The Replacement Nobody Has Explained: Black Rats, Brown Rats and a Benchmark Date

1. Introduction: a practice built on one outcome

Bait station placement, burrow treatment, exclusion at grade, the neophobia our behaviour article described: all of it is written for one animal, which holds its position because of something that happened three hundred years ago.

1.1 Despite is doing a lot of work in that sentence

And the authors put it there.¹

1.1b And the question is not idle

If the replacement happened because buildings or cities changed rather than because one animal beat another, then it is a fact about the built environment and belongs alongside the other articles in this journal that find pest outcomes determined by construction.

1.2 What this article argues

That the replacement is attributed rather than demonstrated, that the dates do not fit together neatly, and that the standard North American arrival figure traces to one nineteenth-century source. Sections 9, 13 and 17 are the case.

2. Two species, two histories

Which is the starting point.

The black rat is one of three rodent species, with the house mouse and the brown rat, to have become globally distributed as a result of its ability to live around human habitations by taking advantage of food and transportation.⁴

2.0b And the strategy is the species' defining trait

What makes all three globally distributed is not tolerance of climate or breadth of diet but the ability to live with people and be carried by them.⁴

2.1 Three species, one strategy

And our house mouse article covered the third, finding a spatial ecology unlike either rat.⁴

3. Where each came from

Different parts of Asia.

Black rats speciated on the Indian subcontinent and brown rats in the northern steppe, which one paper says implies vastly different phylogeographic histories.⁸

3.0b Two origins, two ecologies

An animal that evolved on the Indian subcontinent and one that evolved on the northern steppe begin with different tolerances, and the climate boundary in §23 may be the oldest thing in this article.⁸

That connection is ours.

3.1 And the black rat's expansion is traced

Genetic evidence indicates the global distribution of one black rat lineage began with expansion from the Indian subcontinent into western Asia, followed by separate expansions into Europe and Africa, with derived haplotypes indicating it reached the Americas, Oceania and Africa from Europe.⁹

4. And when each arrived

Black rats reached southern Europe by six thousand years ago and Great Britain by the 300s AD, yet brown rats were not recorded in Europe until the 1500s AD.⁸

4.0b And the incumbent was established, not newly arrived

By the eighteenth century the black rat had been in Britain for fourteen hundred years and in the Mediterranean for several thousand, which is not the position of a species easily pushed.⁸

That framing is ours and it is why §17 bothers us.

4.0c And one date is many times older than the other

Six thousand years in southern Europe against five hundred for the newcomer, which is the scale the chart above cannot show.⁸

4.1 Which is a very long head start

Twelve centuries in Britain and considerably more in the Mediterranean.⁸

5. The black rat came twice

Ancient DNA analysis found the black rat colonised Europe in the Roman period, but its population there declined after the fall of the empire, with a genetically distinct population appearing in the medieval period, perhaps reflecting the re-emergence of long-distance trade.⁶

5.0b And the second population was not the first returning

Genetically distinct, which means a fresh introduction rather than a recovery of whatever survived the first decline.⁶

5.1 Which nobody could have known from the historical record

Two colonisations by genetically distinct populations look identical in any written source, since both are simply rats being present.

6. Which tracked something human

The researchers' own conclusion.

The population dynamics of black rats and humans mirror each other, and could reflect historic and economic events.⁶

6.0b And the post-Roman decline makes the point on its own

An animal whose population falls when an empire collapses is not responding to weather or to predators. It is responding to the volume of grain being moved and the number of settlements storing it.⁶

That reading is ours.

6.1 A commensal population as a record of trade

Which is the same logic our articles on stored product pests and on shipping have used in the other direction, reading pest distribution as evidence about commerce.

7. The evidence for that

Worth describing, because the method is unusual.

DNA was analysed from ancient black rat remains found at archaeological sites spanning the first to the seventeenth centuries in Europe and North Africa, by a team drawing on researchers from more than twenty countries.⁵⁷

7.0b Which is a method our diagnostics article described in another setting

Sequence identity used to establish that separated populations are one thing rather than several, here applied to animals dead for a thousand years instead of to a sample from a building.⁷

7.0c The span is what makes it work

Sixteen centuries of remains means the study observes a population before, during and after each change rather than sampling a moment, which is the same requirement our detection-probability article identified for any claim about a trend.⁵

7.1 And the genetic signal was consistent across the continent

Remains from places as different as England, Finland and Hungary showed it was genetically the same animal spreading to all of them, which one account reports the researchers describing as better evidence than they could have hoped for that the colonisation happened in two big waves.⁷

8. Then the decline

Which is the event this article is about.

Black rats were widespread across Europe until at least the eighteenth century, before their population massively declined, with the brown rat now the dominant species in temperate Europe.⁴

8.1 Massively is the word used

Rather than gradually or partially, which describes a collapse and not an erosion.⁴

8.2 And the reversal is total in temperate Europe

The species that had been there for well over a millennium is now the rarer one across the region, and the newcomer is dominant.⁴

9. What is claimed about why

The decline happened most likely as a result of competition with the newly arrived brown rat.⁴

9.0b And the claim is about a population rather than about individuals

What the evidence records is one species becoming scarce while another becomes common, which is compatible with several processes at the level of the animals themselves.

9.0c And it is offered as the end of one story rather than the start of another

The displacement is where the black rat's history stops, so it appears as a conclusion rather than as a question, which may be why it has been restated more often than it has been examined.

9.1 The same formula appears in every version

A press release, a news summary and an archaeological report all give the cause as most likely competition, which is the same phrase rather than three independent assessments.⁴⁵⁶

10. Note the hedge

Our reading, and it is a small point with a large consequence.

Most likely is the language of an inference from coincidence in time. One population falls as another arrives, and competition is the obvious candidate.

10.0b And the alternative hypotheses are not listed

A displacement coinciding with an arrival has at least two readings: the newcomer drove the incumbent out, or something else reduced the incumbent and the newcomer filled the space. Our sources report the first without mentioning the second.

10.1 Which is not the same as having been shown

Nothing in what we read reports an experiment, a modelled population, or a mechanism by which one species excluded the other.

10.1b Which is not a criticism of the study

A paper tracing one species' colonisation is entitled to note what ended it without establishing why, and the difficulty is in how the sentence travels afterwards. Four of our sources carry it, and a reader meeting it four times has met one claim.

10.2 And the ancient DNA work was not designed to test it

It was designed to trace the black rat's colonisation, and the displacement appears at the end of the story as its terminus.⁵

11. The North American record

Where a study went looking specifically.

Isotopic and protein analysis of archaeological rat remains from the 1550s to 1900 in eastern North America, assembled to provide a large-scale framework for species arrival, interspecific competition, and dietary ecology.¹

11.0b The period covered is the useful part

Beginning in the 1550s, well before any proposed arrival date for the second species, so the record covers the situation before, during and after the replacement.¹

11.1 With the motivation stated plainly

That little is known about the ecology and behaviour of these animals in urban areas due to the challenges of studying animals in city environments, despite their documented impacts and the billions spent annually on control.¹

11.1b And the contrast the sentence draws is the striking part

Billions spent annually on control, set against an admission that the basic ecology is poorly known. A trade that large resting on a knowledge base that thin.¹

11.2 Which our rat counting article said in a different register

That the animal at the centre of a large industry is among the harder ones to study.

12. Earlier than expected

The result.

Brown rats arrived earlier than expected and rapidly outcompeted black rats in coastal urban areas.¹

12.0a And coastal urban areas is the specified setting

Which is where shipping arrives, where density is highest and where the archaeological record is richest, so the finding is about ports rather than about the continent as a whole.¹

12.0b Rapidly is worth noting alongside earlier

The finding is not only that arrival came sooner but that the turnover that followed was quick, which is the opposite of the pattern §17 describes in Europe.¹

12.1 Black rats were there first in the Americas too

Arriving with early European visitors and colonists, including on the first Caribbean crossing in 1492, and proliferating in settlements across the islands and continents thereafter.¹

13. Where the old date came from

Conventional wisdom based on historical accounts suggests brown rats arrived later, likely by the time of American independence in 1776, with most citing a benchmark arrival year of 1775, following a named nineteenth-century author, although earlier dates have been suggested.¹

13.1 Independence is a memorable anchor

Which may be part of why the figure travelled: a date adjacent to a famous one is easier to carry than an arbitrary year.

That is speculation on our part.

14. Which is a pattern this journal knows

And has now found several times.

A figure enters the literature from one source, is cited by the next work, and becomes the benchmark by repetition rather than by evidence. Our articles on callback rate benchmarks, on allergen thresholds and on the claim that most infestations begin through defective piping each found a version of it.

14.0b And the mechanism of propagation is the same each time

A figure that is plausible, specific and convenient gets cited rather than checked, and each citation makes the next one easier, until the original basis is several layers back and nobody has looked at it.

14.1 The difference here is that the review literature is honest about it

Naming the source and noting that earlier dates have been suggested.¹

14.1b And the earlier suggestions were already there

The review literature records that earlier dates had been proposed, so the benchmark was not unchallenged, merely more convenient than its challengers.¹

14.2 And the correction came from a different kind of evidence

Physical remains rather than historical accounts, which is the only thing that could have settled it.¹

15. The trophic problem

Which the study raises about its own conclusion.

The replacement happened despite evidence that the two species occupy different trophic positions.¹

15.0b The word despite signals the authors saw the tension

A finding reported with a concessive clause attached is a finding whose authors expect a reader to object, which is a good sign about the reporting.¹

15.1 Trophic position is what the isotopes measure

Which is why a study using isotopic analysis was able to establish it at all.¹

16. Why that complicates the account

Our reasoning.

Competitive exclusion normally requires overlap: two species contending for the same limiting resource, one better at obtaining it. Different trophic positions means the diets differed, which weakens the simplest version of the story.

16.0b And the finding was possible only because of the method

Historical accounts record which rats people saw. Isotopes record what the animals ate, which is a question no archive could answer and which is what makes the complication visible.¹

16.1 It does not refute competition

Animals compete for harbourage, nest sites and space as well as for food, and direct aggression does not require a shared diet.

16.1b And the two candidates behave differently over time

Competition for a depletable resource produces gradual replacement. Competition for space through aggression can produce a fast one, which is what the North American record describes.¹

16.2 But it does mean the explanation needs specifying

Competition for what is a question the word on its own does not answer.

17. The gap nobody addresses

And this is the thing we found most puzzling.

One source states brown rats became established in Europe by the 1500s. Another states they were not recorded in Europe until the 1500s. The displacement is placed in the eighteenth century.⁸⁹⁴

17.0b And the North American case has the same shape in miniature

Arrival earlier than expected, displacement described as rapid, and a conventional date that sat between them for two centuries of scholarship.¹

17.1 Which leaves roughly two hundred years

Between a species being present and that species displacing the incumbent.

17.1b Or the earlier date refers to something narrower

A first record in one port is not establishment across a continent, and the gap may be the time taken to spread rather than a delay in displacing.⁹

Which would resolve the puzzle and which none of our sources states.

17.2 And a competition account has to explain the delay

If one animal simply outcompetes another, the transition should follow arrival rather than trail it by two centuries.

Sections 17.1 and 17.2 are ours, and no source we read raises the question.

18. What else could explain it

Offered as possibilities rather than as findings.

Something about the eighteenth century may have changed the competitive balance rather than introduced the competitor: building construction, urban density, sewerage, refuse handling, or the volume and pattern of shipping.

18.0b And the eighteenth century is a plausible place to look

It is the period of rapid urban growth, of masonry replacing timber in cities that had burned, and of shipping volumes rising sharply, all of which change what a building offers an animal that climbs versus one that burrows.

18.1 Each would suit one species over the other

Since the two differ in where they live within a structure, which §22 describes.

18.1b Which would make the arrival and the takeover separate events

With separate causes and separate dates, and no requirement that either explain the other.

18.2 And a population already present can expand when conditions change

Which would reconcile an early arrival with a late takeover.

19. Which we cannot adjudicate

Stated plainly.

Section 18 is speculation by a pest control company reading abstracts. We raise it because §17 is a real gap and not because we have an alternative to offer.

19.0b And a negative result would be as interesting

If experimental work found the two species coexisting without exclusion, the historical account would need rebuilding entirely, and we do not know whether such work exists.

19.1 The relevant literature exists and we did not read it

One study's reference list includes a thesis specifically on interspecific competitive interactions between the two species, which is presumably where the question is addressed.²

20. The name

A small matter with a familiar shape.

What is absolutely certain is that Rattus norvegicus does not originate from Norway.⁷ Other names recorded for the same animal include sewer rat, Parisian rat, common rat and Norway rat.⁷

20.0b Which is more than a curiosity for this trade

Common names in rodent control are geographic and misleading throughout: Norway rat for an animal from the steppe, roof rat for one that lives outdoors in warm climates, and house mouse for a species our own article found in fields as readily as in buildings.

20.1 A scientific name known to be wrong and kept anyway

Because nomenclature rules favour stability over accuracy, which is a defensible trade and worth knowing about when a name looks like evidence.

20.1b The name carries an implied claim nobody intends

A species epithet naming a country reads as information about where the animal is from, and in this case it is information about where an eighteenth-century describer thought it was from.⁷

20.2 And the common names locate it by habit rather than origin

Sewer and common being descriptions, and Parisian and Norwegian being guesses.⁷

21. Why the outcome matters here

Which species occupies temperate cities determines what every guidance document assumes about where rats are and how to reach them.

21.1 And the assumption is usually invisible

Guidance rarely says which species it is written for, because in the places the guidance is written it is obvious.

22. The practical divergence

Set out briefly.

The brown rat burrows and works at ground level, which is why bait stations sit at grade along foundations and why our exclusion article concentrated on openings near the ground. The black rat climbs and nests high, which puts it in roof voids, on wires and in vegetation.

22.0b And the detection signs differ with it

Burrow openings, runs along foundations and ground-level rub marks belong to one animal. Droppings in a roof void, damage to overhead wiring and activity in trees belong to the other.

22.1 So placement logic inverts between them

A programme designed for one will miss the other, and the inspection looks for different things.

22.1b And a misidentification costs a whole programme

Ground-level stations against an animal living in a roof void produce no catch and no information, which is the failure mode our article on untouched stations described from the other direction.

22.2 That contrast is drawn from general knowledge

Rather than from the sources here, which concern history rather than control.

23. And the warm-region caveat

Because the replacement was not universal.

The brown rat has spread across the globe except to Antarctica and a few islands, but the black rat rules in warmer regions.⁷

23.0b And it is the best available clue to the mechanism

A replacement that stops at a climate boundary suggests something climate-linked was part of the outcome, whether directly through tolerance or indirectly through what buildings and food storage look like in each region.

That inference is ours, and it is a hypothesis rather than a finding.

23.1 Which means the temperature boundary is the practical one

And guidance written for one climate does not transfer to the other.

23.2 And the black rat has not gone anywhere

Its range shifted rather than shrank to nothing, which is a different outcome from extinction and means the species is available to return if conditions change.⁷

24. What this adds to our plague article

Which examined the attribution of the Black Death to rats and their fleas.

That article found the rat attribution contested and the evidence for human ectoparasite transmission stronger than the popular account allows. This adds that the species conventionally blamed was present in Europe in the relevant period through the second of two colonisations traced by ancient DNA.⁶

24.0b And the two-wave finding has a bearing on the timing

A medieval population genetically distinct from the Roman one means the animals present during the pandemic were recent arrivals rather than a long-settled population.⁶

24.1 Which neither supports nor undermines the attribution

Presence is a precondition rather than an argument.

25. The wider point about commensals

Our reading of what these studies show.

Three species became globally distributed by living alongside people, and the distribution of each records human movement rather than anything about habitat.⁴

25.0b And it explains why the dates are contested at all

An animal moved by shipping arrives where records are kept, and the record of its arrival is a commercial document rather than a biological one, which is why historical accounts and physical remains can disagree as they do in §13.¹

25.0c And it puts a limit on ecological reasoning about them

Asking what habitat suits these species is less useful than asking what people were shipping and where they were storing it.

25.1 Which makes their history a human history

As the mirroring in §6 states directly.⁶

26. What we would take from it

Three things.

The species in front of you is a historical accident. Not a fact about what rats are.

The explanation for it is an attribution. Section 10.⁴

And a benchmark date can be one author repeated. Section 13.¹

26.0b And a fourth, about what the evidence can settle

Written records told us who saw which rats. Bones and isotopes told us when the animals were there and what they ate, and disagreed with the records on both counts.¹

26.1 And one habit worth keeping

When a date or a figure is cited without a source often enough to feel settled, the question worth asking is who first wrote it down.

27. Our own position

The disclosure, which is short.

Nothing in this article affects what we sell or how we work, and we wrote it because the history behind a practice is worth knowing and because the date finding is the kind this journal collects.

27.1 With one qualification

An article whose central move is to question a settled explanation is one we should hold to the same standard, and §§19 and 29 are where we do.

28. The Manitoba position

28.1 The brown rat is the species here

Which follows from §23 and from the climate, and which means every article this journal has written about rats concerns the winner of the replacement.

28.1b Which makes the local history invisible

A species that arrived at a known date elsewhere arrived here at an unknown one, and nothing in the record we could reach fixes it.

28.2 What we could not find

Any date for the brown rat reaching this province, any archaeological rodent work from the prairies, and any record of black rats establishing here.

28.2b Which the rest of the continent's record cannot settle

Coastal port cities have archaeological rat assemblages because they have deep stratified deposits and long occupation. An inland region settled much later has neither, so the method that produced §12 has nothing to work with here.¹

28.3 And the arrival question is genuinely interesting locally

A landlocked region reached overland long after coastal cities, which our article on the prairie rat frontier approached from the control side rather than the historical one.

29. Limitations and open questions

We read abstracts, press releases and news coverage. Both primary studies reach us through their abstracts and through institutional and journalistic summaries, and we did not read either paper's methods or results in full.¹³

That is the most important limitation because §§10 and 15 make claims about what the studies did and did not establish, and an abstract is a poor basis for a statement about what a paper does not contain.

Four of our sources describe one study. A press release, a news article, an archaeology news item and a feature all report the same ancient DNA work, so the repetition of most likely competition in §9.1 reflects a single origin rather than agreement.⁴⁵⁶⁷

The date conflict in §17 may not be one. Established by and not recorded until could both be true if presence preceded record, and our two sources may be describing different things rather than disagreeing.⁸⁹

Two of our sources are preprints. Which we used for speciation origins and arrival dates, and which had not necessarily completed review.⁸⁹

And the control contrast in §22 is unsourced. Recorded as general knowledge in §22.2 rather than taken from anything in this article's references.

Sections 10, 14, 16, 17, 18, 20.1, 22 and 25 are our reasoning. The reading of the hedge, the comparison with other benchmark figures this journal has traced, the argument that different trophic positions complicate a competition account, and the two-century gap are ours rather than sourced positions.

30. Conclusion

Ancient DNA from black rat remains spanning the first to the seventeenth centuries shows the species colonised Europe twice, in the Roman period and again in the medieval, with a genetically distinct population the second time and a decline in between that followed the fall of the empire carrying it.⁶ The researchers report that black rat and human population dynamics mirror each other and could reflect historic and economic events.⁶ Then, from the eighteenth century, the population massively declined, most likely as a result of competition with the newly arrived brown rat.⁴ That phrase appears in four of our sources, which are four accounts of one study, and it is an inference from timing rather than a demonstration. Nothing we read reports a mechanism.

The North American work goes further and complicates it. Isotopic and protein analysis of remains from the 1550s to 1900 found brown rats arriving earlier than expected and rapidly outcompeting black rats in coastal cities, despite evidence that the two species occupy different trophic positions.¹ Competitive exclusion usually needs overlap, so different diets do not refute competition but do require it to be specified: for harbourage, for nest sites, for space, but apparently not for the same food. And the conventional arrival date, a benchmark year of 1775 cited almost everywhere, is attributed in the review literature to one nineteenth-century author, with earlier dates suggested and physical remains now supporting them.¹ That is the fourth time this journal has traced a widely repeated figure to a single origin.

A further gap sits in the middle of the story and none of our sources raises it. Brown rats are described as established in Europe by the 1500s, or as not recorded there until the 1500s, while the displacement is placed two centuries later.⁸⁹ A species that simply outcompetes another should not take two hundred years about it, which suggests something changed in the eighteenth century to alter the balance rather than to introduce the competitor. We cannot say what, and we note that the relevant literature exists and we did not read it.² What we can say is that one species burrows at ground level and the other climbs, that every placement assumption in temperate structural rodent work follows from which one is present, and that this was settled by an event whose cause is still given as most likely.

References

1. The ratting of North America: a 350-year retrospective on Rattus species compositions and competition. Journal article in an open-access general science title, read as its abstract and introduction rather than in full. Source for the description of isotopic and protein analysis of archaeological rat remains from the 1550s to 1900 in eastern North America, assembled to provide a large-scale framework for species arrival, interspecific competition and dietary ecology; for the statement that little is known about the ecology and behaviour of these animals in urban areas due to the challenges of studying animals in city environments, despite documented impacts including disease spread, environmental destruction and billions spent annually on control; for the central finding that brown rats arrived earlier than expected and rapidly outcompeted black rats in coastal urban areas, and that this replacement happened despite evidence the two species occupy different trophic positions; for the account that black rats arrived first in the Americas with early European visitors and colonists including the 1492 Caribbean crossing, proliferating in settlements thereafter; and for the statement that conventional wisdom based on historical accounts places brown rat arrival by the time of American independence in 1776, with most citing a benchmark arrival year of 1775 following a named nineteenth-century author, although earlier dates have been suggested. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC10990262/
2. Publisher's record for the same study, read for its abstract and part of its reference list. Source for the presence in that bibliography of a thesis specifically on interspecific competitive interactions between the two species, of work on the origins of the domesticated brown rat and its pathways to domestication, of a systematic review on the ecology of rat-associated zoonoses in urban centres, and of a map series showing the chronology and pattern of invasion and range extension of brown rats across the continental United States over roughly two centuries from the mid-1700s. https://www.science.org/doi/abs/10.1126/sciadv.adm6755
3. Palaeogenomic analysis of black rat revealing multiple European introductions associated with human economic history. Open-access journal article, read as its bibliographic record and author affiliations rather than for its results, which reach us through the summaries below. Cited to identify the underlying study behind references 4 to 7 and to record that it was produced by a large multi-institution collaboration spanning archaeogenetics, archaeology and palaeogenomics groups. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC9064997/
4. Institutional press release on the spread of black rats and its link to human historical events, issued by one of the research institutes behind the study. Press material summarising reference 3. Source for the statement that the black rat is one of three rodent species, with the house mouse and the brown rat, to have become globally distributed as a result of its ability to live around human habitations by taking advantage of food and transportation; and for the statement that black rats were widespread across Europe until at least the eighteenth century before their population massively declined, most likely as a result of competition with the newly arrived brown rat, now the dominant rat species in temperate Europe. https://www.mpg.de/18595905/spread-of-black-rats-was-linked-to-human-historical-events
5. Science news summary of the same study, published by a research news aggregator. Secondary coverage of reference 3. Source for the description of the method as analysing DNA from ancient black rat remains found at archaeological sites spanning the first to the seventeenth centuries in Europe and North Africa; for the framing that this pieced together a new understanding of how rat populations dispersed following the ebbs and flows of human trade, urbanism and empires; and for the restatement that the species colonised the continent on two occasions, in the Roman and medieval periods. https://www.sciencedaily.com/releases/2022/05/220503141339.htm
6. Archaeology news item reporting the same study, based on a university statement. Secondary coverage of reference 3. Source for the account that a DNA study of rodent bones dated from the first through the seventeenth centuries traced the spread of the black rat across Europe and North Africa; for the finding that the species colonised Europe in the Roman period with its population declining after the fall of the empire, and that a genetically distinct population then appeared in the medieval period perhaps reflecting the re-emergence of long-distance trade; for the statement that the population declined again in the eighteenth century with the arrival of the brown rat, now the dominant species in Europe; and for the researchers' conclusion that black rat and human population dynamics mirror each other and could reflect historic and economic events. https://archaeology.org/news/2022/05/05/220505-black-rat-dna/
7. Feature article on the same study published by a national science news outlet. Secondary coverage of reference 3, written for a general audience. Source for the account that skeletal remains carrying traces of DNA date from Roman times up to the eighteenth century and allowed researchers to say with confidence that the black rat colonised Europe in two distinct waves; for the statement that remains from places as different as England, Finland and Hungary showed it was genetically the same animal, described as better evidence than the researchers could have hoped for; for the statement that the brown rat has spread across the globe except to Antarctica and a few islands off New Zealand while the black rat rules in warmer regions; for the list of common names including ship rat for the one species and sewer rat, Parisian rat, common rat and Norway rat for the other; and for the statement that what is absolutely certain is that the brown rat's scientific name notwithstanding, the species does not originate from Norway. https://www.sciencenorway.no/animal-kingdom-archaeology/heres-why-rattus-norvegicus-has-taken-over-europe/2033745
8. Brown rat demography revealing pre-commensal structure in eastern Asia prior to expansion into Southeast Asia. Preprint posted to a biology preprint server, read as an extract of its introduction, which we flag as not necessarily having completed peer review. Source for the statement that black rats reached southern Europe by six thousand years ago and Great Britain by the 300s AD, yet brown rats were not recorded in Europe until the 1500s AD; for the inference that these dates imply vastly different phylogeographic histories likely related to where each species speciated within Asia, with black rats on the Indian subcontinent and brown rats in the northern steppe; and for the note that previous phylogeographic studies using mitochondrial DNA identified China as the brown rat's ancestral range. https://www.biorxiv.org/content/10.1101/249862.full.pdf
9. Global population divergence and admixture of the brown rat. Preprint posted to a biology preprint server, read as an extract, which we flag as not necessarily having completed peer review. Source for the statement that, based on historical records, brown rats became established in Europe by the 1500s and were introduced to North America by the 1750s, and now occupy nearly every major landmass outside polar regions with human-assisted island colonisation a continuing threat to insular fauna; and for the account that genetic evidence indicates the global distribution of one black rat lineage began with expansion from the Indian subcontinent into western Asia, followed by separate expansions into Europe and Africa, with derived haplotypes indicating colonisation of the Americas, Oceania and Africa from Europe. https://www.biorxiv.org/content/10.1101/065458.full.pdf