On the Rat Trail in Near Oceania: Applying the Commensal Model to the Question of the Lapita Colonization1 E. Matisoo-Smith,2,4,7 M. Hingston,3,4 G. Summerhayes,5 J. Robins,2,4 H. A. Ross,3 and M. Hendy6 Abstract: Presented here are the most recent results of our studies of Rattus exulans, one of the main commensal animals transported across the Pacific by Lapita peoples and their descendants. We sampled several locations in Near Oceania to determine distribution of R. exulans mitochondrial DNA (mtDNA) haplotypes in the region. We also obtained data regarding distribution of other introduced Rattus species to several islands in the Bismarck Archipelago. Our results suggest that there were multiple introductions of R. exulans to the region, which may suggest a more complex history for Lapita populations in Near Oceania. One of the greatest impacts of human ive fauna and numerous endemic species that arrival on previously uninhabited islands is cannot compete with the more recent intro- the introduction of human-associated plants ductions. and animals. The species introduced by hu- Despite the often negative impacts of in- mans can include not only intentional intro- troduced species on island ecosystems, there ductions such as domesticated plants and were clearly good reasons for people to trans- animals, but also a range of unintentionally port their familiar plants and animals to the transported species including weeds, insects, new environments they occupied. The trans- and other pests (Kirch 1982, Lee et al. ported landscapes (Kirch 1984) of Pacific 2007). Often island ecosystems contain a na- peoples also provide a valuable resource for prehistorians. Not only do they allow us to understand and appreciate how humans adapted to the various environments they en- countered, but understanding the history of 1 This research was funded by the Royal Society of the plants and animals that humans trans- New Zealand under Marsden Project UOA510: The ported can provide direct evidence regarding Rat’s Tale: Tracking Lapita Peoples through Phyloge- the history of the humans themselves: Where netic Analyses of Pacific Rats. Manuscript accepted 23 did they come from and when? How many January 2009. 2 Department of Anthropology, University of Auck- introductions and population arrivals were land, Auckland 1, New Zealand. there? Is the appearance of a particular spe- 3 School of Biological Sciences, University of Auck- cies associated with any particular archaeo- land, Auckland 1, New Zealand. logically definable culture? 4 Allan Wilson Centre for Molecular Ecology and Evolution, University of Auckland, Auckland 1, New Beginning in the 1990s a program was Zealand. developed at the University of Auckland fo- 5 Department of Anthropology, University of Otago, cused on determining if tracking the move- P.O. Box 56, Dunedin, Otago, New Zealand. ment of commensal animals introduced to 6 Allan Wilson Centre for Molecular Ecology and Pacific islands by initial human colonists, Evolution, Massey University, Palmerston North Cam- pus, Private Bag 11222, Palmerston North 444, New through analyses of mitochondrial DNA Zealand. (mtDNA) variation, might serve as a proxy 7 Corresponding author (e-mail: e.matisoo-smith@ for tracing human migration in the Pacific auckland.ac.nz). (Matisoo-Smith 1994, Matisoo-Smith et al. 1998). Pacific colonists transported, among Pacific Science (2009), vol. 63, no. 4:465–475 other things, dogs, pigs, chickens, and rats : 2009 by University of Hawai‘i Press when they settled the previously uninhabited All rights reserved islands of Remote Oceania. It is also gener- 465 . 466 PACIFIC SCIENCE October 2009 ally agreed that the Lapita colonists were the spheres in central East Polynesia (see Figure first to introduce dogs, pigs, chickens, and the 1), both most likely originating in a home- Pacific rat (Rattus exulans) to islands in Near land region centered in the southern Cook Oceania (Kirch 2000, Spriggs 1997), though and Society islands. A southern interaction debates continue around the possibility of sphere connected these populations with earlier, pre-Lapita introductions of pigs and New Zealand and the Kermadec Islands, and dogs to New Guinea (Bulmer 1982, 2001, a northern interaction sphere connected this Goreki et al. 1991, Allen 2000, Green 2000, homeland region to the Marquesas and Ha- Bellwood and White 2005, Larson et al. waiian islands. These results were consistent 2007). with archaeological and linguistic data as well It was decided, for a number of reasons, as oral traditions (Green 1966, Kirch 1986, that the Pacific rat, R. exulans, was the best Irwin 1992, Cachola-Abad 1993). Once it animal with which to first develop and test was shown that the Commensal Model did what we now refer to as the Commensal indeed provide a reasonable proxy for track- Model for human settlement of the Pacific. ing human migrations, the range was ex- The Commensal Model is based on the close panded beyond the Polynesian triangle, and relationship between human populations and a diachronic perspective was added through the plants and animals they transported across the application of ancient DNA (aDNA) the Pacific. By identifying the genetic rela- methods to archaeological remains of R. ex- tionship of the various populations of com- ulans (Matisoo-Smith et al. 1997, 1999, 2001). mensal plants and animals, we can model the To further expand this Commensal Model prehistoric migration and interaction patterns and address the issue of Lapita origins, of Pacific peoples. A primary reason for the Matisoo-Smith and Robins (2004) looked at choice of R. exulans for a commensal study is how Polynesian and other Remote Oceanic its near-ubiquitous distribution in the Pacific. samples related to those from Near Oceania In addition, although the dogs, pigs, and and Island Southeast Asia. The results of chickens carried by early Pacific colonists be- these analyses identified three major hap- long to the same species as those brought in logroups of R. exulans, each with a very dis- by European vessels from the 1700s onward, tinct geographic distribution. All three the rats introduced by these same historic haplogroups appeared to be ultimately de- vessels, Rattus rattus and Rattus norvegicus, are rived from Mainland Southeast Asian popu- different species and do not interbreed with lations. Haplogroup I was found only in R. exulans (Mayr 2000, Robins et al. 2007). populations from the Philippines, Borneo, Rattus exulans is not known to stow away in and Sulawesi. Although its presence in Bor- European shipping vessels, and therefore the neo may represent natural expansion of the populations living on Pacific islands today species across the Sunda Shelf during periods are the direct descendants of those intro- of lowered sea levels, Haplogroup I was most duced by prehistoric colonists. We can there- likely transported by humans to both the fore study extant populations as well as Philippines and Sulawesi, which were sepa- archaeological remains of R. exulans from rated from Sunda by deep undersea troughs. across the Pacific to model the prehistoric It was, however, the distribution of Hap- human colonization of Remote Oceania. logroups II and III that provided evidence The first major test of the Commensal of importance to issues of Lapita expansion. Model for human settlement was based on Given the archaeological evidence for a clear an analysis of mtDNA variation in Polynesian link between Lapita sites in Near and Remote populations of R. exulans (Matisoo-Smith Oceania, the results of this study were sur- 1994, Matisoo-Smith et al. 1998). Analyses prising. It was found that the Near and Re- of 132 R. exulans samples collected from mote Oceanic R. exulans populations sampled throughout Polynesia indicated that the were very different. In fact, there were no Commensal Model worked, with results sug- Near Oceanic mtDNA lineages in Remote gesting that there were two major interaction Oceania and vice versa, but both lineages . On the Rat Trail in Near Oceania Matisoo-Smith et al. 467 Figure 1. Map of Polynesia showing the interaction spheres identified through mtDNA analyses of Polynesian pop- ulations of Rattus exulans. were present in Halmahera (Island Southeast followed by a Lapita introduction to small is- Asia/Wallacea) to the west, where they were lets, locations that have not been sampled in presumably introduced by humans at some any previous analyses, morphological or ge- point in prehistory. Studies of morphological netic. Our goal was therefore to test these variation of Pacific R. exulans (Tate 1935, two possibilities through more thorough and Motokawa 2004) have reported a similar lack specifically directed sampling of both modern of continuity between Near and Remote and archaeological R. exulans populations Oceania. This could be (1) real; or (2) due from throughout Near Oceania. to two introductions of R. exulans to Near Archaeological evidence suggests that the Oceania—an early one to the main islands, Lapita people generally targeted small, off- . 468 PACIFIC SCIENCE October 2009 shore islands for settlement, but the Near deed R. exulans was a Lapita introduction, we Oceanic R. exulans samples reported in would expect to find it associated with all Matisoo-Smith and Robins (2004) were pri- islands that have evidence of Lapita occupa- marily from larger islands (e.g., New Guinea, tion. In June and July 2006, our trapping ef- New Britain, and Bougainville), which were forts were focused on the islands in the New not typical early Lapita targets. Rattus exulans Ireland and Manus provinces, all of which ei- is generally assumed to be a Lapita introduc- ther had known Lapita sites or were likely tion to both Near and Remote Oceania, yet sites for Lapita settlement. Rats were trapped the precise dating of its introduction to Near on New Ireland, Lihir, Tatau and Simberi is- Oceania is problematic, because sites from lands in the Tabar Group, New Hanover, and the period just before Lapita arrival are rare.