Treeshrews Defy Evolutionary Rules

A new study has exposed the common treeshrew, a small and skittish mammal that inhabits the tropical forests of Southeast Asia, as an ecogeographical rule breaker. According to the study, published online on January 4, 2017 in Ecology and Evolution, Tupaia glis, the common treeshrew, defies two widely tested rules that describe patterns of geographical variation within species: the “island rule” and “Bergmann's rule.” The open-access article is titled “Rule Reversal: Ecogeographical Patterns of Body Size Variation in the Common Treeshrew (Mammalia, Scandentia).” The island rule predicts that populations of small mammals evolve larger body size on islands than on the mainland, whereas island-bound large mammals evolve smaller body size than their mainland counterparts. Bergmann's rule holds that populations of a species in colder climates, generally located at higher latitudes, have larger body sizes than populations in warmer climates, which are usually at lower latitudes. In order to determine treeshrew body size from populations on the Malay Peninsula and 13 offshore islands, the researchers measured 260 specimens collected over the past 122 years and housed in 6 natural history museums in Europe and North America. The researchers tested multiple variables, analyzing how island size, distance from the mainland, maximum sea depth between the mainland and the islands, and latitude relate to body size in the treeshrew populations. They found that the island rule and Bergmann's rule, which are rarely tested together, do not apply to common treeshrews. The study revealed no size difference between mainland and island populations. It also revealed that treeshrews invert Bergmann's rule: individuals from lower latitudes tended to be larger than those located at higher latitudes.
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