New light on the introduction of shipborne commensal rats and mice in Aotearoa New Zealand, 1790s–1830s

Commensal rodents are those species that have adapted to feeding, breeding and travelling around the world with humans, as opposed to their relatives who remain fully wild. Three species of commensal rodents each invaded New Zealand many times before about 1830. All carry in their genomes evidence of their origins, which not only confirm known precolonial trade routes in the south-western Pacific during the late eighteenth and early nineteenth centuries, but also extend them into conclusions invisible to archival records. Pacific rats (Rattus exulans) arrived with Polynesian colonists from their home islands in eastern Polynesia in about 1280 CE. They have become a mine of genetic information for deducing the history of human colonisation of the Pacific islands. House mice (Mus musculus) and Norway rats (Rattus norvegicus) both originated in Asia, and spread from there along with people to western Europe by the mid-1700s. Both were carried by the early European and American ships among cargo, livestock and immigrants to Australia, and thence to New Zealand. They provide evidence of a hitherto unknown short but important clandestine connection with China over about 20 years after 1800. A fourth species of commensal rodent to reach New Zealand, the ship rat (Rattus rattus), arrived only in about 1830-50.