Using rat-gnawed seeds to independently date the arrival of Pacific rats and humans in New Zealand

Abstract

The Pacific rat (Rattus exulans) was transported throughout the south Pacific with voyaging humans. Thus, the earliest dated evidence of Pacific rat can be used to infer first human contact. Until recently, it was considered that rats arrived in New Zealand with humans in the thirteenth century AD. However, controversial radiocarbon dates on Pacific rat bones now suggest that rats reached the remote islands of New Zealand with people c. AD 50-150. These dates are anomalous because they imply human contact with New Zealand more than 1000 years before any archaeological evidence for human presence, and precede settlement of tropical eastern Polynesia, the ancestral homeland of Maori, the first New Zealanders. The early rat bone dates are controversial for other technical reasons, which have been debated in the literature. Here, distinctive rat-gnawed seed cases preserved in sediments are used as a proxy to independently date the arrival of the Pacific rat and humans in New Zealand. This method effectively bypasses the problems that have plagued rat bone dating and provides a reliable age for rat and human arrival. The oldest dates on rat-gnawed seed cases from widely separated sites are consistent with the Pacific rat arriving at the same time as the initial human settlement of New Zealand in the thirteenth century AD, and not before. The gnawed seed dates lend no support to the argument for an earlier introduction of rats. This dating approach offers a novel way of clarifying island colonization histories throughout Oceania.