Norway rats and house mice are two of the most widespread invasive species worldwide. But where did the Norway rats and house mice in New Zealand come from? Our geneticist Lara Shepherd and colleagues from Manaaki Whenua Landcare Research, University of Waikato, and Place Management NSW have shed some light on this question by sequencing DNA from rodent bones from a 19th-century archaeological site in Sydney.
Norway rats and house mice have been in New Zealand since the 18th century and 19th century, respectively, and they have been causing trouble for our native species ever since.
New Zealand rodents have been well studied genetically but to understand their origins we need more samples from overseas for comparison.
From across the ditch
One of the most important sites for understanding the origins of New Zealand rodents is Sydney, which was the primary port from which European ships sailed onto New Zealand. It has long been assumed that the rats and mice that reached New Zealand were stowaways on these ships from Sydney.
In the 1990s, an archaeological excavation at The Rocks, an early settlement site at the Sydney wharves, found a number of rodent bones dating to between 1788 and 1900. These have provided an opportunity to compare the DNA variants in these old bones to those found in New Zealand.
Our DNA sequencing showed that the most common Norway rat DNA variant in the Sydney bones was identical to the most common variant found in New Zealand’s North Island and offshore islands. This variant is also the most frequent variant in Britain.
However, a different DNA variant has been found in Norway rats from the South Island of New Zealand and this variant was not found in our sampling of Sydney Norway rats. This DNA variant has only otherwise been found in China.
The house mouse results showed a similar pattern. In New Zealand, there are genetic variants shared with Sydney and British Norway rats. But there are also genetic variants in New Zealand that can be traced to eastern Asia and these were not detected in our sampling from Sydney.
Our results are consistent with at least two independent introductions of both species. One introduction was from Britain, likely via Sydney. The other was from eastern Asia, probably through undocumented trade between China and New Zealand prior to 1820.
Veale AJ, King CM, Johnson W, Shepherd L (2022) The introduction and diversity of commensal rodents in 19 century Australasia. Biological Invasions.
Lara, Very interesting, especially the undocumented trade comment. However it would be interesting to know how that conclusion was reached with documented trade and immigration happening from China to the South Island goldfields only a few decades later. Is it possible that was the source or a source? Just curious, thanks Brent
Thanks for your comment Brent. Norway rats and house mice are both thought to have reached the South Island much earlier than when the first Chinese gold miners arrived (1865) and were already widespread by that time. To expand on the undocumented trade comment it has been suggested that both these species may have come with sealers returning from the Canton fur market between 1792 and 1810. These voyages were undocumented (illegal) because direct trading with China was prohibited until after 1813.
Fascinating post and article – congratulations to you and your colleagues on your amazing detective work!
Re the east Asian DNA variant found in Norway rats from the South Island, is there any possibility this could have come via the Cape of Good Hope rather than more directly from China? The reason I ask is because 1/ the Cape Colony had been a major port of call for European traders and voyagers sailing to and from the East since at least the mid-17th century, and 2/ we have two strong candidates for introducing rats from the Cape into the South Island at a very early stage, namely Cook and Vancouver, who visited Dusky Sound in 1773 and 1791 respectively. Both had spent an extended period at the Cape before sailing on to NZ (directly, in Cook’s case) and both then spent several weeks at different locations within the sound – more than enough time to unwittingly embark and disembark any rodent stowaways.
Have there been any DNA studies of rat bones found at archaeological sites in and around Cape Town which would confirm the presence of the Asian variant there prior to c.1800?
Thanks for your comment James! I haven’t been able to find any DNA studies of rat bones from South Africa so that would be an interesting future research project for someone. There has been a study published (https://bmcgenomdata.biomedcentral.com/articles/10.1186/1471-2156-12-26) which looked at a small number of modern Norway rats from South Africa and found, like New Zealand, there have been separate introductions from Europe and Asia. Unfortunately they looked at a different DNA marker to us so we can’t see if their Asian DNA haplotypes match those found in New Zealand.