Oh, Rats!

In disease research, rats have the lamentable circumstance of being poked and prodded as a test subject. Today I am turning the tables to look at rats in a different way – by reading the literature on the evolutionary biology of rats and mice.

A study performed by Fordham University on brown rats in New York City revealed a close genetic linkage to Western Europe. Additionally, uptown rats had distinct genetic structures & clustering when compared to downtown rats. They can be further categorized into specific rat neighborhoods. Rats tend to stay within 30-40 meters of their home but can travel hundreds of meters to find a mate.

A paper published in Molecular Ecology on white-footed mice in the Big Apple found that different mice populations within the city can be categorized by their locations. It was also discovered that the mice populations diverged genetically within the last century (Munshi-South & Kharchenko 2010).

There is disagreement in the literature surrounding the genetic diversity of white-footed mice. Depending on the genetic markers that evolutionary biologists use to determine diversity, separate conclusions may be reached. One study from Evolutionary Applications open access journal arrived at the determination that mice populations within the city had lower genetic variation than those in nearby suburban or rural areas. Central Park was listed as an exception to that rule, containing “one of the most isolated and unique NYC populations, along with Fort Tilden and Jamaica Bay” (Munshi-South, Zolnik, and Harris 2016).

So, it appears as though urbanization caused physical separation, and later on genetic separation of white mice populations throughout NYC. Urbanization can act as a selection pressure for many species, preventing individuals with certain traits from passing on their DNA and allowing others to prevail regardless of whether or not those traits are truly advantageous to their fitness. One example of this last point is the peppered moth.

Maybe you’ve heard of the peppered moth before in a biology class? There are two versions of the peppered moth: black and white. It’s a testament to natural selection because during the Industrial Revolution, black moths were more likely to blend in with their surroundings, while white moths were more likely to get eaten and not pass on their DNA. It appeared as though moths were suddenly becoming black, but the truth was that white ones just weren’t surviving. This story is also relevant in regards to urbanization as a selection pressure. Some survivors, like rats and black moths, aren’t necessarily the “fittest” but they survive because it works in that environment.

Did you know that non-native island rats can have a negative impact on coral reefs? Perhaps the most interesting rat study was one published in Global Ecology and Conservation regarding this exact predicament. Rats are opportunistic feeders that have been known to eat anything from crabs and baby turtles to sandalwood fruit and even bird eggs. When rats eat bird eggs and kill vegetarian crabs, birds can no longer drop guano in shallow water near the reef. As a result, those nutrients cease to benefit fish or corals. Additionally, crabs won’t be able to minimize vegetation growth along the shore and bird habitat will be replaced with trees which aren’t ideal for nesting.

The study goes into depth on which species are impacted by island rats, whether indirectly or directly, but also points out the fact that some species have already gone extinct because of this issue.

“Pre-European losses may exceed 2000 species and are principally of rails, pigeons, doves, parrots and passerines, mainly due to predation by non-native mammals—primarily rats, along with the dogs and pigs brought by Polynesians and hunting by humans themselves.” Harper & Bunbury, 2015

Although it’s pretty difficult to exterminate non-native island rats, a suggested solution for increasing genetic diversity in populations of white-footed mice in NYC was to increase connectedness to combat habitat fragmentation. This issue is applicable to civil engineering and construction as well because highways separate many creatures, but wildlife bridges serve as a way to let them travel safely.

Have a great Sunday, Jess 🙂