Essentially, and I recommend reading the post yourself, they sequenced DNA from mouse and rat remains from early nineteenth century archaeological deposits in Sydney and compared them with the genotypes of the mouse and rat population in both the North and South Island in New Zealand.
They found that in the North Island population the genotype resembled that of the nineteenth century Sydney rodent population.
European mice and rats are of course introduced species in both Australia and New Zealand.
The Sydney population probably derived from those that arrived on convict ships post 1788.
Sydney, in the early part of the nineteenth century was not just a convict settlement, but a base for whalers and sealers, as well as the informal trade with New Zealand.
New Zealand was not formally organised as a British colony until 1841 (with a short period in 1840 when it was governed by Britain as part of New South Wales), but prior to then there was some settlement by missionaries, not to mention trade in guns, rum and tobacco (all the delights of civilisation) with the Maori, principally in the North Island.
The North Island being warmer, supported a larger population.
So, not surprising. Most trade and contact would be through Sydney, so that would be how the mice and rats would have come, and might even have been introduced by a possible ancestor of mine.
If there are still extant rat populations on some of the remoter Antarctic and sub Antarctic islands, it might be worth repeating the experiment comparing these populations with the nineteenth century Sydney population – for, as James Clark Ross describes in his Voyage to the Southern Seas, there were numerous informal whaling stations on these islands.
So far, so good.
But it gets better, when they analysed the population on the South Island, they found that, rather than being related to the Sydney population, they contained genetic markers linking them to a rat population in China.
Not what you would have expected. Given that before the 1860s gold rush in the South Island, many of the European population were graziers and farmers who migrated from Scotland, one might have expected traces of Scottish ancestry in the rodent population, as well as Sydney, but not China.
It could be due to trade, but what was the trade in?
The Maori population of the South Island was comparatively small and agricultural. Other than greenstone, I can’t imagine anything that the Chinese would have wanted to trade for.
It’s possible that the rats arrived with Chinese goldminers in the 1860’s and outbred the small local population and effectively masked their genetic signature.
Conversely, I guess the reverse could have happened in the North Island, if there were Chinese traders, they could have introduced a small population of rodents which was subsequently outbred by the arrivals from Sydney.