Pacific island invasions: How do settlement time, latitude, island area and number of competitors affect body size of the kiore (Polynesian rat) across the Pacific?

The kiore (Rattus exulans) was introduced by Polynesians to many Pacific islands (hence the name Polynesian rat) in the past 3000 years. In the Pacific region, this species is known to vary in size in relation to latitude, island size and number of competing rat species. In this study, we use recently estimated settlement dates of Pacific islands to re-assess the spatial and temporal factors that determine body size (represented by greatest length of the skull) and the length of the upper tooth row (UTR) of the kiore. The relationship between skull size and UTR and their predictors are shown to be complex due to several significant interactions. In any environment, an organism is faced by many factors, each of which may produce a selection pressure whose effect may combine and interact with other factors. The significant interactions we detected indicate shifting trends in the morphology of the kiore under different geographical (i.e. latitude and island size), climatic and other conditions. We suggest that only a comprehensive consideration of all the relevant parameters and their interactions will provide a realistic understanding of how body parameters of an organism are affected by its local environment and settlement history.