Manhattan Has Two Genetically Distinct Groups of Rats

Just like deserts or forests, cities are functioning ecosystems. Although they happen to be dominated by humans and concrete, these ecosystems are still home to many wildlife species that have long been overlooked. Take rats for example. Brown rats are nearly entirely dependent on humans for resources, which suggests that their evolutionary histories might be linked to the ways humans build and organize their urban landscapes. No one knows this like Matthew Combs, a PhD candidate in the Department of Biological Sciences, at Fordham University. His dissertation focuses on the ecology of urban rats. Matthew wants to understand how interactions between rats and their urban landscape change the evolutionary outcomes of rat populations.

Matthew and his colleagues have spent the past two years trapping New York City rats and sequencing their DNA to put together the most comprehensive rat genetic portrait. When he took a closer look at their DNA, the influence of the city on the rat population was quite clear. They found a clinal pattern, meaning that rats further away from each other were less genetically related. In fact, they found two genetically distinct rat subpopulations in Manhattan: Uptown rats and Downtown rats. The two subpopulations were separated by Midtown, where the rats had lower levels of diversity, a reduced population size, and reduced migration rates. These results were encouraging because they showed that the human environment had real effects on the distribution of genetic variation of rats. Midtown has fewer residential spaces and more commercial ones making it a less appealing to rats (who eat household trash and often shelter in backyards).

For their next project they are looking at the rats of Vancouver, New Orleans, and Salvador, and New York, four cities with different climates and development patterns, to see if they see the same evolutionary outcomes. Then they will look at the features of the urban landscapes themselves to identify which aspects (distribution of park space, different types of sewer infrastructures or the socio-economic status of human residents) have the highest impact on rat gene flow.

Admittedly it was not the rats that drew Matthew to Fordham, but rather his now advisor Professor Jason Munshi-South. Jason had been featured in a New York Times article about the biology of cities that Matthew read as an undergrad and was eager to find new students. When it came time to apply for his PhD Matthew was excited to potentially work with someone who had been one of his first inspirations to study urban ecology. While he didn’t have a specific interest in rats, Matthew has always had a fascination with “weird” animals like bugs and parasites. “Other species that some might find gross, I find intriguing. So, when I had the opportunity to study rats I jumped!” he said.

“Rats have been present in human environments for hundreds of years and we can expect that they will remain in our environments for thousands more,” says Matthew. But just because they’ve been around forever doesn’t make them any less annoying. Rats consistently damage buildings and infrastructure, so learning how to manage their movements could have a large economic impact by reducing the need for repairs. Plus, there’s the public health risk from rat-borne diseases. The more researchers learn about rat biology, the better we can manage their populations and reduce the risk of rat-borne diseases. “These types of contributions can lead to safer urban environments for humans,” says Matthew, “And generally increase our understanding of how to study widely distributed and commensal populations.”