Prior to the mid-19th century, squirrels were thought of as fantastic woodland creatures, rather than the urbane, city-dwelling vermin they are today. In fact, the available evidence suggests that, up until this point, there really weren't a whole lot of squirrels living in cities in the United States — at least, not with the ubiquity that they now do. What changed? A couple of things, according to a paper published in The Journal of American History. First, human architects and city planners got really into the idea of urban greenspace for the first time, constructing elaborate parks like Central Park in New York. Second, the humans then imported squirrels from the countryside to add to the bucolic ambiance they were hoping these parks would foster. The rest, as they say, is all rodent breeding and natural selection.
A couple of years ago, Cory posted a really interesting story about the mathematics behind seemingly cause-less traffic jams. It's pretty interesting. Shorter version: The researchers think jams like this are caused by one person braking, and the response to that slow down moves through dense traffic in a way that is mathematically very similar to the shock wave from an explosion. Once you have enough density of cars on a road, jams are inevitable.
Cory's post included a simulation, showing what the mathematics might look like in the real world. Basically, a computer algorithm figured out how drivers would behave if the mathematical theory were correct and turned that behavior into a little cartoon of cars moving around a track.
But here's the really cool thing. This effect has actually been demonstrated in meatspace. Yesterday, a friend sent me a video from 2008, showing real life drivers behaving in almost the exact same way as the simulation video from Cory's post. That's what you see posted above. Now, these are not exactly real-world conditions. A flat circular track may, or may not, be a good representative for what happens on the highway—I, for one, would be interested in seeing how on/off ramps, hills, and curves change the patterns. Also, the drivers in this case were other students and faculty from the Nakanihon Automotive College, and the study doesn't say whether they knew why they were driving in circles. Again, these details could affect the outcome.
I've not been able to find any studies that test this mathematical model by documenting real-world traffic flows. But if you've got links, I'd love to see them! The idea behind this theory certainly makes sense and it would be interesting to know whether it matches up with the reality you and I experience.