Rattus exulans species group. Inadvertent or intentional human introduction or possibly natural rafting is responsible for most of the Pacific insular occurrences (Langdon, 1995; Matisoo-Smith and Robins, 2004; Matisoo-Smith et al., 1998; Roberts, 1991), and for distributions on islands and archipelagos outside of mainland SE Asia, the region where the species may have originated (Musser and Newcomb, 1983). Matisoo-Smith et al. (1998) claimed that R. exulans was among the plant and animal species carried by ancestral Polynesians in their colonizing canoes. They analyzed mtDNA sequences in an array of Polynesian samples, concluding that the sequences "prove to be valuable genetic markers for tracing the migration routes and movement of the first humans entering the remote Pacific" (p. 15149). A subsequent study by Matisoo-Smith and Robins (2004) documents evidence for origins and dispersals of Polynesians derived from mtDNA phylogenies of R. exulans. Standard karyotype (2n = 42, FN = 60) for Philippines, Indomalayan region, Oceania, and Papua New Guinea summarized by Rickart and Musser (1993), and for Taiwan sample by Motokawa et al. (2001a).

In a significant report on variation in skull size among R. exulans, R. rattus, and R. norvegicus in New Zealand and other Pacific islands, Yoram et al. (1999) documented increase in skull size with latitude for R. exulans but not for the other two. Other relationships between skull size and island area as well as species sympatry are also presented; all aspects bear on understanding insular morphological variation in these species, especially R. exulans, and their taxonomy. Reliability of radiocarbon dating skeletal remains of New Zealand R. exulans discussed by Hedges (2000) and Higham and Petchey (2000), a test between radiocarbon dates and those obtained by Carbon 14 accelerator mass spectrometry using R. exulans reported by Holdaway and Beavan (1999), and possible role of burial contamination or diet in radiocarbon anomalies documented by Beavan-Athfield and Sparks (2001a, 2001b; also see references cited there). Reliability of dating techniques bears significantly on first colonization dates of R. exulans to New Zealand. Data presented by Holdaway (1996:226) indicates the Pacific rat was established on both main islands nearly 2000 years ago, much earlier than the time established for human settlement (about 850 years ago) based on unequivocal evidence. Holdaway reasoned that R. exulans was introduced by "transient human visitors, who either left immediately or quickly died out," and noted that for "more than 1,000 years after its arrival, the Pacific rat was the sole exotic predator in New Zealand." Whether R. exulans was spread by human transport or natural rafting throughout the Pacific is a controversial topic, and Langdon’s (1995) discussion of rodent colonization of Easter Isl is relevant in this context (see also Matisoo-Smith and Robins, 2004; Matisoo-Smith et al., 1998).