Checkered rat history in Europe

The once so successful house rat has been almost entirely replaced by the brown rat. © division, CSIRO

An animal career that was marked by ups and downs: Using genetic methods, researchers have gained insights into the distribution history of the black rat, which originally came from Southeast Asia. Accordingly, the rodents settled in Europe in two waves: First, the Romans spread them throughout their empire. After a severe population slump at the end of this era, there was a comeback in the Middle Ages, when the black rat played an important role as a carrier of the plague. From the 18th century their population then declined sharply due to the spread of brown rats, which they have almost completely ousted from us today.

As is well known, humans have made life difficult for many animal species, but there are also counterexamples. As so-called cultural successors, three rodent species in particular have benefited enormously from civilization: the house mouse (Mus musculus), the brown rat (Rattus norvegicus) and the black rat (Rattus rattus) have adapted to the human habitat with its plentiful food supply and also from ours transport systems benefited. In this way, the three invasive rodents were finally able to spread worldwide.

The focus of the study by the international research team led by David Orton from the University of York was now specifically the history of the black rat in Europe. In Germany, this species, which is rather delicate compared to the brown rat, is now considered to be threatened with extinction – but until the 18th century it was quite different: At that time, it fulfilled the role that its cousin, who immigrated from Northeast Asia, has taken on today. However, the black rat is also not a species originally native to us: It came from Southeast Asia, but was then widely distributed by sea traffic, among other things. This has also earned her the alternative name “ship rat”.

Tracking down rats through genetics

To gain clues to the evolutionary history of black rat populations 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 first to the 17th century. By comparing the genetic traits, conclusions could be drawn about changes in rat populations over this period in connection with developments in human society, the scientists explain.

As they report, their results prove that the black rat colonized Europe at least twice: once at the time of Roman expansion, then again in the Middle Ages. “We were able to confirm that the spread of rats was linked to human historical events, and specifically that Roman expansion brought rats north to Europe,” says David Orton of the University of York. What was special about it was that it was obviously an extremely homogeneous rat population, as was evident from the genetic comparisons. “All our Roman rat bones from England to Serbia form a single group genetically,” says the researcher.

After the collapse of the Roman Empire, the archaeological evidence shows a sharp decline or almost complete disappearance of the rats, the researchers report. According to them, this was probably related to the collapse of the Roman economic system. Because the rats were probably heavily dependent on the complex grain trade and the corresponding storage in the empire. However, climatic changes and the so-called Justinian plague in the sixth century could also have played a role in the population decline. Because the rodents could not only have transmitted the pathogen, but also succumbed to it themselves.

Comeback and fall again

But in the Middle Ages, the rodents made a comeback, the researchers report. When cities and long-distance trade revived at this time, the black rat also spread again. The rodents are also associated with the transmission of the Black Death. As the study shows, the rat populations of the Middle Ages did not result from a strengthening of small remnants in Europe: On the other hand, there are signs of a newly immigrated rat group: “When the rats reappeared in the Middle Ages, we see a completely different genetic signature – and in this case too, all our samples from England to Hungary to Finland form a single group. We could not have wished for clearer evidence of the repeated settlement of Europe,” summarizes Orton.

The descendants of the medieval comeback then shaped Europe’s rat populations until the 18th century, the researchers report. After that, however, their stocks declined sharply. Apparently they were displaced by the expanding brown rat, which is now the dominant species in temperate Europe. “The dominance of migration rate today has overshadowed 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 the former black rats now show how closely the population dynamics of these animals were linked to those of humans”.

As the first author He Yu from the Max Planck Institute for Evolutionary Anthropology in Leipzig concludes, the study could even provide clues to the migration paths of humans in history. “This study is a great example of how the genetic background 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, which are often not considered important,” says Yu.

Source: Max Planck Institute for Evolutionary Anthropology, Article: Nature Communications, doi: 10.1038/s41467-022-30009-z

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