Brown rats are found throughout the world, on its continents and islands, affecting human health and biodiversity, but where did they originally come from? Researchers this month sought to answer that question when they released a global phylogeography (genetic map) of brown rats from cities and islands around the world, published in the Proceedings of the Royal Society B, and in the new documentary Rats. Just like Polynesian rats don’t come from Polynesia, and black rats are not always black (but they do come on ships), brown rats, otherwise known as Norway rats, don’t come from Norway, although they are usually brown, except in the laboratory or pet store. In fact, the brown rat originated in northern China and Mongolia.
Using cutting-edge molecular biology tools – single nucleotide polymorphisms (SNPs) – the team of researchers led from New York found that the rats spread in multiple waves out of Asia. One wave spread east through the Aleutian Islands to western North America (brown colours on the map), while another spread west to Europe as early as the 1500s (blue colours on the map). From Europe, the rats became sea-faring and reached the east coast of America, Africa and Australasia rapidly. In New Zealand the rats showed two pedigrees, the southern one still from Europe but more admixed with China (broown and blue colours), potentially from origins with the fur seal trade, also found for house mice in the same region.
Particularly interesting was the result that in cities, brown rats appear to resist genetic immigration. In this regard cities appear to behave like islands, and once one rat lineage has established, it establishes a priority effect, or incumbent advantage, which prevents new arrivals establishing. As well as the implications this has for eradications of rats from islands, it may also have impacts on human health, by preventing new rats transporting new diseases from establishing in local populations.