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Feature

Dating the Rat

Ancient rats show that humans have been around in New Zealand a lot longer than previously thought. Or do they?

Bruce McFadgen

Richard Holdaway has recently reported radiocarbon dates of kiore bones from eight predominantly South Island sites which suggest that rats were living in New Zealand some 2000 years ago. Since it is unlikely that the rats could have reached New Zealand except as passengers on a boat the dates, if correct, would more than double the accepted time since humans first discovered New Zealand.

If the dates are confirmed, then it will be necessary to visit the role of kiore as a predator of native fauna. The implications for interpreting human history are less clear. The rats imply the presence of humans, but do not in themselves give evidence for human colonisation at that time.

However, the rats may not be as old as claimed. In addition to the dates reported by Holdaway, six dates have been reported by Atholi Anderson on rat bones from the moa-hunter site at the mouth of the Shag River in North Otago. The Shag River moa-hunter site is well dated by more than 30 dates on shell, charcoal, and moa eggshell to about 600 years ago. The rat bones, which are from the cultural layers of the site, returned dates of ca 900 years ago and ca 1,500-2,000 years ago. The 900-year old dates were measured at Oxford University and the 1,500-2,000-year old dates at the Rafter Radiocarbon Laboratory at Gracefield.

The discrepancy between the ages of the Shag River rat bones compared with the age of the site itself is regarded by Anderson as evidence that there is something wrong with Holdaway's old rat dates. Anderson has also answered Holdaway's alternative suggestion raised at a seminar in Wellington last year: that the rat bones were present as subfossil remains in the underlying natural sand of the site and were dug up during occupation and incorporated into the cultural layers.

Certainly Holdaway has a point. Bones from sub-fossil natural deposits are not uncommon in archaeological sites, and the sand beneath the Shag River site is shelly and might have preserved such bones. Furthermore, the range of ages given by the two laboratories is consistent with a subfossil origin of the rat bones. But Anderson's reply is that no bones were found in the underlying natural layers when the site was excavated. Rat bones were only found in the cultural layers where they were abundant, and in the intervening natural sand layers where they were scarce, a distribution which is difficult to explain if the bones were derived from the natural sand beneath the site.

If the rat bones from the Shag River site have come from rats which were alive when the site was occupied, then some explanation for their ancient radiocarbon age needs to be found. If the Shag rats had eaten lots of fish this may perhaps account for the 300-year difference between the rat dates from Oxford and the age of the site.

Yet the Gracefield dates are still too old. Anderson suggests that the difference in the dates reported by the two laboratories is a result of incomplete removal of old contaminated carbon by the Gracefield laboratory when the samples were prepared for dating. As all of Holdaway's dates were measured at the Gracefield laboratory any residual old carbon would also explain Holdaway's rat dates. The Gracefield laboratory has taken very seriously the possibility that residual old carbon may have affected the dates and is currently comparing their sample preparation procedures against those of the Oxford laboratory.

To check the consistency of dates on rat bones with dates on other archaeological materials, rat bones from a 400-year old shell midden site near Wellington are being dated. The site near Wellington is on loess, which would not preserve rat bones for one year -- let alone a thousand years -- unless the bones were in a shell midden (thus minimising the possibility of dating subfossil rat bones).

It is standard practice in scientific research, especially when results such as the rat dates reported by Holdaway conflict with a generally held interpretation, that other possible hypotheses which might produce the same result are tested. One such possible hypothesis is that something in the diet of the rats might have given rise to the old rat dates. Accordingly, the effects of rat diet on radiocarbon dating is under investigation by Nancy Beavan at the Gracefield laboratory.

Old Carbon and New Rats

There are many sources of old carbon in the environment which rats may have ingested as part of their diet. Rats get their radiocarbon from the food they eat, particularly protein, and their diet includes invertebrates, young birds, and birds' eggs. Bugs such as weevils, huhu grubs, and wetas, which live on old wood, would be eating old carbon which would be incorporated into rat bones when the rats ate the bugs. Similarly with other species, such as the chicks and eggs of water fowl, it is not difficult to construct a hypothetical food chain whereby old carbon finds its way into rats.

An important part of Holdaway's argument is the date for a rat jawbone excavated from below Taupo Tephra by John Yaldwyn in 1959 and retained in the collections of the then Dominion Museum. The radiocarbon age of the bone, which agrees with its reported provenance, is strong supporting evidence for Holdaway's thesis. Yet the agreement could still be coincidental because of two uncertainties: the first relates to the problems of old carbon already discussed, the second is the interpretation of the stratigraphy in the shelter.

The jawbone was excavated from a rock shelter in Hawkes Bay and seems to be the only independently dated sample earlier than the currently accepted date for human settlement of New Zealand (ca 700 years ago). The Taupo Tephra was erupted ca 1,800 years ago. The rock shelter is small. It is only about 0.6-0.8 metres high, with an overhang of no more than 1-2 metres, and it faces away from the Taupo volcanic centre.

The question is by what process did the tephra get into the shelter? If airfall tephra sealed the jawbone, then its age would be older than the Taupo eruption. If the shelter was protected by the overhang and the tephra was not deposited in the shelter until sometime later, then the jawbone may be younger than the tephra by an unknown and possibly long time.

Other bones excavated from the shelter included petrels, from both above and below the tephra, and the shelter appeared to have been an inland nesting site for petrels. Some petrels are notorious burrowers and it is possible that the position of the jawbone may have been due to burrowing petrels. Whatever the process by which the rat jaw came to be lying beneath the Taupo Tephra, it is perhaps significant that no other rat bones have been reported from under the tephra. They are all on top.

Implications

None of the foregoing discussion proves anything, but it does raise questions which need to be resolved before the rat dates can be accepted or rejected. If the rats are as old as claimed, our understanding of the later stages of New Zealand's unique biodiversity will change.

For New Zealand's human history, the implications are less clear. If the rats are as old as claimed this probably means that humans visited New Zealand very early, but it does not necessarily mean that New Zealand was colonised early. If the rats are younger than claimed it indicates that the dates do not reflect the true age of the material being dated. This cautionary note applies to all materials which are dated and emphasises the need to understand thoroughly the processes involved in the uptake of radiocarbon by organisms when interpreting radiocarbon dates.

Bruce McFadgen works in the Conservation Sciences Centre which is part of the Department of Conservation's Science and Research Department.