An animal career marked by ups and downs: Using genetic methods, scientists gained insight into the distribution history of the black rat, which was originally native to Southeast Asia. Accordingly, rodents settled in Europe in two waves: first, the Romans spread them throughout their empire. After a major population collapse at the end of this era, there was a comeback in the Middle Ages, when the black rat played an important role as a plague carrier. Since the 18th century, their population has declined sharply due to the spread of brown rats, which today have almost completely driven us out.
As is well known, humans have made life difficult for many animal species, but there are also counterexamples. As so-called cultural successors, three species of rodents in particular have benefited greatly from civilization: the house mouse (Mus musculus), the brown rat (Rattus norvegicus) and the black rat (Rattus rattus) have adapted to the human habitat thanks to their abundance. have benefited from our food supply as well as our transport systems. Thus, the three invasive rodents could eventually spread around the world.
The subject of research by an international research team led by David Orton of the University of York was now specifically the history of the black rat in Europe. In Germany, this species, quite delicate compared to the migratory rat, is now considered to be threatened with extinction – but until the 18th century it was quite different: it played the role that its emigrated cousin Northeast Asia adopts today. However, the black rat is also not a species originally native to us: it came from Southeast Asia, but was then widely distributed, inter alia, through shipping. Thanks to this, it also gained an alternative name “ship rat”.
Tracking rats using genetics
To obtain clues about the history of the evolution of the black rat population in Europe, Orton and his colleagues examined traces of genetic material from black rat remains found during archaeological excavations in Europe and North Africa. They cover the period from the 1st to the 17th century. The scientists explain that by comparing genetic characteristics, conclusions can be drawn about changes in rat populations during this period due to the development of human society.
As reported, their results prove that the black rat colonized Europe at least twice: once during the Roman expansion and then again in the Middle Ages. “We were able to confirm that the spread of rats was linked to human historical events, namely that the Roman expansion brought rats north to Europe,” says David Orton of the University of York. What was special was that, of course, this was an extremely homogeneous rat population, based on genetic comparisons. “All our Roman rat bones from England to Serbia are genetically one group,” says the researcher.
After the fall of the Roman Empire, archaeological evidence points to a sharp decline or almost disappearance of rats, scientists say. According to them, it was probably related to the collapse of the Roman economic system. Because the rats were likely to be heavily dependent on the complex grain trade and associated storage of the empire. However, climate change and the so-called Justinian plague in the 6th century may have contributed to the decline in population as well. Because rodents could not only transmit the pathogen, but also succumb to it themselves.
Come back and drop again
But in the Middle Ages, the rodents made a comeback, scientists say. When cities and far-reaching commerce revived at this time, the black rat also spread again. Rodents are also associated with the Black Death broadcast. As the study shows, the rat populations in the Middle Ages were not due to the reinforcement of small remains in Europe: On the other hand, there are signs of a newly migrated group of rats: “When rats reappeared in the Middle Ages, we see a completely different genetic signature – and in this case all our samples from England through Hungary to Finland form one group. We couldn’t have dreamed of any clearer evidence of the resettlement of Europe, ”concludes Orton.
Scientists report that the descendants of the Medieval Return shaped rat populations in Europe until the 18th century. But then their supplies plummeted. They are said to have been superseded by the expansive migratory rat, which is now the dominant species in temperate Europe. “The current dominance of the rate of migration has eclipsed the fascinating history of the black rat in Europe,” says co-author Greger Larson of the University of Oxford. His colleague Alex Jamieson adds: “The genetic signatures of former black rats now show just how closely the dynamics of these animal populations were related to the dynamics of humans.”
As the first author concludes, He Yu of the Institute of Evolutionary Anthropology Max Planck in Leipzig, the study may even provide clues about human migration pathways throughout history. “This study is an excellent example of how the genetic makeup of species living around human settlements, such as the black rat, can reflect human historical or economic events. We can still learn a lot from these little animals that are often not considered important, ”says Yu.
Source: Max Planck Institute for Evolutionary Anthropology, Article: Nature Communications, doi: 10.1038 / s41467-022-30009-z