The common treeshrew (Tupaia glis) — a small mammal native to Thailand, Malaysia, and Indonesia — defies two widely tested rules that describe patterns of geographical variation within species: the island rule and Bergmann’s rule, according to a research team led by Yale University Professor Eric J. Sargis.
The common treeshrew (Tupaia glis). Image credit: Benjamin Balazs.
“There are a number of ecogeographical ‘rules’ that describe patterns of geographical variation among organisms,” Professor Sargis and co-authors said.
“The island rule predicts that populations of larger mammals on islands evolve smaller mean body size than their mainland counterparts, whereas smaller-bodied mammals evolve larger size.”
“Bergmann’s rule predicts that populations of a species in colder climates (generally at higher latitudes) have larger mean body sizes than conspecifics in warmer climates (at lower latitudes).”
In order to determine treeshrew body size from populations on the Malay Peninsula and 13 offshore islands, the scientists measured 260 specimens collected over the past 122 years and housed in six natural history museums in Europe and North America.
They 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 common treeshrews invert Bergmann’s rule: individuals from lower latitudes tended to be larger than those located at higher latitudes.
“Determining the causes of geographical variation within a species is critical to understanding underlying mechanisms of evolutionary patterns,” Professor Sargis said.
“Our analysis demonstrates the need to assess multiple variables simultaneously when studying ecogeographical rules in a broadly distributed species like the common treeshrew, as multiple factors may have influenced how populations evolved.”
The team found that a treeshrew population’s latitude was the variable most related to body size.
“Maximum sea depth between the mainland and islands was a secondary driver of body size, with treeshrews on islands separated from the mainland by deeper waters typically exhibiting larger body size,” the authors said.
“Our study also showed that treeshrews on smaller islands tend to have smaller body size.”
The findings appear in the journal Ecology and Evolution.
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Eric J. Sargis et al. Rule reversal: Ecogeographical patterns of body size variation in the common treeshrew (Mammalia, Scandentia). Ecology and Evolution, published online January 4, 2018; doi: 10.1002/ece3.3682
