The brown rat has managed to populate New York city by the millions, with billions more in other cities across the world. They contaminate crops and food supplied and destroy wires, walls, and cars, while spreading bacteria and disease. However, scientists are not entirely sure as to how Rattus norvegicus became so rampant. Dr. Munshi-South and colleagues finished a study of brown rats that began to answer the question of what is a New York city rat and where did it come from? They found that these rats began spreading slowly for thousands of years, but in the past three centuries they spread much more rapidly. Dr. Munshi-South contacted researchers from around the world and obtained DNA from hundreds of brown rats, from Japan, New Zealand, Brazil, and the Galapagos Islands to name a few, to compare to that of the rats of NYC. The study revealed that brown rats originated in northern China or Mongolia and fed on wild plants and small animals, until farming in China began, during which rats found a reliable food supply and moved from open plains to farms and villages. The rats then began to expand to other parts of Asia, and eventually west to Europe. It is these rats that are the ancestors of the inhabitants of New York today. As European countries colonized the Western world, they took the rats with them.
Dr. Munshi-South thought that he would find that New York's rats have a mix of genes from ancestors from all over the world, but he found little evidence of genetic mixing in New York or any other city. It seemed that there was not many migrants that arrived and reproduced after a city was already populated with rats. This could be because brown rats are territorial and simply mean, as noted by missing eyes and scars observed by Dr. Mushi-South in many rats. The theory is that the first rats to arrive in a city reproduce rather quickly, so when new rats come along, they are no match for the residents that have already inhabited the city. This is a good thing because it is unlikely that new diseases on new rats can come into a city that is already populated with rats.
Based on the results of the study, I think that we should not take action to eliminate brown rats. Although they do spread bacteria and destroy wires and walls, they are keeping out lots of potential disease because of their territorial nature.