Are sharp teeth necessary to survive the rat race?

Two warm-blooded mammal species have triumphed in the fast-paced environment of New York City: human beings and rats. Robert Sullivan was fascinated with how eager many people are to save whales or dolphins, while they are rather revolted by an animal they live right alongside: rats.  In his book, Rats: Observations on the History and Habitat of the City's Most Unwanted Inhabitants, Sullivan explored rat adaptations to city life.(1)

Sullivan explored a phenomenon that is largely invisible to human eyes. Like a bacterial infection, rat infestation is no less important because it is off the radar. For his research, Sullivan selected an alley a few blocks from Wall St. He found an abandoned McDonald's which seemed empty-at first. When he walked past again, he noticed that the place was covered with rats.  There were so many rats that his eyes had at first been unable to discriminate single rats from the hordes in the background! Fascinated, he parked himself in an alley nearby where he would log countless hours over the following months to study rat adaptations to life in Manhattan.

Sullivan's alley was actually a place where fast food restaurants unloaded their garbage every night. Since rats are nocturnal, their day begins when the sun goes down. They come out initially for a breakfast-type meal, and then retreat to their subterranean passageways. Later, they emerge to continue foraging. Sullivan observed one rat that seemed determined to get a particularly attractive pile of garbage, on a ledge that was over a foot high from the ground. No matter, this normally squat rat jumped all the way up there to reach his lunch!

Rats like to eat the same things humans do.  Even though most of us associate rats with garbage, they will not eat rancid food.  They prefer fried and fatty foods over vegetables. In fact, rat populations often develop a palate that matches up with the ethnic cuisine of the neighborhood they inhabit! Rats only require about 3-4 ounces of food a day, but will continue to eat until a food source is gone. They basically hijack an environment and clean it out. When resources run low, fighting may break out. The runts that cannot hold their own in rat colonies are booted out of the colony. While the group continues to forage at night, this outcast must scurry around during daytime, when things are less competitive. So, if you see a rat during the day, there are two likely reasons: 1) the population is so large that some individuals are being kicked out of the colony, and 2) the rat you're seeing is the runt, so imagine the size of those remaining underground!

The public health issue

Rats breed in filth and carry disease. When the Black Plague spread from rats to humans in the 1340's, it wiped out one-third the population of Europe. Actually, the Plague was carried by fleas that live on the rats, which is an important distinction when we consider modern-day extermination efforts.

After September 11th, a huge proportion of Manhattan's population was suddenly gone. Public health officials worried that the unpopulated and rubble-filled areas at Ground Zero would invite and host an enormous rat population. To preempt a rat takeover, exterminators placed rat poison around the perimeter of Ground Zero, which has helped keep the rat population in check.

Remember all those red and orange alerts? Public health researchers also worried that bioterrorist groups might use the rat population to spread the Plague. Interestingly, experts decided that the smartest way to get rid of rat-borne diseases would be to wipe out the fleas themselves. If one rat population is decimated, fleas might just jump to another rat population.  Even worse, fleas carrying the Plague could jump to the next closest warm-blooded mammal: humans.

Overall, rats are pretty well controlled in Manhattan.  In fact, there's a guy named Derek who lives in an alleyway, and is able to direct hundreds of rats at a time. Sullivan watched him orchestrate the movements of the rat population by beating a piece of metal with a stick.

But when Derek is not around, it's hard to control rats because they are so well adapted to the urban environment.  Rat teeth are extraordinarily strong. They can cut through steel and concrete. Rat teeth score 5.5 on Mohr's scale of hardness -- which is harder than steel! Exterminators now add glass to concrete when they patch holes to discourage rats from chewing through - the glass irritates their mouths. Rats have also evolved to be thigmophilic, or touch-loving. They rely on touch to navigate an environment. For example, if the alley walls were taken down, rats would continue to travel the same path due to the muscle memory telling them where the wall had been the day before.

Rats have evolved to master the same environment that humans inhabit, and thrive on the scraps that humans discard. Luckily, the aggressive efforts of the Public Health Departments that keep rat populations in check are evidence that humans have sharp teeth too.

For more on adaptation and phenotypic variance, take a look at http://www.nature.com/scitable/topicpage/Adaptation-and-Phenotypic-Variance-1132

1 Terry Gross, interview with Robert Sullivan. Fresh Air from WHYY, April 5, 2004. http://www.npr.org/templates/story/story.php?storyId=4571639