Brown rats – those fuzzy blurs in the tunnels of New York, Hong Kong, London and everywhere in between – originally hail from Mongolia. Though they are one of the most widespread species in the world, second only to humans, they actually immigrated fairly recently. While black rats and house mice spread with agriculture over the course of millennia, it is only within the last few centuries that brown rats became globetrotters. Now, for the first time, a team of scientists are investigating their journey to world domination.
‘The brown rat did not appear in Europe until the 1500s,’ says Emily Puckett, an evolutionary biologist at Fordham University. ‘This means that their range expansion was a response to relatively recent increases in global trade.’ With a team of genetic researchers, Puckett has discovered how the brown rat scurried out of Mongolia along five different trade routes. One group headed to Southeast Asia, while another took the Silk Road to Europe. Meanwhile, two groups made it to North America by ship: the first taking a direct route straight across the Pacific, the second by island-hopping along the Aleutian islands to Alaska. Finally, the group that had spread into Europe then expanded into Africa, Australasia, South America and eastern North America. Their ubiquity followed the growth, and often colonisation, of human populations into other regions.
“We thought that port cities would have high genetic diversity, but it seems that once they fill the available habitat of a city, new immigrants may be excluded”
So why were brown rats slower to travel than other rodents? Puckett believes this is because Mongolia did not connect to global trade routes until later in history. The house mouse, for example, stems from the fertile crescent region in the Middle East, an area that provided more opportunity to spread. More surprisingly was how little the rodent’s gene pool mixed once the populations were established abroad. ‘For example, we thought that port cities, being globally connected, would have high genetic diversity,’ Puckett says, ‘but it seems that once they fill the available habitat of a city, new immigrants may be excluded.’
This was published in the December 2016 edition of Geographical magazine.