural selection that we are misunderstanding.

I encourage you to read the full article, though it is a little technical at places.