This Rule-Breaking Treeshrew Defies Two Laws Of Evolution

kajornyot wildlife photography/Shutterstock

Meet the critter that breaks the rules – well two rules of island living, anyway. The common treeshrew, otherwise known as Tupaia glis, is a small, squirrel-like creature that spends its days foraging for food in the forests of Thailand, Malaysia, and Indonesia. Now it seems it breaks two ecogeographical rules – rules that describe patterns of geographical variation in animals – the island rule and Bergmann’s rule.

The island rule proposes that when one species inhabits both islands and mainland, individuals will not be uniform across these habitats. Instead, small animals will be larger on islands, due to fewer predators, and large creatures will get smaller on islands, due to limited resources. Examples include the giant man-eating monitor lizards, or Komodo dragons, on Indonesia’s island of Komodo, the giant tortoises of the Galapagos, and the Borneo elephant, which although still large, is significantly smaller than both its Asian and African counterparts.

Here be dragons - the monitor lizards of Komodo island can grow to 3 meters (10 feet). Adhi Rachdian/Shutterstock

The new research, published in Ecology and Evolution, found that the common treeshrew doesn’t follow this rule as its size remains constant across the islands and mainland of the Malay Peninsula. The team analyzed 260 treeshrew specimens that had been collected over 122 years and kept in various natural history museums.

Using the specimens, they also tested whether the treeshrews conform to another ecogeographical rule, Bergmann’s rule. This states that populations and species of animals will be bigger in colder, higher latitude environments, but smaller in warmer, low-latitude environments. Just think of the huge emperor and king penguins of chilly Antarctica compared to the little beach-dwelling penguins of south-western Africa. 

However, to the team’s surprise, the treeshrews don’t follow this rule either. Instead, they invert it. Treeshrews from lower latitudes were generally bigger than those living at higher latitudes. Therefore, although many species follow the trends of the island rule and Bergmann’s rule, it is clear they cannot be generalized to all living creatures.

The team found that body size among tree shrews was most affected by latitude. The maximum depth of the sea between islands and the mainland was also found to influence body size, with animals isolated by deeper waters generally being larger. Tree shrews also appeared to be smaller on smaller islands.

“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,” said lead author Eric J. Sargis, professor of anthropology at Yale University.

“Without well-documented and curated voucher specimens collected from numerous localities and in large enough numbers to assess statistical significance, we simply could not have done this research,” added co-author Link E. Olsen. Excitingly, studying treeshrew specimens has also allowed the team to discover several new species in the past five years.

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