Scientists work to reveal the secret lives of urban rats

September 22, 2016

Urban areas suffering from seemingly ever-increasing rat populations is nothing new. However, because rats are small, vigilant and live mainly underground, even behavioral ecologists know very little about how they move through cities and interact with their environments.

That’s a problem, one article explains, 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.

Recently, a group of scientists studied urban rats to help fill some gaps in our knowledge, especially pertaining to when and where particular pathogens enter a given rat population. The researchers explained in the article that these studies are the first ever to analyze wild city rats at the level of the individual in a major U.S. metropolitan area.

Rats and diseases go hand-in-hand because rats often make their way into homes from parks, subways and sewers, transporting microorganisms they pick up from decomposition of wastes along the way. They are known to spread diseases directly by passing infectious agents through their blood, saliva or wastes, and indirectly by serving as hosts for disease-carrying arthropods such as fleas and ticks.

Rats are known vectors for Lyme disease, Rocky Mountain spotted fever, Toxoplasma, Bartonella, Leptospira and other microorganisms, many yet to be named. In fact, a seminal 2014 study found 18 novel viruses in 133 rats collected in Manhattan alone.

While obviously disturbing, the problem is amplified because 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.

Although densely populated, rats are still difficult to study due to their size, and lifestyle. However, researchers have made some progress learning more about them by luring city dwelling rats through the use of pheromones – natural scents that they find irresistible. Once captured, the researchers implanted radio-frequency identification (RFID) microchips under their skin, so they could identify each animal individually.

After the researchers release the microchipped rats, they used scents to attract them back to specific areas and monitored when and how often they returned. Using camera traps and a scale that the rats walked across, the researchers could track their health by assessing weight changes and looking for new wounds and bite marks. The researchers repeatedly collect biological samples, including blood, stool and DNA, to document the rats' potential to carry pathogens.

One specific finding the researchers noted was that male rats forage basically 24 hours per day, while females do so only during late mornings. In addition, females and males were equally attracted to scents from lab rats introduced in the study, and females responded to pheromones at the same rate as males.

The researchers have published their detailed methods as a roadmap, so other scientists can replicate this research. Using this approach, the researchers believe they can in the future learn when and where particular pathogens enter a given rat population. Having this information could be very beneficial when it comes to discovering new ways to control rat populations and the spread of disease to humans.

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