In an era when we can decode language among animals and design coatings that make military weapons virtually invisible, it may seem that there are few things science cannot accomplish. At the same time, we are surprisingly ignorant about some things that are much more ordinary. For me, perhaps the most intriguing example is city rats, which in many ways are the most important species of urban wildlife in our increasingly urbanized world.
Because rats are small, vigilant and live mainly underground, even behavioral ecologists like me know remarkably little about how they move through cities and interact with their environments. That’s a problem because rats foul our foods, spread disease and damage infrastructure. As more people around the world move to densely packed cities, they become increasingly vulnerable to rat behaviors and diseases. That makes it critically important to understand more about rats and the pathogens they carry.
I decided to study urban rats to help fill some gaps in our knowledge of how they use their sense of smell to seek favored resources (food and potential mates), and how this attraction influences their fine-scale movements across particular types of corridors.
Small Animals With Big Impacts
Rats like to feed on small quantities of human rubbish while remaining just out of sight, so they have been associated with humans since the rise of agriculture. The ancestors of today’s urban rats followed humans across the great migratory routes, eventually making their way by foot or ship to every continent.
In cities, rats can enter buildings through openings as small as a quarter. They also may “vertically migrate” upward and enter residential dwellings through toilets. Because rats often make their way into homes from parks, subways and sewers, they can transport microorganisms they pick up from decomposition of wastes, thus earning the colloquial nickname of “disease sponges.”
Unlike humans, rats are not limited by the density of their population. In population biology, they are referred to as an “r-adapted species,” which means they mature rapidly, have short gestation periods and produce many offspring. Their typical life span is just six months to two years, but a female rat can produce up to 84 pups per year, and pups reach sexual maturity as soon as five weeks after birth.
Like other rodents (derived from the Latin word “rodere,” to gnaw), rats have large, durable front teeth. Their incisors rank at 5.5 on the Mohs scale, which geologists use to measure minerals’ hardness; for comparison, iron scores around 5.0. Rats use their constantly growing incisors to gain access to food. They can cause structural damage in buildings by chewing through wood and insulation, and trigger fires by gnawing on wiring. In garages, rats often nest inside cars, where they will also chew through insulation, wires and hoses.
