The common treeshrew may look unremarkable, but it breaks evolutionary rules

The common treeshrew – a small mammal native to Southeast Asia – appears fairly unremarkable at first glance.

However, a new study has found that, in fact, the animal is an evolutionary rule-breaker which defies two rules that describe patterns of geographical variation within species – the island rule and the Bergmann's rule.

The former describes how small mammals evolve larger bodies on islands than on mainland, whereas the opposite is true for large mammals.

Meanwhile, Bergmann's rule dictates that populations of a species in colder climates (usually in higher latitudes) have larger bodies than populations in warmer climates [which tend to be in lower latitudes].

Researchers examined 260 treeshrew specimens collected from the Malay Peninsula and offshore islands over the past 122 years, finding that both of these rules do no apply to common treeshrews.

They found that there were no size differences between mainland and island populations and also, individuals from lower latitudes tended to be larger than those at higher latitudes, inverting Bergmann's rule.

"Determining the causes of geographical variation within a species is critical to understanding underlying mechanisms of evolutionary patterns," said Eric J. Sargis, first author of the study and professor of anthropology at Yale.

"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."