Prehistoric Mobility in Polynesia: MtDNA Variation in Rattus exulans from the Chatham and Kermadec Islands

Abstract

Irwin (1992) has suggested that island accessibility in the Pacific, in terms of latitude and safety of return voyaging, for example, affects their degree of contact with other islands and their role in Pacific prehistory. We present results of mtDNA variation in both ancient and modern populations of the Pacific Rat (Rattus exulans), an animal that was transported by humans as they settled the Pacific islands. We argue that the varying levels of genetic diversity in R. exulans populations on Pacific islands will to some degree reflect the level of prehistoric human contact with those islands, and thus will be tied to island accessibility. A high level of mtDNA variation is reported for the Kermadec Island R. exulans populations, but there is marked lack of variation in Chatham Island rats. This is consistent with predictions based on the relative degrees of accessibility of the Kermadecs and the Chathams. High levels must be the result of either multiple introductions by humans or in situ evolution over an extended time frame; however, lack of variation could conceivably be the result of recent population crashes, and may therefore not be reflective of low levels of human mobility. Analysis of mtDNA from archaeological R. exulans samples shows a direct link between ancient and modern populations on Chatham Island. This result (1) confirms relative prehistoric isolation of Chatham Island; (2) allows for rejection of the in situ evolution explanation for New Zealand and Kermadec levels of variation; and (3) supports the use of Rattus exulans mtDNA variation as an assessment for accessibility and contact of prehistoric Pacific populations. KEYWORDS: Rattus exulans, mtDNA, ancient DNA, prehistory, Polynesia.