Rat bones tell us of the prehistoric dispersal of Polynesians to to New Zealand

Afarensis has just blogged on this new PNAS paper, “Dating the late prehistoric dispersal of Polynesians to New Zealand using the commensal Pacific rat.” The goal of this paper is to clarify when New Zealand was peopled. There are several hypotheses floating out there, two of which indicate New Zealand was either peopled 1,200 years ago or 800 years ago.

One hypothesis, suggested in this 1996 Nature article, “Arrival of rats in New Zealand,” indicates people arrived with rats roughly 2,800 years ago. This was established using carbon dating of rat bones. Rats, as explained by Afarensis, are often used as a proxy to understand human migratory patterns, because rats and humans have a longstanding commensal relationship. In February, I shared some news on how rats have been used to understand migrations of humans in the Neolithic.

I’m not a rat biologist, and I don’t fully know their ecological independence. Through my personal experiences, I understand rats have been pretty dependent on humans but because these rat bones were dated to be 2,000 years older than the first human remains in New Zealand and were excavated with no supporting ecological or archaeological context, the dates have been hotly contested.

The authors of this new PNAS revisit this study and included carbon dating of rat gnawed seeds from two caves in New Zealand. The authors hope to re-clarify the presence of human occupation of New Zealand because it just doesn’t seem feasible that rats could live for so many thousands of years without humans.

Here’s a summary of the results, from Afarensis’ blog post,

“The research specifically focussed on plant seeds from plants that had been driven to extinction by the Pacific rat (or the rats have been implicated as a possible cause). All the rat bones date to 1280 AD or younger. The un-gnawed seeds are the oldest, whereas, none of the gnawed seeds date to before approximately 700 years BP. This current research dovetails with dates on rat gnawed snails.”

The new dates all confer with the time humans are understood to be present in New Zealand, but one thing that wasn’t discussed in the PNAS paper is other sources of the ratting of New Zealands. Like I said, I’m not well versed in rat ecology. I know they live in close association with humans but that doesn’t mean rats are dependent solely on humans… they just benefit from living next to humans.

See the genus Rattus is thought to have emerged from the Murid family about 3.5 million years ago in Asia. This is well before human ancestors ventured into Asia. They did fine for a couple millions of years there, living in colonies independent from humans. It is possible that rats can live in areas without a human presence.

So if we agree that rats can live by themselves, how did they get to New Zealand? 3,000 years ago, sea levels were about the same level as they are now. Janet Wilmshurst, pictured above, and one of the authors of this paper, told the press that this particular species of rat, the kiore, “cannot swim very far, it can only have arrived in New Zealand with people on board their canoes, either as cargo or stowaway. ”

I feel that Wilmshurt is treading on very fine line, speaking with such vindication that these rats had to have come to New Zealand along with humans. Many of the non-marsupial mammals of Australia came by island hopping and rafted from the north, it is possible these rats did the same.

    Wilmshurst, J.M., Anderson, A.J., Higham, T.F., Worthy, T.H. (2008). Dating the late prehistoric dispersal of Polynesians to New Zealand using the commensal Pacific rat. Proceedings of the National Academy of Sciences, 105(22), 7676-7680. DOI: 10.1073/pnas.0801507105

7 thoughts on “Rat bones tell us of the prehistoric dispersal of Polynesians to to New Zealand

  1. “it is possible these rats did the same”. Not really. The distances are too great. The rat expansion through the Pacific islands largely coincides with human expansion. Where it doesn’t I suspect dating for one or the other is wrong. That is what the researchers are going to turn to next, looking at places where arrival is ambiguous. Sure, once rat have reached an island they can survive without close contact with humans, in fact have done so on offshore islands around NZ for centuries.

    Anyway 2000 years ago humans hadn’t yet reached Eastern Polynesia so if rats got here that long ago they must have been carried by people from Western Polynesia. No evidence for contribution to Maori NZers, or rats, from that region. Not impossible of course.

    For some reason there is a great deal of opposition in some quarters in NZ to the idea that humans have been here only 700 or 800 hundred years. I can understand the indigenous inhabitants arguing against the idea but many people of European extraction also seem unable to accept the evidence. Got any ideas of motivation?

  2. Hi Terry,

    Thanks for the comment. As you can tell, I’m not too sold that rats couldn’t have made it to New Zealand by themselves. I’m not denying the evidence, I just think rats, even the kiore are capable of coming to New Zealand independent of humans.

    I say that because rodents are the only placental order, other than bats and Pinnipeds, to have reached Australia without human introduction. Certainly bats and seals and sea lions flown or swam over to Australia. But rodents had to have navigated the seas to some extent, even the though sea levels were different in the Pliocene, when rodent fossils first appeared in Australia. The Wikipedia entry on the kiore confirms what Wilmshurst said,

    “[The Kiore] cannot swim over long distances and are therefore considered to be a significant marker of the human migrations across the Pacific, as the Polynesians accidentally or deliberately introduced them to the islands they settled.”

    But these rat’s didn’t have to have necessarily swim. It is just as likely they got to New Zealand by way of hitching a ride on a piece of driftwood or some other raft, just as they did by hitching a ride on a human watercraft.


  3. Yes but. The kiore didn’t manage to reach many offshore islands. In fact this meant that many plants, birds and invertebrates that died out on the mainland survived on the offshore islands that the rats failed to reach. Where the rats were present on islands during pre-European times it is obvious that humans had lived on the islands at some time. Besides which the new research indicates the earliest rat presence in NZ coincides with a great deal of other evidence for human arrival. So it would be a huge coincidence if “they got to New Zealand by way of hitching a ride on a piece of driftwood or some other raft”.

    Their arrival on many other Pacific islands also coincides with dated human arrival. The evidence is pretty convincing overall.

  4. Fair enough, Terry. You’re right, it would be a huge coincidence to have rats getting to New Zealand before humans given the fact that pre-European times, there’s evidence of humans and rats were living on other islands.


  5. The gap between Australia and New Guinea across the Torres Strait is full of islands – the biggest gap is (I guess) a few tens of kilometres. The gap from Australia to New Zealand is about 2000 kilometres. That’s a bigger step for ratkind.

  6. I find this interesting because there have been several papers recently on the introduction of other animals into Oceania by humans. Pigs are an obvious example of this, but as I recall there are others too. I wrote about a paper that had come out on the introduction of chickens not so long ago. With this information about the rat, it seems like the picture is gradually being filled in. But it is also clear that there is a lot of work still to be done.

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