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MARGINALIA

The Rat's Tale

A historical "document" isn't always a writing sample

Pat Shipman


Seeds of Change

In 2001 Wilmshurst recognized a new sort of document of human presence: woody seeds that had been gnawed by rats. She had found many such seeds in her botanical samples and could easily tell rat-gnawed seeds from those broken by birds. She and dating specialist Thomas Higham of the Oxford Radiocarbon Accelerator Unit tried to resolve the debate by using rat-gnawed seeds as proxies for rats. If rats had been present for more than 1,000 years earlier than the Wairau Bar site, there ought to be plenty of older rat-gnawed seeds. But they found none older than Wairau Bar in the two North Island sites.

Urged to look for evidence on South Island, Wilmshurst added Anderson and avian paleontologist Trevor Worthy from the University of Adelaide to her team for a three-year project that used a two-prong approach. First, they re-excavated Holdaway's oldest two sites—called Predator Cave and Earthquakes #1—to collect new rat bones for dating. They also re-examined bones, now in museums, that had been originally collected by Holdaway. Second, Wilmshurst and Higham collected and dated woody seeds from a further eight sites, some on South Island.

As the team reported in a 2008 paper in the Proceedings of the National Academy of Sciences, all of the rat bones they tested were more recent than A.D. 1280. The 48 gnawed seeds they dated were all more recent than A.D. 1250. Another 48 intact or bird-broken seeds ranged in age from about 2000 B.C. to about A.D. 1800. A volcanic ash from North Island, in which they found rat-gnawed seeds, provided an independent check as it was dated to A.D. 1314. Combining all of their data, Wilmshurst and her colleagues concluded that the earliest presence of rats in New Zealand was securely dated to about A.D. 1290 to 1380, a range closely matching the dates of the older archaeological sites and confirming the short chronology. Apparently, Polynesian rats took less than 80 years to spread through out North and South Islands—about the same time span recorded for the spread of the European rat, Rattus norvegicus, after its arrival.

With the short chronology established, the massive ecological impact that has been called the "dreadful syncopation" of human arrival, faunal extinction and deforestation is also validated. Within a few centuries of their arrival, the people of eastern Polynesia devastated the animals they found in their new habitat and cut down and burned much of the forested land.

And yet, as Anderson points out, from the point of view of the invading species, this was not a disaster but a triumph.

"The first duty of colonizers is to survive. That requires rapid population growth sustained by consuming the richest resources. Relentless slaughter of large flightless birds and breeding seals, and sustained burning of the forest to open up land for edible ferns and gardening was often the result, but without such environmental change people could hardly have inhabited Remote Oceania at all."

The same might be said of rats.

Bibliography

  • Holdaway, R. N. 2006. Arrival of rats in New Zealand. Nature 384:225–226.
  • Wilmshurst, J. M., A. J. Anderson, T. F. G. Higham and T. H. Worthy. 2008. Dating the late prehistoric dispersal of Polynesians to New Zealand using the commensal Pacific rat. Proceedings of the National Academy of Sciences 105:7676–7680.



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